42 EVOLUTION OF PLANTS 



strictly unicellular, the higher ones are multicellular, 

 this being most marked in the genus Volvox, where there 

 are several thousand cells in each individual, all con- 

 nected by protoplasmic threads. We have seen, too, 

 that, in the simpler types, all the cells are alike, and 

 there is no clear distinction between vegetative and 

 reproductive cells, nor between sexual and non-sexual 

 ones, while in the higher ones special cells are set apart 

 for reproductive purposes, and sexuality is well marked. 

 It must be remembered, however, that the evolution 

 of the plant body in such specialized Volvocinese, as 

 Volvox, is in a direction away from that which leads up 

 to the more typical plants, and there is no evidence 

 that this peculiar line of development has ever advanced 

 beyond such forms as Volvox, which seems to represent 

 the highest expression of this type of structure. We 

 must regard the whole series of the Volvocinese as an 

 offshoot of the main line of development of plants. 

 The simplest of the group, such as Chlamydomonas, are 

 closely related to the lowest of the typical green plants, 

 the so-called Protococcacese, and may be considered to 

 represent a primitive stock which has given rise to two 

 branches, one, the Volvocineae, culminating in Volvox, 

 the other, the Protococcaceee, which leads directly to 

 the higher green plants. 



THE PEOTOCOCCACE^E 



The Protococcacese, employing this term in its widest 

 sense, form a rather poorly defined group of unicellular 

 plants, some of which are of doubtful autonomy, since 

 many supposed members of this group have been shown 



