Smith . — Cytological Studies in the Protococcales. II. 473 
The formation of the swarm-spores and the number of cells in a colony 
are greatly influenced by cultural conditions. Sixty-four- and thirty-two- 
celled colonies predominate among those produced during the first two 
weeks in cultures containing 0-2 per cent. Knop’s solution, while in cultures 
a month old 16- and 8-celled colonies predominate. At the end of two 
months few new colonies are formed, the cells of the old colonies in the 
culture containing large amounts of ‘ stroma ’ starch. The production 
of colonies with large numbers of cells in fresh vigorous cultures, and the 
later production of those with fewer cells, is a fact that has been noted by all 
investigators of coenobic algae under cultural conditions. 
The observations of Caspary ( 3 ) and Braun ( 2 ) have shown that any 
cell in the colony is capable of giving rise to a daughter colony. Since cells 
in the same colony are of exactly the same age and have been under 
the same external environmental conditions, the natural expectation would 
be that all cells would form zoospores at the same time. Any one familiar 
with field collections of Pediastrum knows, however, that generally at most 
only a few cells of a colony are empty as a result of zoospore formation. 
The question arises, then, whether the other cells of the colony will form 
zoospores on some succeeding night, or whether, on - the other hand, only 
a few cells of a colony will ever produce them ? Upon observation, however, 
it was found that zoospores are formed and liberated from different cells in 
the same colony at different times (Fig. 17). 
Since my observations agree in the main with Braun’s ( 2 ) account of 
the swarming, little need be said on this point. In my cultures, however, 
I find that the changes taking place after the swarm-spores come to rest are 
much more rapid than Braun describes them. The rapidity with which 
these changes take place is shown in Text-fig. 3, A-D, which are outline 
camera lucida drawings made at three-minute intervals. These sketches 
should be regarded as being only approximate, since in each instance less 
than a minute was devoted to the outlining of the eight cells in the colony. 
The sketches show that within the few minutes following the cessation of 
zoospore motion changes take place in the cell that according to Braun’s 
description require a period of hours for their completion. 
For the observation of the cytological details of zoospore formation 
I selected month-old cultures which were forming 8- and 16-celled colonies, 
because the small number of nuclei made the process comparatively easy to 
follow. The details are essentially the same as in the formation of 32- and 
64-celled colonies. The first stages in zoospore formation are found in the 
early hours of the night, and the first indication of what is to occur is 
a rapid division and redivision of the nuclei so that their number is increased 
fourfold. These nuclear divisions are always simultaneous ; the number of 
nuclei present is therefore always a multiple of two. Nuclear division takes 
place only in the cells that are to form zoospores, so that a surface view of 
K k 2 
