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Tuatara aims to stimulate and widen interest in the natural sciences in New Zealand, by publishing articles which (a), review recent advances of broad interest; or (b), give clear, illustrated, and readily understood keys to the identification of New Zealand plants and animals; or (c), relate New Zealand biological problems to a broader Pacific or Southern Hemisphere context. Authors are asked to explain any special terminology required by their topic. Address for contributions: Editor of Tuatara, c/o. Victoria University of Wellington, Box 196, Wellington, New Zealand. Enquiries about subscriptions or advertising should be sent to: Business Manager of Tuatara, c/o. Victoria University of Wellington, Box 196, Wellington, New Zealand.
is the journal of the Biological Society, Victoria University of Wellington, New Zealand, and is published three times a year. Editor: H. B. Fell. Assistant Editor:
In the preceding issue ofTuatara, Dr.M. C. Probine reviewed recent advances in our knowledge of the structure of the plant cell wall. The present paper is an attempt to do the same for the cytoplasm and in forthcoming issues of this journal, Mr. G. K. Richards will review the nucleus of the cell.
It is somewhat unfortunate that a multiplicity of terms has often been used to describe a single part of the cytoplasm. In other cases, recent work has revealed that two different cytoplasmic components, with different names, are actually different phases of a single component. Where possible I have endeavoured to indicate synonyms for terms.
In recent years, sections of the cytoplasm have been examined up to magnifications of 300,000 times with the electron microscope. With improved methods of biochemical analysis it has become increasingly obvious that each of the various organelles in the cytoplasm is a compartment in which specific physiological activities occur, different from but interrelated with those occurring in other organelles. It is also becoming increasingly apparent that many cytoplasmic components which seemed clearly distinct to the optical microscopist, are in fact not entirely independent. For
This term is preferred to the older ‘nuclear envelopenuclear membrane’, because there is actually a double membrane delimiting the nucleus.endoplasmic reticulum — a discontinuous membrane system which is spread through the ground substance of the cytoplasm.
In a review of this scope, space does not permit a discussion of some cytoplasmic components, for example pyrenoids, fat bodies and phragmosomes (see Manton, 1961, for information about this last component). In general only the cytoplasm of higher plants is considered and thus the chloroplasts of algae, which in many cases are unlike those of higher plants, will not be discussed. All of the electron micrographs which are reproduced here are from animal cells, but they have been chosen to show components which are similar to those found in plants.
The following will be discussed in turn:— plasmalemma and tonoplast; vacuoles; ground substance; mitochondria; chloroplasts; lysosomes; endoplasmic reticulum. Golgi bodies, ribosomes; the nuclear envelope and its fate during cell division.
In contrast to animal cells, most plant cells are surrounded by a cellulose wall. Internal to this is the cell membrane or plasmalemma (plasma membrane). Minerals in the soil water surrounding roots, freely move into intercellular spaces and into the meshwork of the cell wall. There is evidence that there are charged sites within the wall which are able to bind certain ions, before they are transported into the cytoplasm (see Epstein, 1960). The chemical elements within cells are present in quite different proportions from those in the water and soil surrounding the roots, and from those in the sea in which plant life first began.
‘Cells, tissues, and organisms are microregions of the world containing atoms derived from the environment in proportions differing characteristically from any found in the non-living world. Almost any grouping of atoms can be identified as being part of non-living nature or having been assembled by living cells, by mere determination of the proportions of carbon, oxygen, hydrogen, nitrogen, potassium, phosphorus and sulfur that it contains. Membranes are the boundaries where the living cells abut on the environment.’ (Epstein, 1960).
Despite the high concentration of sodium chloride in sea water, there is little salt in sea weeds, in fact a high salt concentration prohibits many enzyme reactions essential for plant life. Plant cells therefore need a barrier against the free entry and exit of chemicals and the A few years ago, with the inception of the ‘Apparent Free Space’ theory, it was considered that the plasmalemma was not a barrier to free diffusion. Experiments revealed that there appeared to be a larger volume within plant tissues open to free diffusion than that occupied by the cell wall and intercellular spaces. It was suggested that only the plasmalemma is this barrier against free
tonoplast (vacuolar membrane) might be selectively permeable. Subsequent work has revealed experimental errors, e.g. insufficient drying of tissues, which gave values that were too high.carrier molecule mechanism has been hypothesised to explain such absorption. It is suggested that there are different types of carrier molecules each containing a site for a particular ion, or chemically related ions such as potassium and rubidium, which compete for the same site. The carrier molecules traverse the plasmalemma from the cytoplasm and the ion is bound into the molecule which then passes back through the plasmalemma and releases the ion, into the cytoplasm.
In the electron microscope, the plasmalemma and tonoplast each appear as two dark (electron dense) lines separated by material of a less dense nature. The total thickness of the membrane is about 70-100A. An Angstrom unit (abbreviated here as ‘A’) is 1/10,000 of a micron, which in turn is 1/1000 of a millimetre.
The tonoplast and plasmalemma possess different permeabilities. Ions passing through the tonoplast into the vacuole can be stored there until required for cellular metabolism.
Vacuoles are membrane-bound bodies which are filled with liquid. The vacuole has the highest water content of the cell's components and they contain up to 98% water. Vacuoles also contain sugars, organic and inorganic salts, organic acids, pigments, proteins, lipoids and other compounds. Many solutes are concentrated and stored in the vacuoles until required for cellular metabolism.
Manton (1962) has studied the growth of vacuoles. She noted that in immature cells, vacuoles may escape recognition under the electron microscope and in fact have been erroneously identified as ‘lipoid bodies’ and ‘dense masses of unknown nature’, by some workers. In Anthoceros (a Bryophyte) in young epidermal cells of the sporophyte, the small vacuoles are star shaped with numerous narrow tubular projections. When the tentacle-like outgrowths of adjacent vacuoles meet they fuse to form a network of tubules, and eventually the adjacent vacuoles become completely fused together.
Part of the cytoplasm appears structureless even at the highest magnification of the electron microscope. This is known as the ground substance or matrix. Some authors include the endoplasmic reticulum within the term ‘ground substance.’ Separation of the organelles in the cytoplasm from the ground substance has not been achieved with enough certainly to permit any firm conclusions about its specific chemical composition (Whaley et al, 1960). Porter (1961) has commented that with improved methods there is no reason to believe that this apparently structureless part of the cytoplasm will not in time be shown to contain complex organisations of macromolecules.
