CHAPTER IX. 



PTERIDOPHYTA (Vascular Cryptogams). 



I. EQUISETALES. II. SPHENOPHYLLALES. 

 III. LYCOPODIALES. IV. FILICALES. 



The Pteridophytes include plants which vary in size from a 

 few millimetres^ to several metres in height. The spore on 

 germination gives rise to a small thalloid structure, the pro- 

 thallium, on which the sexual organs are developed ; this is the 

 gametophyte or sexual generation. The sexual organs have the 

 foi"m of typical archegonia and antheridia. From the fertilised 

 egg-cell there is developed the Pteridophyte plant or sporo- 

 phyte, which bears the spores. This asexual generation shows 

 a well-marked external differentiation into stem and leaves, and 

 bears true roots. Internally the tissues exhibit a high degree 

 of differentiation into distinct tissue-systems. True vascular 

 bundles occur, which may or may not be capable of secondary 

 thickenin'g by means of a cambium, i.e. a definitely localised 

 zone of meristematic tissue. The sporangia are borne either on 

 the ordinary foliage leaves or on special spore-bearing leaves 

 called sporophylls, which differ in a greater or less degree from 

 the sterile leaves. 



The majority of the best known and most important 

 Palaeozoic genera are either true Vascular Cryptogams, or 

 possess certain of the pteridophytic characteristics combined 

 with those of higher plants. It is not merely the commoner 

 and more familiar recent genera with which the student of 

 extinct types must be acquainted, but it is extremely important 

 1 e.g. the Fern Trichomanes Goehelianum Gies. Giesenhagen (92) p. 157. 



