SUMMARY AND CONCLUSION 285 



One important reason for considering the ciliated 

 Volvocinese as primitive forms in the very frequent 

 reversion to this condition exhibited at times by very 

 many of the higher green plants, whose reproductive 

 cells, zoospores, and gametes very generally are extraor- 

 dinarily similar in structure to the simpler Volvocinese.' 

 The persistence of motility in the reproductive cells 

 is very remarkable, being found in members of all the 

 groups. The spermatozoids of the Archegoniates — 

 mosses and ferns — illustrate this, and the recent dis- 

 covery of similar motile cells in the lowest of the seed 

 plants extends this phenomenon to the highest sub- 

 kingdom. 



-Starting with this primitive motile unicellular organ- 

 ism, there have evidently arisen a number of indepen- 

 dent lines of development resulting in very divergent 

 types of structure. The first step in the evolution of 

 what may be termed the typical green plants is the loss 

 of motility in the vegetative cells through the suppres- 

 sion of the cilia and the development of a firm cell-wall. 

 The latter precludes the active locomotion, so charac- 

 teristic of most animal forms, and makes the plant 

 assume the more stable condition typical of the vege- 

 table organism. 



In these lowly organisms there is no clearly marked 

 line between vegetative and reproductive cells. An 

 individual by simple fission gives rise to two new in- 

 dividuals like itself. Many of these, however, show 

 two kinds of cell-division, a purely vegetative one by 

 fission into two equal parts, and a modification of this, 

 internal cell-division, by which a number of individuals 

 may arise by simultaneous division of the protoplasm of 