Mitochondria (chondriosomes) have been called the ‘powerhouses’ or ‘furnaces’ of the cell. It is within these ‘packets’ that the potential energy in foods, manufactured in photosynthesis within the chloroplasts, is released for metabolic processes by respiration. For example, energy is released for the formation of
Plant and animal mitochondria are very similar. They have been found in all groups of plants except the blue-green algae, red algae and photosynthetic bacteria (Novikoff, 1961a). However, it would seem likely that in these three groups there are simple membrane systems which perform the functions of mitochondria.
Mitchondria may be rod-shaped (chondriochonts), oval or spherical and range in length from 0.2 to 3.0 microns. There may be hundreds of them within a single cell —- approximately 1000 mitochondria were counted in a rat liver cell. In animals, they are present in greatest numbers in cells which undergo the greatest respiratory activity, e.g. insect flight muscles. There has been little investigation of the number of mitochondria per cell in various plant tissues. According to Mercer (1960) a few observations suggest that they are present in high numbers in the companion cells of the phloem. The companion cells are considered to have a high rate of metabolism because they supply energy for the unknown mechanism by which foods, mainly sucrose, are transported in the sieve tubes.
In most cells the mitochondria are continually moving and changing their shape.
From a number of observations, it seems that mitochondria may fuse and divide. These observations were made from a study of living tissue culture cells, seen with the phase contrast microscope. Novikoff (1961) in his excellent and detailed review, stresses that these mitochondria might well be exhibiting abnormal behaviour when fragmenting or fusing, since tissue, culture cells may be living under stress. Mitochondria are known to be very sensitive to changes and it has been discovered that merely holding a tissue between forceps may cause mitochondria to break into granules.
As seen under the electron microscope, a mitochondrion is bounded by an outer membrane about 40-60A thick which is separated by a less electron dense region about 60-90A thick from an inner membrance as thick as the outer. This inner membrane has many folds which project as cristae into the body or ground substance of the organelle. (Fig. 4). In most plant cells the cristae are flat plates; in some they are more tubular in section. In many animal cells and in those of some plants, e.g. Elodea canadensis (Buvat, 1958), the mitochondria appear to have at least some cristae which seem to extend right across the body of the mitochondrion. The infolded inner membrane provides a large surface area of membranes along which chemical reactions can occur. Each of the two mitochondrial membranes is considered to have a broadly similar structure to that of the plasmalemma.
It is remarkable that mitochondria can swell 4-5 times in volume without losing their internal solutes. It has been suggested that this property is due to the convoluted nature of the inner membrane. Recently Chandra (1962) published photographs which appear to show continuity, or rather a reversion, between inner and outer membrances (Fig. 3). Such an interchange of membranes (which I find difficult to visualise in three dimensions) would more readily explain the considerable swelling of mitochondria which can occur before there is a thinning out and rupturing of the membranes.
20-30% of the dry weight (which is about 33% of the ‘wet’ weight) is lipoid and 65-70% is protein. Some of this protein occurs as the major component of enzymes. Mitochondria do not have DNA (deoxyribose nucleic acid) the hereditary material in chromosomes. There has been controversy as to how much, if any, RNA (ribose nucleic acid) is present. Recent chemical methods indicate that about 5% dry weight of the mitochondrion is RNA and this component (see on) would be important in any protein synthesis which may occur in the mitochondria.
The following processes occur primarily or exclusively in mitochondria and all play a part in respiration.
The Krebs cycle (Citric acid cycle; Tricarboxylic acid cycle). This cycle is the terminal phase in respiration.
Oxidative phosphorylation.
The electron transfer chain in respiration.
Fatty acid oxidation. Fatty acids are oxidised in several distinct steps in which the final product is acetyl Coenzye-A.
The overall equation for respiration in which, for example, a gram-molecule of a simple sugar is decomposed to water and carbon dioxide with a release of energy can be depicted as:—
C6 H12 O6 + 6 O2 ——— 6 CO2 + 6 H2 O + about 690,000 calories
The above reaction does not occur in a single step, but in a series of about 18 reactions, each catalysed by its own specific enzyme. There are also additional steps in which hydrogen is combined with oxygen to produce water. There are scores of other chemical reactions associated with respiration, e.g. the production of enzymes, and molecules which accept and transfer electrons and which store the energy released in respiration.
The first series of reactions in respiration is known as glycolysis. These steps are common to anaerobic (respiration in the absence of available oxygen) and aerobic respiration. The end product
2 COCOOH). In anaerobic respiration (fermentation), the pyruvic acid is transformed into lactic acid or ethyl alcohol, and sometimes other compounds.
In the Recent work has indicated that this particular cytochrome may not be on the main pathway of the electron transport system (Novikoff, 1961 a).Krebs cycle terminating respiration, the pyruvic acid loses carbon dioxide and the degradation product is combined with oxaloacetic acid, with the aid of enzymes and acetyl Coenzyme-A (the final product of fatty acid oxidation). This combination forms citric acid. The citric acid is gradually decomposed with the aid of enzymes, to a series of other acids. During some of these reactions carbon dioxide is released and hydrogen is removed from substrates by DPN (diphosphopyridine nucleotide), forming DPNH (reduced diphosphopyridine nucleotide). At the close of the cycle, oxaloacetic acid is again formed. During the glycolysis steps and Krebs cycle, all of the carbon and oxygen in the original sugar molecule is released as carbon dioxide and the hydrogen has been transferred to DPN. Now in the electron transport chain the DPNH is oxidised back to DPN by a flavine compound which is reduced in the process. The reduced flavine is in turn oxidised by a member of a class of compounds known as cytochromes (cytochrome b)3 or cytochrome C oxidase) takes up oxygen which combines with hydrogen ions (formed earlier in the electron transport system) to form water.
The most important step in respiration for the plant is the storing and utilisation of the energy released. Whether or not the chemical energy released is needed immediately for cellular metabolism, it is first stored as an energy-rich bond in a compound known as ATP (adenosine triphosphate). When this decomposes to ADP (adenosine diphosphate) it releases energy. ATP is formed from ADP and inorganic phosphate at a number of places during the steps in respiration. This process whereby energy-rich ATP is formed is called oxidative phosphorylation. The energy obtained from respiration is available for metabolism when ATP is decomposed to ADP and inorganic phosphate, with the aid of enzymes.
ATP——ADP + inorganic phosphate + about 10,000 calories.
