PTERIDOFHYTA 



245 



B 



cells divide into four spores, precisely as in the Bryophytes, and the 

 ripe spores have the same structure 

 (Fig. 211). 



The nearest approach among Bryo- 

 phytes to the segregation of the 

 sporogenous tissue found in the 

 Pteridophytes occurs in the Antho- 

 cerotales, where the groups of sporo- 

 genous cells are separated by layers 

 of sterile tissue somewhat as in 

 Ophioglossum. 



Apogamy and Apospory. In 

 several Ferns the sporophyte has 

 been observed in certain instances 

 to arise as a vegetative bud from 

 the gametophyte instead of from 

 the egg-cell. This non-sexual ori- 

 gin of the sporophyte is known as 

 Apogarny. 



Apospory is the development of 

 the gametophyte as a bud of the 

 sporophyte. These abnormal pro- 

 thallia have been observed to develop 

 from the margin or from the surface 

 of a leaf, or in some cases from the 

 transformation of a young sporan- 

 gium into a prothallium. 



Distribution of Pteridophytes. 

 The sporophyte in most Pterido- 

 phytes is terrestrial, but a few 

 forms are aquatics. Thus Isoetes 

 lacustris is completely submersed, while Salvinia and Azolla are 

 floating forms. Marsilia is usually amphibious. The Pteridophytes, 

 like the Hepaticae, reach their greatest development in the moist 

 mountain forests of the Tropics, where they constitute an important 

 and most beautiful feature of the vegetation. A small number, 

 like the little Gold-back Fern (Gymnogramme triangularis), are 

 adapted to a dry region, and can endure complete desiccation with- 

 out injury. 



The living Pteridophytes are usually divided into three classes, 

 Filicales, Equisetales, and Lycopodiales. Of these the first com- 

 prises much the greater number of existing species. The Equise- 

 tales, which during the Palaeozoic age were numerous and varied, 

 now comprise but a single genus. The Lycopodiales were also at 

 this period much better developed than they are at present. There 



FIG. 210. Lycopodium clavatum. 

 A, leafy shoot having two cones 

 composed of sporophylls, sp (re- 

 duced) . B, a single sporophyll, with 

 sporangium, sp, enlarged. C, cross- 

 section of the stem. 



