Smith—The Organization of the Colony . 1195 
of cells then unrolls so that they all lie in one plane. There 
are different combinations possible in the unrolling of these cells, 
so that a particular cell may he either terminal or median in the 
mature coenobe. I was unable to distinguish any horns 
on the cells of the young coenobe before the rupture 
of the mother cell wall, but have found stages similar 
to that which Senn (35) has figured, where he shows 
that the two cells lying next to the split in the 
mother cell wall form horns while the other cells do not. If 
the splitting should occur on the other side of the mother cell 
wall, the two cells that are the inner pair in the first case would 
be the outer cells and consequently deyelope horns while the 
other cells did not. Thus we have a totipotence of every cell 
similar to that which Harper (21) has described for Pedias- 
trum , where he finds every cell of the coenobe capable of form¬ 
ing spines and that only the marginal cells are able to do so 
easily, but that the inner cells of the coenobic plate form them 
when they can. The formation of horns in Scenedesmus quad - 
ricauda may well result from some interaction between the 
cells which is of the nature of a contact stimulus and response. 
As a result of the totipotence of the cells we have the stimulus 
causing horns to be formed on the median cells of the coenobe as 
well as the terminal cells. 
A much greater response to changes in external environment 
is shown by Scenedeismus quadricauda than S. acutus . This 
is especially the case where cultures are made in a nutrient 
solution of comparatively high osmotic pressure. All of the 
drawings in Plate LXXXIX, were made from colonies of 
S, quadricauda grown in 1.0 per cent. Knop’s solution to which 
T. O per cent, sodium chlorid had been added. The nutrient 
medium affects both the shape of the individual cell and the 
cells of the colony. In the individual cells monstrosities ap¬ 
pear more frequently than they do- in S. acutus. These mon¬ 
strosities are not so much in the'form of abnormally shaped 
cells as in the form of giant cells. Xormally these cells are 
from 3-8 by 4-12 microns but in cells that have been grown in 
solutions of high osmotic pressure the cellular measurements 
