SMITH: TETRADESMUS, A NEW COENOBIC ALGA 83 
mother cell. Asa result the nucleus of each daughter cell is not 
found in the center of the cell but nearer the end that was originally 
in contact with the apex of the mother cell wall. Fic. 15 shows 
this fact, but it must be borne in mind that a portion of the upper 
pair of cells is concealed by the lower ones. At the beginning of 
this elongation there is no pyrenoid present in any of the daughter 
protoplasts (FIG. 15). The further steps in the maturation of the 
daughter cells consist in the formation of pyrenoids and cell walls. 
The daughter cells are without pyrenoids until they have reached 
their full length, and then a pyrenoid suddenly appears in each 
(FIG. 16). Such an origin of pyrenoids de novo is contrary to the 
current conception, according to which a pyrenoid always arises 
by the division of a preéxistent pyrenoid but is in harmony, as 
will be later pointed out, with Timberlake’s observations in 
Hydrodictyon. The formation of the new pyrenoid possibly may 
be a result of the metabolic activities of the nucleus, since the 
pyrenoid at its first appearance is often very close to the nucleus 
(FIG. 17). In other cases, possibly representing later stages, the 
pyrenoid is at a greater distance from the nucleus (FIG. 16). 
At about the same time as the appearance of the pyrenoid a cell 
wall is formed around each of the four protoplasts while they are 
still within the old cell wall. 
The liberation of the daughter colony is through a longitudinal 
break in the mother cell wall. End views, such as those shown in 
FIG. 19 and 20, are especially favorable for studying the relation 
of the daughter coenobe to the mother cell wall. The opening 
through which the young colony escapes is a fissure that extends 
the whole length of the mother cell wall (Fic. 18). The cause of 
this rupture is probably the growth of the young colony within 
the old wall. It is not an irregular break but a definite longitu- 
dinal split, which is always at the side turned away from the other 
cells of the mother colony. After liberation the young coenobe 
is composed of cells which are of nearly full length but which are 
much more acicular than the mature cell (FIG. 17). The further 
history of the coenobe consists in the growth of the individual 
cells until they have assumed the shape characteristic of the adult 
stage. 