It has been estimated that the efficiency of utilisation of the energy released in respiration is at least 55% (Lehninger, 1961). That is to say, of the 690,000 calories released when a gram-molecule of a simple sugar is decomposed to cardon dioxide and water, some 380,000 calories are incorporated in ATP. Lehninger
Mitochondria have been fragmented into smaller parts by various methods to find out how many of their chemical functions can be performed by isolated cristae for example. Much of this work was done by D. E. Green and co-workers. Their results indicated that particles apparently derived from the external mitochondrial membrane were only capable of electron transport. Other particles which were considered to be derived from cristae, were also able to perform oxidative phosphorylation. Only complete mitochondria were able to carry out the Krebs cycle and fatty acid oxidation and it has been suggested that many of the enzymes needed for these processes are located in the ground substance of the mitochondrion. Using a new techniue of negative staining, Parsons (1963) and Stoeckenius (1963) obtained electron micrographs showing what were apparently one or more enzyme molecules about 85A diameter attached by a narrow stalk (40-50A long) to the membranes of the cristae and probably also on the side of the mitochondrial envelope which faces the matrix.
As I have stated, it seems that mitochondria are able to divide. Manton (1961) states that multiplication of mitochondria by division is ‘most certainly so’ in the small flagellates where the single mitochondrion can be traced throughout a cell division. If this is the only way in which they originate, they resemble nuclei in being self-perpetuating bodies. On the other hand, there have been a number of suggestions that they are formed from other protoplasmic components. It has been suggested that they (1) arise from ‘microbodies’ in the cytoplasm; (2) are formed from Golgi bodies; (3) originate in the nucleus; (4) are formed from the nuclear membrane; (5) are formed from the plasmalemma. This last possibility was suggested to me by Dr. S. G. Wildman, University of California, in 1961. Others too have considered this a possibility and Novikoff comments, ‘It is likely that more attention will be given to the presently unorthodox view that in higher cells mitochondria may arise from infoldings of the cell membrane’.
Only the chloroplasts of higher plants will be discussed. For details of other types of plastids see Granick's (1961) detailed review.
Chloroplasts contain the pigments and enzymes necessary for photosynthesis. In this process light energy from the sun is converted to potential energy when carbon dioxide and water are combined to form sugar. All life depends on photosynthesis for its continuing existence. The overall equation for photosynthesis, in which a simple sugar is formed, is:—
6 CO All of the oxygen evolved is derived from water molecules.2 + 12 H2O6H12O6 + 6 O2* + 6 H2O
Chloroplasts are considerably larger than mitochondria and vary in size, shape and number per cell. Unlike mitochondria, they are relatively immobile in most plant cells. The chloroplasts of green algae are among the most variable in shape e.g. spiral (Spirogyra), net-like (Oedogonium) and star-shaped (Zygnema). In most higher plants they are disc-like with convex ends. They are about 5 microns in diameter and 2-3 microns thick. The chloroplast is bounded by a differentially permeable double-layered membrane about 100A thick, in the centre of which is an area of low electron density. Little is known of the detailed structure of the membrane. It has been suggested that it consists or two layers of protein separated by a bimolecular lipid layer as in the plasmalemma.
When examined under high magnification with the light microscope, chloroplasts appeared to contain a number of small discs which were called grana. Electron microscope studies have shown that each granum consists of a pile of flattened vesicles (lamellae) arranged one on top of the other, like a pile of pennies. They are embedded in a ground substance called the matrix or stroma. There are lamellae (stroma lamellae) or tubules present in low density in the matrix interconnecting the grana (Fig. 9).
There are densely staining bodies, 20A to 0.2 microns in the stroma (Mercer, 1960). McLean has tentatively identified them are a carotenoid lipid phase. They have been called ‘osmiophilic droplets’. Starch grains, manufactured in photosynthesis, also occur in the stroma.
The double membrane components in a granum have been called discs. Grana are about 0.3-1.0 microns in diameter. There have been different opinions about the fine structure of the chloroplast. Hodge. McLean and Mercer (1955) consider that chlorophyll occurs on all lamellae, but Thomas (1958) and Frey-Wyssling (1957) believe it to be localised in the lamellae of the grana.
Opinions also differ as to the structure of the grana discs. Steinmann and Sjostrand (1955) from a study of Aspidistra chloroplasts consider the grana are flat hollow discs interleaved between lamellae which are continuous with the stroma lamellae (Fig. 5). Hodge, McLean and Mercer (1955, 1956) studied maize (Zea mays) chloroplasts and concluded grana and stroma lamellae were identical and grana are simply regions where the stroma lamellae have divided (bifurcated) and become more highly oriented (Fig. 6).
When considerable swelling occurs during fixation of material the intergrana membrane system becomes fragmented into small, round, closed vesicles and the grana are seen as disjunct ‘piles of pennies’.
Recent work by Weier. Stocking, Thomson and co-workers (1963) on tobacco (Nicotiana rustica) and bean (Phaseolus vulgaris) chloroplasts has given evidence for a different pattern of chloroplast structure. Their results indicate that the stroma does not invade between the discs of a granum so that discs do not alternate with interdisc space. Thus the discs (membrane bounded loculi) are tightly appressed together. They also concluded that adjacent grana are connected not by intergrana lamellae but by a network of flattened tubular channels which they called frets (Fig. 8). Their photos indicated there are connections between discs in a granum by means of these channels. Their model seems to more readily explain the ‘pile of pennies’ configuration, when the membrane system (tubules) in the stroma is ruptured by swelling treatments.
The chief components are, proteins 35-55% dry weight; lipid 20-30%; pigments (chlorophyll a, chlorophyll b, xanthophyll and carotene) 13.5%; RNA 2-3% (from Granick, 1961). It is not yet certain whether DNA is present. However Ri and Plaut (1962) using new techniques revealed ‘microfibrils’ which appeared to be DNA macromolecules in the chloroplast of Chlamydomonas, a green alga. They have undertaken preliminary studies on chloroplasts of higher plants, e.g. maize, which indicate that there are small (25A) fibrils which are DNA macromolecules. They suggest that these fibrils represent the genetic system of the chloroplast.
As in all cytoplasmic organelles, structure and function are closely related. The molecular structure of the membrane system (at present not known in detail) is such that there is an efficient transfer of energy absorbed by the pigments from the sunlight. All of the pigments absorb energy from the sun and this is
The pigments are bound into lipoprotein complexes in the lamellae. It appears that the enzymes which ‘fix’ CO2 and convert it finally to starch are localised in the stroma. The photodecomposition of water occurs in the grana. For further details, see Granick's review (1961).
