220 Chlorococcineae 



than the zoogonidia, which conjugated in pairs (fig. 149 K and L) to form 

 polyhedral zygotes. The latter then gave rise to new coenobia by the segmen- 

 tation of their contents. These observations have not been confirmed. 



In the genus Euastropsis, originally established by Lagerheim ('94), the 

 coenobium consists of two flattened cells closely attached along their straight 

 inner margins, the outer margins being widely notched (fig. 144 A and B). 

 The entire coenobium has a superficial resemblance to a minute species of the 

 Desmidian genus Euastrum, and was, in fact, originally described as such. 

 Each cell has one large parietal chloroplast with a single pyrenoid. Lagerheim 

 states that there is one nucleus in each cell, but suggests that there may be 

 more. Reproduction occurs by ovoid zoogonidia which swarm in a vesicle as 

 in Pediastrum. From 2 to 82 of these are formed by successive divisions of 

 the contents of the mother-cell. On becoming quiescent they arrange them- 

 selves in pairs, each pair gradually developing into an adult coenobium 

 (fig. 144 G E). Thus, in this genus, as many as 16 daughter-coenobia may 

 be produced from one mother-cell. 



The only genera are Pediastrum Meyen, 1829, and Euastropsis Lagerheim, 1894. The 

 former genus occurs more particularly in small ponds and ditches, and not infrequently in 

 quiet bog-pools. Certain forms are constant and regular constituents of the freshwater 

 plankton. Euastropsis occurs only in bogs and is a very rare Alga. 



Sub-family HYDRODICTYE^E. Of this sub-family only the one genus 

 Hydrodictyon Roth (1800) is known. The coenobium is macroscopic, attaining 

 a length of 20 centimetres, and consists of a net-like sack floating freely in 

 the water. The meshes of the net vary much in size, and each one is bounded 

 by five or six large cylindrical coenocytes, the angles being formed by the 

 junction of three coenocytes (fig. 145 A and B). The protoplasm of each 

 coenocyte forms a fairly thick lining layer containing many nuclei, the central 

 part of the segment being occupied by a large sap-vacuole. In Hydrodictyon 

 reticulatum there are no definite and distinct chloroplasts. Both Artari ('90) 

 and Klebs ('91) were wrong in describing a reticulated chloroplast, Timberlake 

 ('01) having shown by means of sections that the chlorophyll is evenly dis- 

 tributed throughout the peripheral protoplasm of the coenocyte. Numerous 

 pyrenoids are present, and Timberlake has shown that they are directly the 

 seat of the processes resulting in starch-formation. 



Reproduction occurs normally by the formation of a very large number 

 (7,000 to 20,000) of zoogonidia within the mother-coenocyte, which swarm 

 while still within the parent-wall and then become quiescent. They at once 

 form a reticulated daughter-coenobium by the apposition of their extremities 

 (fig. 145 G). The old wall is then ruptured and the young coenobium is set 

 free. The zoogonidia are biciliated with one nucleus and a single pyrenoid ; 

 they are formed by the breaking-up of the contents of the mother-segment 

 into large multi-nucleated masses, which in turn become subdivided into 



