THE PTERIDOPHYTA : FILICALES, THE FERNS 



493 



The output of spores from a mature Dryopteris plant is of the order of 

 five hundred milHon in a single season. Only a minute percentage of these 

 ever develop into new plants, but they may remain dormant in the soil for a 

 long time, and owing to their numbers they must be almost ubiquitously 

 distributed. 



D 



Fig. 482. — Drxopteris filix-mas. Dehiscence of sporangium. A, Sporangial 

 wall splitting through stomium. B, Annulus retracted and spores being 

 discharged. C, Cells of the annulus before retraction. D, Contracted 

 annulus after dehiscence. {After Boner.) 



Development of the Prothallus 



The spores lie dormant in the ground for some time, and at length 

 germinate. The spore wall breaks and a green cell emerges, from which a 

 colourless branch grows into the soil and forms a rhizoid. The green cell 

 then divides and a filament is formed (Fig. 483) which grows by means of 

 an apical cell, that is, a terminal cell which at first only cuts off^ cells behind 

 it. By subsequent lateral divisions of the apical cell the flat prothallus or 

 prothallium is formed (Fig. 484). The prothallus is usually heart-shaped, 

 but this depends to a large extent upon its environment. When a number 

 of prothalli are crowded together they never reach the heart shape but remain 

 more or less filamentous and minute. In a well-developed prothallus only 

 the central part is more than one cell thick : this part is called the cushion, 

 and on its under surface rhizoids develop, attaching the prothallus to the 

 soil. The formation of the cushion is associated with a stage in development 

 at which the original apical cell is replaced by a group of initial cells. During 

 the growth of the prothallus the cells cut off from the apical cell expand 

 rapidly, thus leaving the actual apex at the bottom of an indentation and 

 producing the characteristic heart shape. 