J. W. Lyttelton (1962) of the New Zealand D.S.I.R. isolated ribosomes from chloroplasts. Although most protein synthesis occurs in the endoplasmic reticulum and in ribosomes free in the cytoplasm, it was known that mitochondria and chloroplasts could synthesise proteins from amino acids. Ribosomes have also been isolated from mitochondria and nuclei.
‘Mature plastids in many pigmented algae can multiply by fission and habitually do this to keep pace with normal cell division. In higher plants and in algae with specialised growing points this capacity seems to have been lost’ (Manton, 1961).
Chloroplasts of higher plants arise by the growth of small bodies called proplastids and multiplication occurs at this proplastid stage. Young proplastids are amoeboid, colourless bodies, 0.4-0.9 microns in diameter and are surrounded by a double membrane (Fig. 7a). They divide by elongating and ‘pinching’ in half. When they are about 1 micron in diameter. the inner membrane buds off spherical or elongate vesicles (Fig. 7b). These increase in number, fuse, widen and in some areas the vesicles thicken and a pale green colour develops. The proplastids continue enlarging and become lens shaped. Vesicles are still formed from the inner membrane (Fig. 7c). Numerous double membraned lamellae now extend the length of the plastid with slight differentiation into grana and non-grana regions (Fig. 7d). Then in regions where the grana are becoming differentiated. the vesicles increase in thickness and become arranged close together, forming grana (Fig. 9).
When seedlings are grown in the dark, the vesicles accumulate to form a dense body, the primary granum or prolamellar body which has a three-dimensional lattice of beaded or tubular strands. When the seedling is placed in the light. normal grana are formed. It has been considered that the formation of the primary granum was typical of any developing chloroplast, but most workers now believe that it is an atypical structure formed only when seedlings are kept in the dark.
Less than 10 years ago a new cytoplasmic component was found by de Duve and co-workers, using improved centrifugation techniques. They discovered spherical particles, about 0.4 microns in diameter which they named ‘lysosomes’ because of their richness in hydrolytic enzymes (Novikoff, 1961). It has been deduced that they are bounded by a lipoprotein membrane. There is also evidence for the existence of other particles of similar size but not chemically identical to lysosomes. Most of the research has been on mammalian tissues, especially liver cells, but recent work indicated that plants too apparently have acid phosphatase located in granules (Novikoff. 1961b). At present any granules which stain for acid phosphatase are considered to be lysosomes and considerable work is being undertaken to clarify the nature of them.
It has been suggested that together with Golgi bodies, lysosomes play a part in the formation of many kinds of secretion products. Brachet (1961) states that de Duve has shown that the lysosome, ‘contains the digestive enzymes that break down large molecules, such as those of fats, proteins and nucleic acids, into smaller constituents that can be oxidised by the oxidative enzymes of the mitochondria.’
Extending throughout the ground substance of the cytoplasm is a network of membrane-bound vesicles — the endoplasmic reticulum. This endoplasmic reticulum (ER) or ergastoplasm, is a more or less labile (changing) structure which in many meristematic cells is an elaborate network of tubules. In older cells it may be a less extensive system of membrane-bound vesicles. The bounding membrane of the ER is lipoprotein and is considered to be of a broadly similar structure to the plasmalemma. The material enclosed by the membranes appears structureless under the electron microscope. The membranes provide a large surface area within the cytoplasm which would allow an ordered distribution of enzymes and substrate. The ER probably represents, ‘various and varying packets of metabolites and enzymes’ (Porter, 1961).
There are two types of ER, a smooth or agranular form and a rough or granular one. The smooth form consists of a complex of tubules with a diameter of 500-1000A. The rough type which is especially prominent in cells undergoing considerable protein synthesis, has vesicles which are flat rather than tubular. There are always small round particles (sometimes called Palade granules) on the outer surface of the membranes. They also occur on the outer side of the nuclear envelope. These sperical particles which are rich in RNA seem similar to or identical with ribosomes. Ribosomes are small spherical cytoplasmic bodies. 150-200A in diameter, which are involved in protein synthesis. Continuity between the smooth and rough forms of the ER has been repeatedly demonstrated (Porter, 1961).
When the ER is centrifuged. it is broken up into small phospholipidribonucleoprotein particles. 500-2000A diameter, to which Claude gave the name ‘microsomes’. For a time, it was believed that ‘microsomes’ were integral components of the cytoplasm but it has now been established that they represent a breakdown of smooth and rough ER.
The Golgi bodies (Golgi apparatus, dictyosomes) of plants each consist of a ‘stack’ of about six flattened plate-like sacs at the edges of which are associated small spherical vesicles, apparently budded off from the edges of the Golgi-sacs. The Golgi apparatus resembles smooth ER except that the sacs are somewhat smaller, more closely appressed together and may not stain with the same density. There are many Golgi bodies in a meristematic cell. It is not clear how they reproduce, but daughter cells have as many or more Golgi bodies as their parent cell and thus multiplication must occur at about the time of division (Whaley et al., 1960).
At present it is not clear just how closely related the ER and Golgi system are. Periodic continuity has occasionally been shown between them but it is not yet clear whether the Golgi apparatus represents a special differentiation of the ER. ‘Stacks’ of rough endoplasmic reticulum have also been reported in some cells.
In plant and animal cells it has been found that the outer membrane of the nuclear envelope (i.e. the outer layer of what is also called the nuclear membrane) is continuous with the ER. In places, this outer membrane bounding the nucleus extends out into the cytoplasm as part of the ER. Porter (1961) comments, ‘this striking fact … makes it proper to regard the nuclear envelope as part of the endoplasmic reticulum.’
At present these functions are inadequately known. There is no doubt that considerable protein synthesis occurs in the rough form of the ER. Its associated granules are rich in RNA. There is also evidence of RNA in membranes of the smooth ER. Pioneer work by Brenner et al., (1961) indicated the probable role which
It has also been suggested that the ER may function in the transport of metabolites, e.g. from sites of synthesis to sites of breakdown. In meristematic cells, elements of the ER extend to the cell surface and occasionally at least through the wall into neighbouring cells (Whaley et al., 1960).
The smooth form of the ER is common in cells engaged in the synthesis of lipoids and there is an elaborate development of this reticulum along surfaces where cell walls are being formed (Porter, 1961).
Porter (1961) noted that a membrane enclosed space would allow development of electrical membrane potentials of possibly great significance in life processes.
