2] THE VARIOUS GROUPS OF PLANTS 63 



The Gymnosperms are distinguished by having ovules which are 

 borne ' naked ' on leaf-like organs (cone-scales). Although not 

 enclosed in an ovary, the ovules are often well protected by the 

 mutual contact of the cone-scales or by the development of other 

 special structures. For fertilization a small gap is left or an opening 

 occurs, and for liberation of the ripe seeds the cone-scales or other 

 protective structures simply spread apart. The living Gymnosperms 

 are all perennial woody plants, ranging from small shrubs to the 

 very largest of trees. Their internal structure is generally different 

 from, and more primitive than, that of the other, remaining group ; 

 and they are far less numerous in species and individuals, tending 

 to be less widely dominant. They show, however, effective internal 

 differentiation and external division of labour, allowing conduction 

 to the tops of the world's tallest trees (Coastal Redwoods, which 

 reach 364 feet in height^), and, externally, involving highly specialized 

 roots, stems, leaves, and intricate organs of reproduction. The 

 roots are generally much-branched and fibrous, the stems essentially 

 columnar at least below (though they may vary from tuberous in 

 some Cycads to much-branched and shrubby, especially in Ephedras), 

 while the leaves vary from very large and subdivided in Cycads to 

 small and needle-like in many Conifers. The plants most commonly 

 grow as trees, the leaves being usually evergreen, lasting for several 

 years. Examples of Gymnosperms are shown on pages 64-67. 



The structures that ultimately produce the male and female 



gametes are borne on modified leaves, usually aggregated into terminal 



cones of one sex and developed either on the same (hermaphrodite) 



or on different (male and female) plants. The female gametophyte 



consists of a mass of cells developing within the megaspore and, 



like it, remaining hidden in the sporophyte. In this gametophyte 



develops the egg, which, with the immediately surrounding envelopes 



comprising the ovule, forms the seed after fertilization. Sexual 



fusion is effected by male gametes which may be motile spermato- 



zoids or merely passive nuclei, and which are normally enclosed 



within a pollen tube growing towards the egg ; these gametes are 



formed by a microscopic and vestigial male prothallus developed on 



germination of the microspore. This last is the pollen grain, 



produced in great numbers in special organs, the pollen sacs, and 



each with some infinitesimal chance of being carried by the wind 



to the vicinity of a receptive ovule. The seed contains the embryo 



sporophyte and, after liberation from the parent and given suitable 



1 This goes for living trees, though some Eucalypts in south-eastern Australia 

 appear to have exceeded this figure in the recent past — see note on p. 379. 



