56 PLANT-LIFE 



In very dry weather they seem to rest, and are easily 

 dispersed by wind; but in damp conditions they show 

 much activity in reproduction by division. One cell 

 divides into two, repeated divisions follow, and the 

 resulting individuals may either cohere in groups for a 

 time, or separate soon after formation. Each cell is 

 perfectly independent. 



We have yet much to settle in regard to the life-history 

 of Pleurococcus, and future observations will probably 

 confirm indications that, in addition to the simple 

 mode of reproduction by division, motile zoospores and 

 sexual cells are produced; moreover, that filaments of 

 cells occur. Thus, the life-cycle of this Alga may be 

 much more complicated than has hitherto been sup- 

 posed. It is customary among evolutionists to regard 

 Pleurococcus and its allies as the modern representatives 

 of a stage of development intermediate between the 

 Chlamydomonas stage and several higher forms. The 

 vegetative Pleurococcus cell may safely be regarded as 

 derived from a cell of the Chlamydomonas type which 

 lost its motile flagella, and became invested in a con- 

 tinuous membrane of cellulose in adaptation to terres- 

 trial conditions. It would not be at all surprising to 

 find such cells producing motile cells in an aquatic en- 

 vironment. But the developments leading to higher 

 plants would most probably take place in water. We 

 may imagine a Chlamydomonas type of cell losing mo- 

 tility and coming to rest. It adheres by a mucilaginous 

 secretion to some object, grows, and divides. The 

 daughter-cells do not disperse, but remain in contact, 

 either in rows, so forming filaments, or in groups, due to 

 cell division in two directions, thus producing a plate- 