In general it seems that the Golgi apparatus has a secretory role in plants and animals. Mollenhauer. Whaley and Leech (1961) found a function for the Golgi apparatus in outer rootcap cells of maize (Figs. 10 and 11). They observed that the Golgi-sacs swelled at their edges and blebbed off vesicles, larger than the ones characteristically associated with the Golgi apparatus. These vesicles enlarged to about 1000A, became more electron dense and appeared to develop an internal fibrillar structure (Mollenhauer and Whaley, 1963). They moved to the surface of the cytoplasm and passed through the plasmalemma. When seen outside the plasmalemma the vesicles lacked bounding membranes and it was assumed that the membranes of these Golgi-produced vesicles are incorporated into the plasmalemma (Fig. 10). The bodies outside the plasmalemma became packed together and finally became part of new cell wall material (Fig. 11). In another paper, Whaley and Mollenhauer (1963) suggested that the Golgi apparatus in
4, embedded in Araldite and stained in KMnO4. Further details in text.
This double membraned structure delimits the nucleus. The space between these two membranes (perinuclear space) is 200-400A wide, ‘a sort of moat around the nucleus’ (Porter, 1961) and each membrane is 50A thick. It has been clearly shown that there are pores (annuli) in the nuclear envelope 500-1000A in diameter and larger. Their position seems to coincide with places where the nucleoplasm extends to the nuclear surface, i.e. pores are not found where the chromatin of the chromosomes abuts onto the membrane. Current evidence suggests that the pore is an opening which allows the passage of relatively large particles between nucleus and cytoplasm. Feldherr (1962) injected small gold particles of up to 55A diameter into the cytoplasm of an amoeba (Chaos chaos) and obtained electron micrographs showing the particles in the nucleus and within the pore of the nuclear envelope. The inner and outer membranes of the envelope join to form the circumference of the pore. The extensions of the outer membrane into the cytoplasm which become part of the ER do not have pores.
Barer et al., (1960, 1961) studied the division of spermatocytes in insects and snails. (a) They found that mitochondria cluster around the nuclear envelope and in some cases appear to pull parts of the outer membrane of the envelope into the cytoplasm, where it forms into vesicles and apparently becomes part of the ER. They also suggest that the mitochondria may secrete enzymes which are involved in the breaking up of the nuclear envelope. Mazia (1961) comments that lysosomes might be a more likely source of such enzymes.4 embededd in Methacrylate and stained in KMnO4.
Porter and Machado recently studied division in onion root tip cells and also concluded that the nuclear envelope was reformed from the ER. In contrast to these conclusions, Manton (1960) from a study of cell division in the meristem of Anthoceros cautiously suggested that the new nuclear envelope appeared to be derived by fusion of tubular elements formed from Golgi bodies.
Our knowledge of plant and animal cells has been greatly extended over the last few decades. There has been an ever increasing volume of literature published and it is becoming common for research projects to be duplicated, especially with research on animal tissues. Techniques are still rapidly improving and within a few years our knowledge of the cytoplasm will be greatly extended. Recent work emphasises the following points. (a) There are many detailed similarities between the cytoplasm of plants and animals. (b) A close relationship between structure and function of cytoplasmic organelles. (c) Many organelles are more closely interrelated than was once thought.
Fig. 12 is a diagrammatic representation of plant cell structure, which summarises the ultrastructure of the plant cell.
Fig. 13 is an electron micrograph of a section through part of two spermatocytes of the weta, Pachyrhamma fascifer. Magnification about 20,000. N = nucleus; CY = cytoplasm; PL = plasmalemma, separating the two cells: NE= nuclear envelope: P = pore in nuclear envelope (to the left of the letter); C = continuity between nuclear envelope and endoplasmic reticulum (below the letter); M = mitochondrion.
Fig. 14 shows a section through part of a weta spermatid. Magnification about 50,000. G = Golgi body; N = nucleus; C = cytoplasm: E = nuclear envelope.
Fig. 15 was chosen to show both rough endoplasmic reticulum (R) and smooth endoplasmic reticulum (S) in a single cell. P = the plasmalemma (plasma membrane), separating two cells; N = nucleus; M = mitochondrion. The section is of sheep liver hepatic cells, magnified about 20,000 times.
I wish to thank Mr. W. S. Bertaud, Electron Microscope Section, Dominion Physical Laboratory, Lower Hutt, and Dr.
4 embedded in Methacrylate and stained in KMnO4.
general references.
Third Residential Course on Forest Ecology, Totaranui, Tasman National Park, Nelson Province. Nov. 19-26, 1963. Tutor: A. E. Esler.
Fourth Field Course for Naturalists (Ecology), Stratford Mountain House, Egmont National Park. Jan. 4-11, 1964. Tutors: P. F. Jenkins,
Second Field Course on Geology, Wairarapa, St. Matthew's Collegiate School, Masterton. Jan. 4-13, 1964. Tutors: P.
All three schools are residential. Full information concerning these schools is obtainable from the Director of Adult Education (Victoria University of Wellington), P.O. Box 2945, Wellington.
This handsome flower is often erroneously known as the ‘Mount Cook Lily’. although in fact it belongs to the same genus as the common buttercup. It is a buttercup of grand dimensions with stems up to five feet tall, peculiar, nearly circular leaves up to one foot across and flowers up to three inches in diameter. The photograph was taken at Arthurs Pass and the species occurs elsewhere in moist places on mountains throughout the South Island and in Stewart Island. The pure white of the petals is unusual. Of the 43 native species of Ranunculus in New Zealand only one other species (R. buchananii) has white petals. The remainder are yellow. A few years ago Mr. W. B. Brockie crossed Ranunculus lyallii with the large, yellow-flowered R. insignis and obtained a handsome sterile hybrid with pale-lemon petals.
The genus Ranunculus is one of a number of alpine genera in New Zealand which are strongly represented in the north temperate regions and may have originated there. Other such genera are Gentiana. Epilobium and Myosotis and in these, unlike Ranunculus, white flowers are the rule in New Zealand with few exceptions.
This is the alpine cushion plant commonly referred to as the ‘vegetable sheep’. Such extreme growth forms are quite common in the New Zealand alpine flora, having evolved independantly in about 19 genera. In many cases, including Haastia pulvinaris, the plant is basically a shrub, profusely branched, with the densely leafy, ultimate twigs pressed together lengthwise so that their tips form a continuous and often very firm surface. In the illustration, the more or less circular areas on the surface of the cushion are the branch tips and these are often so firmly compressed that they assume a hexagonal outline. The leaves are densely woolly, and only those at the surface are living. The interior of the cushion between the branches is filled with the decayed remains of older leaves which form a felty humus with a high water retaining capacity.
