The New Zealand flora exhibits a number of unusual features and notable among these is the relatively high proportion of flowering species having unisexual flowers. Not all species have been fully investigated, but Millener (1961) has estimated that about 25% exhibit unisexuality in New Zealand compared with only 8% in the British flora. The contrast becomes even more striking when certain cosmopolitan genera and families are singled out for consideration. In Clematis, for instance, the New Zealand species are dioecious (male and female plants), while elsewhere in the world this sexual pattern is so uncommon in the genus that a European species possessing it has the name Clematis dioica. Rubus in New Zealand is a similar case.

Among families unisexuality is prevalent in the New Zealand Umbelliferae. In this family elsewhere in the world flowers are predominately hermaphrodite, although in some cases male flowers may be mixed with hermaphrodite in the same inflorescence. Of the 94 species currently recognised for the family in New Zealand, at least 54 (58%) are dioecious (Anisotome, Aciphylla, Coxella), probably 8 (8%) are gynodioecious i.e. with female and hermaphrodite plants (Gingidium), and 32 (34%) are hermaphrodite (Daucus, Apium, Lilaeopsis, Oreomyrrhis, Eryngium, Hemiphues, Centella, Hydrocotyle, Schizeilema). In New Zealand unisexual flowers apparently occur only in the subfamily Apioideae for which 72 native species are recognised. The corresponding percentages for the subfamily are 76% dioecious, 10% gynodioecious and only 14% hermaphrodite. Eight of the 14 genera of the Umbelliferae in New Zealand belong to the subfamily Apioideae and of these 4 genera (Anisotome, Aciphylla, Coxella, Gingidium) exhibit unisexuality.

Unisexuality then is a common condition in the New Zealand Umbelliferae and it is also relatively common in the New Zealand page 68 flora as a whole. Why should this be so? In dioecious species outcrossing is mandatory so individual variation is at a maximum in such species. When environmental conditions change drastically variable species could be expected to have a selective advantage over those less variable, particularly in isolated, relatively small land areas such as New Zealand. On continents species may survive adverse conditions by migrating. On isolated islands such escape routes are still available, but being largely over water, are much more difficult to negotiate. In these circumstances variability becomes a major factor in survival, as Rattenbury (1961) has pointed out, and among the variable species would be included those with some form of unisexuality.

During the Pleistocene the climate fluctuated widely and there must have been a high rate of extinction at some times and a high rate of speciation at others. Possibly the dioecious element in the New Zealand flora began to expand at this time.