The technique described in this note is essentially the same as the Avena straight-growth method developed by Bentley and Housley (1954); modifications have been introduced to make the technique suitable for class experiments. Advantages of the method are:—

Oat grains are soaked overnight (the variety Onwards, kindly supplied by Crop Research Division, D.S.I.R., has been found satisfactory). These are then sown fairly thickly on the surface of moistened ‘vermiculite’ in a seed box and are covered thinly with a mixture of approximately equal volumes of vermiculite and sand. The substitution of vermiculite for sand (as used by Bentley and Housley) has two advantages. Firstly, the amount of water used in the initial wetting of the substrate is far less critical since the margin between providing sufficient water for growth on the one hand, and avoiding water-logging with consequent loss of aeration on the other, is greatly increased. Secondly it disposes of the necessity for subsequent waterings during the growth period.

In order to maintain the high humidity necessary for the growth of the test seedlings, the seed box is supported above free water contained in a deep tin, which substitutes for the constant humidity room used by Bentley and Housley. Blotting paper dipping into the water, lines the sides of the tin and a sheet of glass is used for a cover (Fig. 1a). The tin is placed in the dark in an incubator at a temperature of about 25° C.

When the coleoptiles emerge through the surface a red photographic safety lamp supported above the glass plate is switched on. This prevents excessive growth of the first internode. When the coleoptiles are 1.4-1.5 cm. long (about 80 hours after sowing the grain under the conditions described) the seed box is removed and coleoptile sections are prepared in a dark room using a red light which is phototropically inactive.

For the preparation of the sections the following are required:—

Coleoptiles are first broken off between finger and thumb at ground level, those of an unsuitable size being discarded. Ten coleoptiles are then laid side by side on the moistened surface of the slide. These are arranged

Fig. 1 — A: humidity chamber for growing the oat seedlings; a, glass cover; b, blotting paper lining walls; c, tin container; d, seed box; e, vermiculite; f, stand; g, water. B: cutter; a, razor blade screwed to wood. C: coleoptiles ready for cutting; a, 3 mm. line on slide.

so that their tips are at the edge of the slide subtended by the 3 mm. line (Fig. 1c). The slide is then upturned and the coleoptiles lowered on to the cutting edges so that one of the latter corresponds in position to the 3 mm. line on the slide. The 3 mm. tips are discarded and the 10 mm. sections lifted from between the razor blades with a pair of forceps. The difficulty of gauging accurately the position of the cuts makes this method of preparing the sections relatively slow. Use of a cutter of the type described by Bentley and Housley would overcome this but page 26 its construction would demand facilities not normally available in the laboratory.

The sections are floated on the solutions to be assayed which are contained in petri dishes. These are incubated in the dark at a temperature of 25° C. for 24 hours. At the end of this period the length of the coleoptile sections is measured to the nearest mm. using a flexible celluloid rule.

The results shown in Fig. 2 are based on an experiment in which five sections were floated on 15 ml. of solution in a 9 cm. petri dish. Three

Fig. 2 — The effect of 3-indolylacetic acid on the elongation of coleoptile sections. The vertical lines represent twice the standard error of the means.

dishes were used in each treatment. Comparison with results reported by Bentley and Housley (1952) suggest that simplifications and modifications of method have not impaired the sensitivity of the assay.