Haastia pulvinaris belongs to the Compositae or daisy family and can be found in exposed rocky situations above 4.000 feet on the mountains of Marlborough and adjoining areas of Nelson Province. The photograph was taken on Mount Cupola near the head of the Travers Valley.
In the Andes of South America there is a similar array of cushion plants, but they are quite unrelated to those of New Zealand.
This short review is intended primarily as an introduction for students not acquainted with the literature of ethology. Within the space available it has been necessary to be selective; I have attempted at least to mention all the major aspects of the subject but it has not been possible to give equal attention to them. My personal interests and opinions have no doubt introduced some bias and placed emphasis where others would not put it. For this I make no apology.
I have chosen to treat the subject in a roughly historical fashion: firstly because in a short period ethology has undergone such rapid development that it already serves as a good example of how sciences progress; secondly because some of the older ideas of ethologists are still in current use in some quarters, although they have been superseded in the opinions of most present day workers. I think the newer ideas can be best presented by indicating how they grew out of the older.
Large scale study of animal behaviour by zoologists is a relatively new development; it lagged behind the study of animal behaviour by psychologists and physiologists. This statement should perhaps be qualified on two counts: firstly, the distinction between zoologists, psychologists, and physiologists is a relatively modern one and, even now, it is not always easy to draw; secondly, students of animals from at least the time of Aristotle more than occasionally took note of behaviour — in the cases of such people as
By and large the psychologists had looked for behaviour, in animals, that conformed to the categories of learning patterns that had been worked out for humans, and had found little else There are, however, numerous exceptions, particularly among American psychologists. For example some of the early work of Watson (1908), Yerkes (1912), and Lashley (1915, 1938) treated the behaviour of sub-human animals as sui generis and, more recently, Schnierla and his associates have emphatically argued the case for keeping in mind the differences in behaviour between animals at different phylogenetic levels (e.g. Schnierla, 1949).
It was largely a reaction to these teachings which established the existence of ethology. Konrad Lorenz (e.g. 1935, 1937 a & b, 1950), building on the work of such people as C. O. Whitman, Oscar Heinroth, Wallace Craig. Edmund, Selous, Eliot Howard, Gestalt or ‘purposivist’ psychology (the European school that denied the possibility of behaviour analysis without introspection), and of behaviourism (the composite of learning theory and Pavlovian reflexology that flourished in America during the thirties) would not do when applied to the majority of animals in nature. The Gestalt people were convicted of vitalism, of retreating into mysticism before the limitations of a scientific analysis had been tested. The behaviourists were praised for their tough-mindedness but censured for their narrow-mindedness. If they had taken the trouble accurately to observe their animals, Lorenz claimed, they would have seen that much of the behaviour was spontaneous — not dependent on changes in the immediate external stimuli — and that reaction to a stimulus was rarely constant. The facts, for Lorenz, indicated a measure of internal control that is independent of the external stimuli of the moment. A further set of facts indicated that certain aspects of this internal control are independent of the stimuli of any moment, i.e. they are inborn rather than acquired by experience — innate rather than learned.
Properly to understand a piece of behaviour we have to appreciate its function in the life of the animal and its position in the whole behavioural repertoire of the animal. This necessitates study of the animal in its natural situation or in conditions that do not disguise the biological relevance of its behaviour. Studies of this
sort, by such people as Selous (e.g. 1905), Howard (e.g. 1929), Huxley (1914) and Verwey (1930), had shown that an animal's behaviour is as nicely adapted to its environment and way of life as are its structure and physiology. To a zoologist brought up on Darwinian principles, adaptation suggests natural selection. This line of thought was strengthened by the observations of Whitman (1899, 1919) and Heinroth (1910, 1930) that many of the acts of birds (most of the early observations of behaviour were made on birds I are stereotyped, are performed in exactly the same way by all members of the species, and can be recognised as homologous with similar acts in related species, the degree of similarity corresponding to taxonomic affinity The Shorter Oxford Dictionary lists three meanings for ethology the closest to the present connotaion being J. S. Mills' use of it for ‘the science of character’ (System of Logic, 1843). Lorenz and his followers adopted the word from Heinroth (1910) and this probably links with Mills' use through Heinroth's emphasis on species specific aspects of behaviour. At the Macy conference on Group Processes. 1954, Lorenz claimed that Heinroth's meaning was ‘the study of innate behaviour. Species-specific drive activities’. Tinbergen recommended that ‘ethology’ be understood as ‘the biological study of behaviour’ (ibid.: 77).ritualisation. In social hostile contexts displays clearly are of selective value because they avoid the risk of physical injury that animals expose themselves to in actual fighting. The selection of species specific courtship patterns seems frequently to have been involved in the evolution of sexual isolation between diverging populations (see Mayr, 1942). These facts of adaptation and the taxonomic distribution of behavioural characteristics strengthened the case for saying that much of the variation of behaviour between individuals and between species corresponds to variation in the germ plasm.
Finally there were observations of animals performing complicated behaviour patterns perfectly at the first opportunity without previous experience of practice or imitation. For example Grohmann (1939) reared a group of pigeons in narrow tubes so that these birds were prevented from carrying out the flapping movements of the wings which young pigeons perform before they can fly. At the age when pigeons are normally able to fly these experimental birds were released and flew immediately as well as unconfined controls. A similar experiment was carried out by Spalding (1873, 1954) on young swallows. Carmichael (1926, 1927) raised a number of tadpole eggs in a solution of chloretone, a substance that produces anaesthesia of striped muscle but permits normal growth. This prevented the practice of swimming movements by the developing tadpoles, but when they were eventually placed in pure water they swam as prefectly as controls of the same age that had been reared in normal conditions. To this list could be added the cases where trial and error learning is ruled out because unless a response is performed perfectly at the first time of asking, the animal is killed. For instance, unless the courtship dance of a male salticid spider inhibits the feeding responses of the female, he will be killed and eaten at his first attempt at mating. These cases, Lorenz argued, could not be explained as instances of learning from experience in the life of the individual; they could be explained only in terms of the history of the species and its genetic endowment.
