THE STEM OR SHOOT. 



141 



entire plants on leaves, but the plant in this case 

 belongs to the sexnal, the leaf producing it to the 

 sporophyte generation. Druery first described the 

 phenomenon in a variety of lady-fern (Athyrium Filix- 

 foemina var. clarissima). Wollaston discovered it in 

 Poh/stichum angulare var. puJchevrimum. Bower later 

 investigated the entire phenomenon in these and other 

 ferns, e.g. Trichomanes alatum. In the first-mentioned 

 and in the bracken (Pteris aquiiina) (PI. XT, fig. 3, 

 and fig. 39) the prothalloid growths are formed from 



Fig. 39. — Pteris aquiiina. Prothallus (pr) growing- aposporously from 

 sporangium (sp). (After Farlow.) 



the sporangia which become thereby arrested in their 

 development, and never produce spores. In Poly- 

 stichum there are three distinct methods of production 

 of these prothalloid growths : (a) from the arrested 

 sporangia ; (b) from the base of the sorus, i. e. quite 

 distinct from the sporangia ; in this case also the 

 sporangia of the same sorus are arrested in their 

 development ; these prothalli may be in part plate- 

 like and in part filamentous ; (c) from the pinnule of 

 the frond, i. e. quite independently of both sporangia 

 and sorus, viz. from the vegetative portion of the 

 frond ; of this category there were prothalli which 

 sprang from the surface of the pinnule in the region 

 of a vein, and others which arose from the margin as 

 a direct continuation of its growth ; a case of this 



