Pediastrex 219 



themselves. In all known forms of the genus the coenocytes forming the 

 periphery of the disc differ in shape from the remainder, generally in the 

 possession of marginal processes. The number of coenocytes in a ecenobmm 

 varies from 4 in one of the common forms of Pediastrum Tetras (fig. 143 G) 

 to 128 in some of the large forms of P. duplex. The few recorded instances 

 of coenobia of two coenocytes probably refer to Euustropsis. The coenocytes 

 are often arranged in fairly distinct rings around a central one, 8, 16, 32, or 

 more being the number constituting the coenobium. Nageli ('49) long ago 

 pointed out that the coenobia were usually constructed as follows : colony of 

 8 = 1 + 7; colony of 16 = 1 -f 5 + 10 ; colony of 32 = 1 + 5 + 10 + 16 ; but 

 this arrangement is not always observed and there are many irregularities 

 due primarily to the death of one or more of the gonidia. In P. Tetras this 

 sometimes results in a curious coenobium of 3 ccenocytes (G. S. W., '07). 



Each coenocyte contains from 3 to 6 nuclei, and a massive parietal 

 chloroplast with one pyrenoid (fig. 144 F). 



Some species of Pediastrum are furnished with long bristles which play 

 the part of a buoyancy apparatus and augment the floating capacity of the 

 cosnobia (fig. 144 H). The bristles are rigid and elastic, and are mostly 

 attached in tufts, largely at the ends of the marginal processes and therefore 

 in the plane of the ccenobium. There are, however, many other tufts and 

 also solitary bristles, attached to the inner coenocytes, which project at right- 

 angles to the plane of the coenobium (Petersen, '11). The bristles are very 

 delicate, but may sometimes be seen in plankton-specimens of P. Boryanum 

 and P. duplex without any special treatment and with ordinary illumination. 

 Dark-ground illumination usually shows them up very well. Petersen has 

 shown that in P. simplex they are seasonal, disappearing during the winter. 



The normal method of reproduction is by the successive division of the 

 contents of a coenocyte to form a number (4, 8, 16, 32, etc.) of zoogonidia, 

 which are liberated into a delicate external vesicle through a slit in the wall 

 of the mother-coenocyte (fig. 143 G). The zoogonidia swarm in the vesicle 

 for a time and then become quiescent, arranging themselves in one plane as a 

 new coenobium. In many cases, however, the gonidia do not appear to 

 ' swarm ' ; they pass into the vesicle but are not ciliated and exhibit no 

 movements except that they gradually arrange themselves as a plate. This 

 fact is of considerable interest as it shows how the autocolonies of certain of 

 the AutosporacefB may have arisen. In fact, this method of reproduction in 

 Pediastrum is really by autocolonies, and there is no doubt in this case that it 

 is merely a modification of the normal method due to a suppression of the cilia. 



Resting aplanospores ( hypnospores) are not infrequently observed in 

 certain species of Pediastrum (fig. 143 E). Their formation and subsequent 

 germination were worked out by Chodat and Huber ('95). 



Askenasy ('88) described the formation of biciliated gametes, much smaller 



