Hydrodictyacece 209 



short rest forms two or four large biciliated zoospores, which on 

 coming to rest assume a polyhedral form. The repeated division 

 of the cell-contents of these polyhedral bodies results in the 

 formation of numerous zoogonidia, which by apposition give rise 

 to new net-like coenobia. 



The division of the protoplasm of the adult cells of this genus 

 to form either zoogonidia or gametes is a splendid example of free- 

 cell-formation. 



The only known species, H. reticulatum (L.) Lagerh., which has received 

 the name of the " Water-net," is a very rare plant in Britain. The average 

 length of the adult ccenocytes just before they become zoogonidangia is 4 or 

 5 mm., but they are known to attain a length of 1 cm. The length of the 

 quiescent zoogonidia at the time of their apposition is 13'5 25 p.. The 

 swarming zoogonidia are 10 p in length by 8 p in breadth and the gametes are 

 a little smaller. Fig. 84. 



Sub-family II. PEDIASTRE.E. 



The plants of this sub-family are microscopic in size and consist 

 of a number of small coenocytes united to form a flat, disc-like 

 ccenobium. The zoogonidia swarm in a vesicle which is protruded 

 from the mother-coenocyte and the new coenobium is thus formed 

 outside the old coenocyte. 



Genus Pediastrum Meyen, 1829. The coenobium is always a 

 free-floating, flat plate, disc-shaped or star-shaped, and consists 

 of a single layer of small coenocytes which is rarely duplicated in 

 the centre. The coenocytes are either parenchymatous and closely 

 united, or there are perforations of variable size between them 

 which give the coenobium a sieve-like aspect. The marginal 

 coenocytes are of different form from those in the centre and they 

 are generally furnished with a pair of diverging processes. There 

 is a single chloroplast in each coenocyte, containing one pyrenoid. 



The number of coenocytes in a coenobium varies from 2 to 64, 

 or even more. Coenobia of two coenocytes are very rarely observed, 

 and possibly belong to the genus Euastropsis, but in one species 

 (P. tetras) four is the regular number. The coenocytes are often 

 arranged in distinct rings round a central one, 8, 16, 32, or more 

 being the number in the coenobium. Nageli 1 pointed out that 

 the coenobia were usually constructed as follows : Colony of 



1 Nageli, Gatt. einzell. Alg. Zurich, 1849. 

 W. A. 14 



