Smith—The Organization of the Colony . 1177 
her of cells are formed only under conditions very favorable 
for growth. 
Coelastrum, A Pybamidal Isodiametbic Colony. 
The cells of Coelastrum microporum ISTaeg. are not always 
perfectly spherical; but since the axial differentiation is usually 
very slight and often quite negligible, it seems best to classify 
this species among those with isodiametric cells. 
Under ordinary conditions the cells of a Coelastarum colony 
are approximately symmetrically arranged, the cells being in 
tiers, each tier in one half of the colony corresponding to a 
tier of the same number of cells in the other half. In an 
eight-celled colony there are two possible symmetrical arrange¬ 
ments. The cells may either be in four tiers, the respective 
tiers containing 1—3—3—1 cells, or the cells may be in two 
tiers of four cells each. Both of these arrangements were 
found in the eight-celled colonies in my cultures. Figures 2, 
6, and 13, Plate LXXXV, shows colonies whose cells are 
arranged in two tiers of four cells each. Figure 6 shows the 
most perfectly symmetrical colony, the small central intercellu¬ 
lar spaces being of the same size in the two tiers. In the 
colonies of this type shown in Figures 2 and 13 the cells in one 
tier are much closer together than are those in the other tier. 
Figure 2 shows a colony in which the tier of cells with the 
small intercellular space is below and the upper has a large in¬ 
tercellular space. Tn Figure 13 this arrangement is reversed. 
It may very well be that in these two cases we are not dealing 
with eight celled coenobes but rather with fragments of sixteen- 
celled colonies. The coenobes with the I— 3 —3—1 arrange¬ 
ment of the cells have a single small central intercellular cavity 
(Fig. 10). This figure also shows the symmetrical nature of 
the 1—3—3—1 type of colony. 
In sixteen-celled colonies there are five possible symmetrical 
arrangements of the cells in tiers. There may be two tiers, 
each of eight cells; or four tiers containing varying numbers of 
cells. The possible arrangements in four tiers are 1 —7—7—1, 
