CHAPTER XX 

 FERNS 



THE Bryophyta, though exhibiting a marked advance in organ- 

 isation as compared with the Thallophyta, differ from the 

 remaining flowerless plants, comprised in the third class of the 

 Vegetable Kingdom, or Pteridophyta, in lacking roots and true 

 vascular tissue. In the Pteridophyta, moreover, the gameto- 

 phyte is usually thalloid in form and short-lived, whilst the 

 spore-producing phase in the life-history is far more conspicuous 

 and more permanent, and is quite independent of the gametophyte 

 except in its very earliest stages. In the sexual process and 

 the general structure of the sexual organs, however, there is 

 considerable correspondence between the two classes, the Pteri- 

 dophyta, like the Bryophyta, possessing motile male cells, and 

 thus depending on the presence of liquid water at the time of 

 fertilisation. The Pteridophyta, as a matter of fact, in general 

 favour moist habitats, although quite a considerable number 

 can grow under relatively dry conditions. Included in this class . 

 are the Ferns (Filicales), the Horsetails (Equisetales) , and the 

 Clubmosses (Lycopodiales) , of which the first-named will be 

 considered in the present chapter. 



In many common Ferns the stem is an underground rhizome, 

 which is relatively insignificant in comparison with the con- 

 spicuous, usually deeply divided, pinnate fronds which alone 

 appear above the surface ; a notable exception is, however, 

 afforded by the Tree Ferns of subtropical regions, which have 

 well-marked trunks. The subterranean stem is either horizontal 

 (as in the Bracken, Pteris aquilina, Fig. 158, B), or compact 

 and more or less erect (e.g. Lady Fern, A thyriu m filix-fcemina ; 

 Royal Fern, Osmunda regalis). It rarely shows branching, 

 though this occurs at intervals where the rhizome is elongated 

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