THE GREEN ALGiE 647 



to produce a multicellular individual at the sacrifice of the 

 motile stage. This is the main characteristic of the peculiar 

 group known as the Hydrodictyaceae, the two best-known 

 genera of which are Pediastrum (Askenasy 3) and Hydrodictyon 

 (Klebs 39). In the former generally sixteen or thirty-two 

 cells of more or less irregular shape are arranged to form a 

 flat plate, the outermost cells of the colony being provided 

 with characteristic blunt processes (fig. 3, d). In Hydrodictyon, 

 on the other hand, we have large cells united in great numbers 

 to form a very characteristic and often extensive network 

 with wide-open meshes. In both genera asexual reproduction 

 takes place by subdivision of the contents of any or all of 

 the cells to form a number of biciliate zoospores (fig. 3, d). 

 These exhibit a slight jerking movement within the mother-cell, 

 but this soon ceases; the zoospores then become arranged 

 in the way that is characteristic of the genus and acquire 

 cell-membranes, and thus a new daughter-colony may be 

 produced from each cell of the mother-colony. The analogy 

 with Endosphcera is obvious, but it seems to me that it can be 

 carried further. Asexual reproduction of the type just described 

 may go on for many generations in the Hydrodictyaceae, but 

 under certain conditions gametes are produced. Their formation 

 is identical with that of the zoospores, but unlike the latter they 

 become completely free from the mother-cell and after swarming 

 about for a time fuse in pairs to form a zygospore, which is 

 somewhat angular and has a thick wall. After a period of rest 

 a limited number (generally two to four) of large zoospores are 

 produced from this zygospore ; but these soon cease to move, 

 and, assuming a somewhat angular outline, become enveloped by 

 a thick membrane. These so-called polyhedra go on increasing 

 in size for some time until ultimately their contents subdivide 

 to form numerous zoospores, which, without being liberated, 

 acquire cell-membranes and arrange themselves as a new 

 Pediastrum- or Hydrodictyon-colony. Apart from complications, 

 the resting-cells of these Hydrodictyaceae thus give rise to a 

 multicellular colony just as do the resting-cells of Endosphcera. 

 The cells of this colony are capable of reproducing daughter- 

 colonies in the same way through numerous generations ; but 

 this is the only essential respect in which they differ from 

 Endosphcera, for sooner or later the cells of the Pediastrum- or 

 Hydrodictyon-colony become gametangia, just as do those of 