The writings of Lorenz stimulated field naturalists and zoologists to pay close attention to the behaviour of animals and to think of it in terms of biological function and evolutionary origin. Detailed descriptions were produced such as the ethograms of Makkink (1936, 1942), Tinbergen's studies of birds (1935, 1939), insects (Tinbergen et al, 1942), and fish (Ter Pelkwijk & Tinbergen, 1937), Baerend's work on digger wasps (1941) and cichlid fishes (Baerends & Baerends van Roon, 1950). The functional significance of such things as the bill colour of gulls (Tinbergen, 1949. Tinbergen & Perdeck, 1950), countershading in caterpillars (de Ruiter. 1955), the red breast of the Robin (Lack, 1943), the
Though much of this new information could be explained in terms of ultimate causes — biological utility and phylogeny — there remained the questions of proximate causation — the factors and mechaninisms acting here and now which directly determine what an animal is doing. The emphasis Lorenz placed on the inateness of behaviour implied a degree of independence of behavioural control from the vagaries of the external world, and this was made explicit in the kind of mechanism that he suggested for this control. He claimed that, far from being a stimulus-bound reflex machine, an animal is a spontaneously active thing driven from within by endogenously generated energy. He started from Wallace Craig's (1918) observation that many behaviour patterns can be described as a chain of variable, striving, goal-directed responses ( Some end acts have been analysed into an externally oriented component — the appetitive behaviour) which terminates in performance of a simple stereotyped response (the end act or consummatory act). The appetitive acts are oriented by external stimuli (releasers or sign stimuli) and, once released, runs its course without further mediation from external stimulitaxis — and a component independent of external cues after release— the fixed action pattern, e.g. the egg-retrieving of the Grey-lag Goose, Lorenz & Tinbergen. 1938.vacuum activity, as he called it, Lorenz (1937) cited a captive starling that he had and which he consistently fed by hand. This bird would perform a complete sequence of prey-catching and eating reactions although there were none of the normal releasing stimuli present. (Tinbergen, 1951: 61-62, mentions a number of similar examples).
As further support for his belief that much behaviour is the expression of endogenous co-ordination and energy fluctuations, independent of afferent input, Lorenz cited the work of von Holst, Weiss and W. R. Hess. Von Holst (1932, 1933) had shown that the isolated nerve cord of an earthworm, deprived of all afferent stimulation, continues to send vollies of impulses along its length and that the timing of these rhythmical vollies corresponds exactly to the contraction waves that pass down the segments in normal locomotion. A spinal eel with its nerve cord isolated from all proprioceptive input continues to perform perfectly co-ordinated swimming movements (von Holst. 1937). In experiments on the growth of nerve fibres in axolotls. Weiss (1941) contrived a transplated limb graft that received connections with motor nerve fibres from the nerve cord before it had received any connections with sensory fibres; such a limb graft began making perfectly co-ordinated walking movements as soon as the motor nerves made their connections. Hess (e.g., 1956) electrically stimulated the mid-brain of cats with implanted electrodes and found that it was possible, by this means. to produce fully co-ordinated behaviour patterns, including appetitive sequences terminating in consummatory act, identical with normal behaviour. In Lorenz's view such experiments as these could not be accounted for by a chain reflex theory of integration.
From the fact of the specificity of the stimuli releasing a response, Lorenz argued that there must be. in the animal, a releasing mechanism for each such response. which is selectively responsive to only a narrow range of external stimuli. This is referred to as the In a recent review of the concept. Schleidt (1962) has pointed out an interesting difference in the senses of angeborene auslosende Schema (AAM) or, in Tinbergen's translation of the term, the innate release mechanism (IRM)Schema and Mechanismus in German and that Tinbergen's translation also shifted the meaning. Schema was von Uexküll's term and it signified simply a correlate or image of the releaser, that must be carried inside the animal. Tinbergen's IRM signified a kind of structural organisation linking a specific stimulus to a specific reaction.reaction specific energy: corresponding with each response pattern there is an internal source, generating energy, which activates the appetitive behaviour when it reaches a certain
Ubersprungbewegungen (Tinbergen, 1940) or displacement activities (Armstrong, 1947, 1950). According to this theory a response could thus be caused in two ways: it could be caused by ‘its own factors’ as when feeding behaviour is consequent on hunger and the presence of food; or it could be caused by the factors belonging to another behaviour pattern, in which case it was a displacement activity. Kortlandt (1940 a & b) coined the terms autochthonous and autochthonous to distinguish the two kinds of causation.
Lorenz's scheme was elaborated by Tinbergen (e.g. 1951). He gave a more neurophysiological ring to it by renaming the action specific energy as motivational impulses and by referring to the sources of these impulses as centres in the central nervous system. Tinbergen (1942, 1950) and Baerends (1941) also introduced the notion of hierarchy into the system. Behavioural functions can be classified in a hierarchical fashion. An act can be described as belonging to a series of progressively more comprehensive classes. For instance a particular movement might be labelled as ‘digging’; this, together with others such as carrying material, will be further classified as ‘nest building’; nest building, together with classes on the same level such as courtship, territorial fighting, care of offspring, can be classed together as reproductive behaviour. Tinbergen claimed that the course of many behavioural sequences is a descent through such a hierarchy of functional classes. To return to an earlier example, the hawk that flies over the country is showing appetitive feeding behaviour; the precise behaviour that this leads to will depend on the kind of stimuli that is discovered — if the hawk encounters a flock of small birds it will do one thing, if it encounters a lone pigeon it will do another, if it sees a young rabbit will do a third thing. Whatever the kind of prey encountered, the result will be a switch to a more restricted class of actions (‘starling catching’ behaviour for example). Once the prey is secured the new set of stimuli at
A prominent place was given to innate elements in these theories. It was believed that learning could affect appetitive parts of a behaviour pattern to some extents but the consummatory act was regarded as purely innate — its constancy of form in the life of the individual, and in each individual of the species, pointed to the stability of species genotype rather than the uncertainty of environmental influences. The releasing mechanisms were labelled as innate for the same reasons.
Lorenz emphasised that learning could, for particular behaviour patterns, be confined to crucial short periods during development. He (e.g. 1935) found that in many birds, such as ducks and jackdaws, if the young are exposed to certain stimuli during a critical period in early life, these stimuli become irreversibly linked to certain behaviour patterns. Thus a duckling can be made to treat a green box as if it were its mother, and a jackdaw can be made to direct all its courtship behaviour to a man. This phenomenon Lorenz called imprinting.
Ethology, then, in the early 1950s, could be identified with a school of animal behaviour students who approached behaviour from the direction of ecology, evolution, taxonomy and comparative anatomy; who consequently emphasised the roles of genetic components in the development and control of behaviour; who insisted on thorough study of the whole of an animal's behavioural repertoire, preferably in its natural setting, and developed a set of new technical terms for classifying and describing behaviour; and who interpreted their findings in terms of models of energy generation, flow, accumulation and exhaustion.
