244 BOTANICAL GAZETTE Loctoser 
of the shoot has been determined by selection during phyloge- 
netic development; its growth is the enlargement of a proto- 
plasmic mass along lines predetermined. The putting in of the 
cell walls, being a purely mechanical process following the 
simple rule of rectangular intersection, can in no way be 
regarded as determining the shape of the shoot. The tip grows 
just as does the Vaucheria tip, the formation of individual cells 
being a purely secondary matter. The independence of the 
single cells, their growth and division, is entirely subordinated 
to the growth of the entire organ. It is an open question 
whether the rectangular intersection of cell walls in vegetative 
points, assuming that the facts are as Sachs describes them, is 
sufficient basis for so important a conclusion as that above 
stated. 
Jennings has characterized the law of rectangular intersect 
tion as ‘‘hardly to be considered as more than a statement of a 
condition commonly found.’ There is nothing inherently 
impossible in the assumption that the habit of forming — 
sive cell plates so that they intersect at right angles has itself 
been acquired by the cells as individual units. The form of oe 
shoot may then depend on this power of the cells, assuming 
always a further regulation of their activities by reciprocal 
stimuli between the cells themselves as well as by stimuli from 
their environment. 
There can be no question that at least the details of cleavage 
in the sporanges I have studied have been modified in the course 
of phylogenetic development with especial reference “2 
needs of the organisms concerned. I have elsewhere pout i 
out the correlation between the abbreviated cleavage poe 
Sporodinia and its more rapid spore development. The gi 
pation of vacuoles in the formation of the cleavage aa 
Pilobolus is another example of such modifications. Ps nt 
cleavage processes are after all so similar in the ae 
Sporanges studied is doubtless due to the fundamental phys 
and chemical properties of the protoplasm, but the process 
4 
ne 
: Nn. 
none the less to be conceived of as modified by select? 
