642 SCIENCE PROGRESS 



but in S. caudatus, although the mucilage becomes uniformly 

 developed in the presence of abundant oxygen, the cells cohere 

 together as cylindrical structures and do not become rounded oflf. 

 In the genus Coelastrum, in which the colonies consist of 

 sixteen or thirty-two cells joined to form a hollow spherical 

 reticulum, the species show similar differences in this re- 

 spect. In C. microporum (fig. 2, m) the colonial tendency is 

 little pronounced, and free cells are almost as frequent as 

 colonies ; in the latter the cells are connected by little round 

 cushions of mucilage, which correspond in adjacent cells. The 

 same is the case in C. reticulatum, but here the connections 

 between adjoining cells are more pronounced, and colonies 

 are the rule. In both these species the isolated cells are 

 practically identical in shape with those in the colonies. This 

 is, however, not the case in a third species — C. proboscideum — 

 in which the cells of the colonies are polygonal, while the 

 isolated ones are rounded ; with this change there goes hand 

 in hand a very pronounced colonial tendency, isolated cells 

 being obtained only with difficulty. In both Scenedesmns and 

 Ccelastrum, therefore, we can distinguish between species in 

 which the colonial tendency is not yet very pronounced and 

 species in which colonies are the rule. With the latter we 

 may place the genus Dictyosphcerium, in which isolated cells are 

 only formed when there is a great excess of oxygen present. 

 Each cell of the colony in Dictyosphcerium is provided with a 

 thick mucilage-envelope, showing a radiate structure, while the 

 cell-contents are like those of a Chlorclla. In division the cells 

 nearly always divide into four daughter-cells, which become free 

 by the rupture of the wall of the mother-cell into four basally 

 connected lobes ; a daughter-cell loosely coheres with the apex 

 of each of these lobes, the whole being held together by the 

 enveloping mucilage (fig. 2, n). As this process is repeated, 

 we obtain colonies of quite considerable size. Dictyosphcerium l 

 is thus merely a special kind of colony, derived from the 

 Chlorella-type of individual, and the same applies to other 

 genera of the Scenedesmaceae (Oocystis (Wille yo),Schizochlamys, 

 Raphidium, Kirchneriella, Sorastrum, etc.), which space does 

 not permit of considering here. Numerous allied genera, 

 classified by Blackman and Tansley (6) as the Phyteliaceae, 



1 Dictyosphcerium has been stated to reproduce also by zoospores, but this 

 awaits confirmation. 