These models exerted considerable influence and still provide the conceptual basis for analysing and thinking about behaviour in some quarters. They had the virtue of introducing order into a wide range of otherwise unconnected facts; their elegance and comprehensiveness carried considerable appeal of a sort that might he called aesthetic.
(To be continued)
In View of the Frequently Suggested Subtropical Affinities of a part of the New Zealand flora a close look at the flora of our nearest neighbour in a tropical direction, New Caledonia, is clearly called for. New Caledonia lies 1.000 miles north-west of New Zealand at a point approximately half-way between New Zealand and New Guinea. The island's dimensions, about 250 x 30 miles, are very similar to those of the North Auckland peninsula, although the former is much more mountainous with altitudes up to 5.500 feet. Geologically New Zealand and New Caledonia have had related histories and the two countries are at the present time connected by a series of submarine ridges. The number of seed plants native to New Caledonia is estimated at about 3,000 far exceeding the approximately 1,750 species native to New Zealand. Furthermore, if the comparison were restricted to comparable types of vegetation the discrepancy would be even greater, as the New Zealand alpine vegetation has no counterpart in New Caledonia.
Melaleuca woodland, East coast. Note pale bark blackened at the base by fire.
Dracophyllum sp. centre, Meryfa sp. right.
Despite the lack of alpine habitats the island is hy no means entirely forested. The prevailing winds are easterly, so rainfall is highest on the east coast as well as on the upper slopes of the mountains and rainforest is largely restricted to these situations. On the drier western side of the island the prevalent vegetation cover is a dry open woodland dominated by Melaleuca leucadendron (Niaouli). This overall pattern is complicated by the occurrence of large areas of serpentine rock. The largest such area occupies all of the southern third of the island apart from a western strip and there are also smaller ‘islands’ of serpentine spaced along the north-western coast. These latter cause a striking change in the vegetation cover as Melaleuca leucadendron is rarely found on serpentine, being replaced there by a lower, denser cover of Acacia spirorbus. The pattern of vegetation on serpentine in the wetter areas, notably the south-east, is much more complex. Rainfall in the south-eastern serpentine area is mostly over 100 inches which would be more than adequate for forest under normal circumstances. However at lower elevations, below about 1,500 feet on the average, the vegetation is ‘scrub’ formed by a surprising variety of shrubs and some herbs of a distinctly xerophytic appearance. The terrain clothed by serpentine scrub is of relatively low relief with a deep, red soil of sandy texture. According to Sarlin (1954) this soil may be metres to dozens of metres thick and he attributes the xerophytic nature of the vegetation to the extreme permeability of the soil, which he feels more than counter balances the high rainfall. On the other hand Birrell and Wright (1945) suggest that the absence of forest on these soils may be due to toxicity of the chromium and nickel compounds present.
Above 1.500 feet the slopes become much steeper and support extensive, species-rich forests. Rainfall is higher and according to Sarlin this factor, combined with the steep slopes, allows heavy haching of the serpentine minerals and their rapid transport to lower levels. The soil is skeletal, consisting of a thin layer of clay without any red colouration.
The presence of forest at higher levels on serpentine is puzzling as it is generally thought that with steep slopes and skeletal soils the toxic effect of serpentine increases, while here the reverse appears to be the case.
My main aim in visiting New Caledonia was to examine the forests there in order to compare them with those of New Zealand. Two main types of forest are recognised in New Caledonia. These are termed by Sarlin (1954) ‘middle altitude forest’ and ‘conifer forest’. The former ranges from approximately 1.300 feet to 3.250 feet in altitude and appears to fit the concept of tropical rain-forest as described in Richards (1952), the latter ranges from 3.250-5.400 feet and has much in common with the New Zealand
I was able to examine lowland forest on the Mount Kohgi Range near Noumea and also patches of regenerating forest of this type on the east coast near Poindimié. A good example of montane forest was studied on Mt. Ignambi in the far north.
Although sharing the same range of plant forms with the New Zealand lowland forest — trees, shrubs, lianes and vascular epiphytes — the lowland rain forest in New Caledonia differs from ours in several respects. On entering the forest two differences are immediately apparent, firstly general leaf size is much greater, a fact which is first observed in the leaf litter, and secondly lianes and epiphytes are neither so abundant nor so luxuriant. Shrub epiphytes appear to be absent. In structure the forest is similar to ours, being multi-storied with an upper level of emergent trees and below that upper and lower canopy layers and a shrub layer. The emergents however are flowering plants while ours, with the exception of Metrosideros robusta, are conifers. The Metrosideros becomes an emergent by virtue of establishing itself as a so-called ‘strangling’ epiphyte on an emergent conifer.
The number of species in this New Caledonian forest is bewilderingly great. In one small valley on Mount Kohgi I collected 60 species of trees, shrubs and lianes and the collection was by no means complete.
Ferns, bryophytes and lichens appeared to be less common than in our forest and tree ferns in particular were quite infrequent. In gullies were one might expect to find tree ferns there were instead impressively large plants of Marattia. In some cases the root stocks were several feet high and wide with fronds up to 20 feet long.
In my notes on the Mt. Kohgi forest I describe the emergent trees as being 20 or more feet apart and up to 100 feet high with relatively slender, gradually tapering trunks 2-3 feet in diameter. A few of the emergent species have elaborate plank buttresses at the base, while others are only slightly buttressed. The trunks for the most part are free of epiphytes and climbers. Occasional plants of Asplenium nidus (Bird's nest fern) occur at branch forks and also the more diffuse fern Drynaria rigidula, but there does not seem to be any tendency for these to be aggregated together into ‘epiphyte gardens’ as is the case with Astelia and Collospernum in New Zealand.
The main canopy trees are closer together, lack buttresses, are mostly no more than a foot in diameter and range up to 70 feet high.
The sub-canopy trees are up to 40 feet high with trunks six inches or less in diameter.
An occasional large tree supports a strangling fig (Ficus spp.). These differ from the New Zealand Metrosideros robusta in that the descending roots form a complete network about the trunk of the host. In Metrosideros robusta the roots are usually disposed to one side of the host trunk.
When we reached the montane forest on Mt. Ignambi the similarities with New Zealand lowland forest were immediately apparent. Leaf size was greatly reduced, species were fewer, and most of the canopy trees belonged to familiar genera —