398 A TEXTBOOK OF THEORETICAL BOTANY 



Under favourable conditions the capsule bursts by the splitting of the 

 wall, forming four petal-like valves which spread out horizontally. By 

 this means the spores are liberated and the separation of the spores from 

 one another is assisted by the elaters. The elaters are hygroscopic, that is 

 to say, they expand and contract according to the degree of atmospheric 

 humidity, during which process they twist and turn rapidly, and it will be 

 realized that by this means they stir up the spores and separate them. The 

 elaters are arranged in radiating lines from a solid tuft at the base of the 



Oospore 



Amherozoids 



fidium 



Pellia Thallui 

 Fig. 380. — Life-cycle oi Pellia epiphyllo. 



theca, and when all the spores are shed this tuft of elaters, in the centre of 

 the four spreading valves of the wall, produces the semblance of a minute 

 flower. 



On liberation the multicellular spore begins to grow at once, for, like 

 other spores containing chlorophyll, it has no resting period. A cell at one 

 end of the spore becomes the apical cell, the others form the ordinary cells 

 of the thallus, and the various parts of the Pellia thallus soon become 

 differentiated. 



Alternation of Generations 



From our study of the life history of Pellia we see that the sexual and 

 asexual reproductive phases are very closely connected ; far more closely 

 than they are in a type like Dictyota or Polysiphonia. In either of these algal 

 types the asexual plant possesses an independent life, whereas in Pellia the 

 sporophyte lives in and upon the gametophyte. 



The life history of Pellia may be summarized by the above diagram 

 (Fig. 380). 



