Cytological Studies in the Protococeales* 
II. Cell Structure and Zoospore Formation in Pediastrum 
Boryanum (Turp.), Menegh, 
BY 
GILBERT MORGAN SMITH, 
With Plate XII and four Figures in the Text, 
T HE family H yd rod icty aceae of the Protococcales is of exceptional 
interest because of the peculiar formation of daughter colonies by 
means of motile zoospores. The cytological details of the process have 
been studied in Hydrodictyon by Timberlake ( 17 ), Klebs ( 7 ), and others ; in 
Euastropsis by Lagerheim (8) ; but nothing is known regarding the process 
in Pediastrum. This is due to the fact that although the alga is widely 
distributed in nature, it rarely occurs in sufficient abundance to fix for 
cytological study. Even general laboratory cultures do not produce it in 
abundance, so that only by the application of special culture methods is 
a sufficient number of colonies obtained to embed in paraffin. 
The source of the alga, whose study is reported in the present paper, 
was the plankton of Lake Mendota at Madison, Wisconsin ; and it was 
obtained in pure culture by means of the methods which I have described 
in another connexion ( 14 ). The cultures were grown in o*2 per cent. Knop’s 
solution, and fixed in Flemming’s weak osmic-acetic-chromic acid mixture 
diluted with an equal volume of water. The washing and dehydration 
of the material were by means of osmosis through a celloidin film placed 
over the end of a vial in the manner which I have described elsewhere (12), 
The material was embedded in paraffin, and cut on a microtome in sections 
5-8 fx in thickness. Flemming’s triple stain gave the best differentiation of 
the pyrenoid and a good differentiation of the nucleus. Heidenhain’s iron- 
alum-haematoxylin also stains the nucleus satisfactorily. 
Practically all descriptions of colony formation are based upon the 
observations of Braun (2). He found in P. Boryanum that the division 
of the mother-cell contents and the liberation of the young colony took 
place in the afternoon. According to his account, the cell divides into 
halves by cleavage. The halves then redivide, but not always simultaneously. 
[Annals of Botany, Vol. XXX. No, CXIX. July, 1916.] 
