62 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Although mainly favouring shady and humid situations, Ferns 

 occupy a wide array of habitats ranging from dryish heaths and 

 crevices of rocks to wet mud and open fresh water, and from forest 

 floors to quite lofty branches or crutches of trees. They are plentiful 

 especially in the damper tropical and temperate regions and reach 

 the southern portion of the Arctic in fair array, while a very few 

 species persist northwards to almost the highest latitudes of land. 

 Vegetationally they are chiefly of importance as subsidiaries in humid 

 habitats from the tropics northwards and southwards to temperate 

 regions, though arborescent types may contribute substantially to 

 the forest, for example in New Zealand. North of the temperate 

 regions in the northern hemisphere they tend to become scarcer, 

 being often absent from dry areas and almost negligible as com- 

 ponents of vegetation in most boreal and arctic regions. Their 

 economic significance is chiefly aesthetic and horticultural ; many 

 are among the most beautiful of living things, and consequently 

 fern-growing is a popular hobby. Some are minor constituents of 

 animal fodder or may be employed as food by humans or cut and 

 dried for litter, but probably far outweighing these uses is the 

 nuisance-value of others — particularly the common Bracken, which 

 is a pestilential weed that widely overgrows pastures and young 

 tree plantations. 



Spermatophyta 



Gymnospermae : This, the more primitive of the two classes (by 

 some considered subdivisions) of the Spermatophyta (Seed-plants), 

 includes the Conifers, Cycads, and Gnetales among living groups, 

 and many extinct fossil representatives that were of great importance 

 as components of vegetation in earlier geological ages (see Chapter V). 

 The Seed-plants are the main phylum existing on land today, 

 providing the vast majority of dominant species and the great 

 preponderance of vegetation in most situations. They are, of 

 course, vascular plants, being, briefly speaking, those which bear 

 seeds. A seed is an organ peculiar to this ' highest ' division of the 

 plant kingdom and is the product of a fertilized ovule, consisting 

 of an embryo that is often embedded in a nutritive tissue and is 

 normally enclosed by one or two protective seed-coats. The large, 

 complicated visible plant is always the sporophyte generation, the 

 usually microscopic female gametophyte being embedded within it 

 and never having a separate existence, while the male gametophyte 

 is even more reduced. 



