SUMMARY AND CONCLUSION 285 
One important reason for considering the ciliated 
Volvocinee 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, zodspores, and gametes very generally are extraor- 
dinarily similar in structure to the simpler Volvocinee. 
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 
