Hydroclk'tyacecK 207 



The method of reproduction by the apposition of quiescent 

 zoogonidia to form new coenobia distinguishes the Hydrodictyacese 

 from all the other Protococcoidese, and the ccenocytic nature of the 

 ccenobium also distinguishes them from the coenobic forms of the 

 Protococcaceee (or Autosporacese). 



It is most likely that Hydrodictyon and Pediastrum have no 

 direct affinity, the resemblance being due to convergence of 

 modification, but until more is known concerning the phylogenetic 

 relationships of these genera the Hydrodictyaceas are best divided 

 into the two following sub-families : 



Sub-family I. Hydrodictyetx. New ccenobium formed by apposition 

 of zoogonidia within the mother-coenocyte. Plants macroscopic, coeno- 

 cytes arranged in the form of a net. 



Sub-family II. Pediastrece. New ccenobiurn formed by apposition 

 of zoogonidia outside the rnother-ccenocyte. Plants microscopic, coeno- 

 cytes arranged to form a flat plate. 



Sub-family I. HYDRODICTYE^. 



This sub-family includes only the one genus Hydrodictyon. 

 The plants are macroscopic and consist of very large ccenocytes 

 which are disposed so as to form a more or less cylindrical net. 

 The zoogonidia swarm and become quiescent within the wall of 

 the mother-coenocyte, and there they become apposed to form the 

 new coenobium. 



Genus Hydrodictyon Roth, 1800. The coenobium is a net- 

 like sac, freely floating in the water, and reaches a length of 8 10 

 centimetres. The meshes of the net are of variable size and each 

 one is bounded by either five or six ccenocytes, the angles being 

 formed by the junction of three ccenocytes. The protoplasm of 

 each ccenocyte is confined to a lining layer containing many nuclei, 

 the central portion of the segment being occupied by a large 

 vacuole. There are no definite chloroplasts, the chlorophyll being 

 more or less diffuse through the whole protoplasm, but numerous 

 pyrenoids are present. 



The normal method of reproduction is by the formation of a 

 very large number of zoogonidia within the mother-ccenocyte, 

 which swarm within the wall of the segment and then become 

 quiescent, immediately forming a reticulated daughter-ccenobimn 

 by the apposition of their extremities. The old cell- wall then 

 ruptures and the young ccenobium is set free. The zoogonidia are 



