192 



THE 



The fertilized egg becomes an oospore within the oogonium 

 and is further protected by a cellular envelope (Fig. 185, B) 

 developed from short filaments which grow up around the 

 structure (Fig. 185, A, /), making a conspicuous fructification. 



The oospore germinates the 

 following spring, forming 

 within itself a small group 

 of ceUs (Fig. 185, C), each of 

 which gives rise to a zoo- 

 spore. The decay and ruptur- 

 ing of the fructification allows 

 the zoospores to escape and 

 start new Coleochcete plants 

 at the beginning of each sea- 

 son. The fructification of Co- 

 leochcete serves to multiply 

 FIG. 185. Coleochcete pulvinata 



the number of zoospores pro- 



A, filaments with an oogonium o, an thend- . . 



ium a, and a sperm above ; /, fertilized duced by the Oospore, which 



egg in its oogonium becoming surrounded ig clearly an advantage, 

 by short filaments from the cell below ; * 



Coleochcete stands at the 



B, oospore completely inclosed in a eel- 



lularenvelopederivedfromfilamentssuch 



as are shown in A,f; C, germination of 



oospore, each cell in the interior develops fined lines of evolution in the 



a zoospore.-After Pringsheim star( . g afc 



best _ de _ 



the lowly level of the Ulothricacece and runs upwards through 

 the Chcetophoracece. Authors have generally regarded these 

 forms as leading almost directly to the bryophytes, with Coleo- 

 chcete just a little below the liverworts; but, as we shall see 

 later, there are serious difficulties in the way of this view. 1 

 Nevertheless, these forms are perhaps nearer than any other 

 living algae to the theoretical main line of ascent. 



1 The fructification of Coleochcete has been regarded as similar to the so- 

 called fruit or sporophyte of the liverworts, but, as explained in Sec. 336, 

 there is strong evidence against this interpretation. The one-celled sexual 

 organs of Coleochcete are also very different from the many-celled sexual 

 organs of the bryophytes, and this is also evidence against the existence of a 

 close relationship between the groups (see Sec. 297). 



