24 



Algcc 



cell-division gradually replaced the formation of zoogonidia as the 

 chief method of multiplication. The first group of Alga evolved 

 in this direction was the lowly family of the Palmellacea?, in which 

 the cells are grouped together either in all directions of space as in 

 Palmella, or regularly disposed in one plane as in Tetraspora. 

 The cells are enveloped in a general mucous envelope formed by 

 the confluence of the special gelatinous cell-walls, and in every 

 stage the cells on leaving the envelope are capable of swarming as 

 zoogonidia. It is to this family of the Palmellacea? that we must 

 look for the origin of most of the other families of green Algae. 



The Endosphasrine tendency in the Chlamydomonad-type has 

 given rise to certain plants in which vegetative cell-division is 

 absent, the multiplication of individuals taking place only by the 

 formation of zoogonidia or gametes. These plants belong to the 

 family Endosphgeraceaa and are almost strictly unicellular. 



So far, then, the phylogeny of the green Alga3 can be illus- 

 trated by the following simple diagram : 



Volvocaceas 



A 



Endosphaeraceae 



Palrnellaceae 



Chlamydomonas 



The Volvocine tendency has resulted in no higher development 

 than the Volvox-co\ony, but a very reasonable suggestion has been 

 made by Blackman as to the origin of the Siphonese by a further 

 development of the EndosphaBrine tendency and formation of a 

 thallus, which, although essentially coenocytic. is structurally uni- 

 cellular and lacks the solidity acquired by septate cell-division. 

 He remarks that "nothing appears to have been evolved from it 

 of higher status than an Alga. While the Tetrasporine tendency 

 has given rise to all the higher green plants, the Endosphaerine 



