ALG^E 51 



vessel in which they are placed. After a short period 

 of active movement, they settle down, become invested 

 with a cell-membrane, and enter what may be called 

 the Protococcus stage, in which they sometimes remain 

 for a long time, giving rise to large colonies of unicellu- 

 lar plants by repeated fission and separation of the cells. 

 Indeed these unicellular stages of many algse have been 

 given special names, under the mistaken impression that 

 they were really autonomous forms instead of simply 

 transitory stages in the development of a filamentous 

 ala. Usually, however, the zoospore, after coming 

 to rest, elongates, and, by the formation of a transverse 

 wall, becomes two-celled, and, by further elongation 

 and repeated cross-divisions, assumes the filamentous 

 form of the adult plant. (Fig. 9, D, E, F.) 



THE CONFERVACE^E 



The order which seems to be most directly connected 

 with the Protococcaceae is that known as the Confer- 

 vacese, especially important in a study of the evolution 

 of plants, as it probably represents, more nearly than any 

 other existing group, the direct ancestral forms of the 

 higher plants. 



The lowest members of the order are simple un- 

 branched filaments, composed of perfectly similar cells 

 (Fig. 8, A). Somewhat higher in point of develop- 

 ment are a number of common forms, e.g. Chsetophora, 

 Cladophora (Fig. 8, B), which are branched, while many 

 of these, as well as such of the unbranched forms as 

 are attached, often show a modification of the basal cell 

 into a root-like organ. Where this is the case of course 



