FAMILY HYDRODICTYACEAE 



The members of this coenocytic family are morphologically very 

 distinctive, although extremely variable. All forms are free-floating, 

 but some are found only in the tychoplankton. In one genus, 

 Hydrodictyon, there are cylindrical cells arranged to form a mac- 

 roscopic, closed cylindrical net. In other genera the cells are 

 triangular or polyhedric in outline and are arranged to form flat or 

 spherical coenobia. The number of cells in the colony varies from 

 2 to 64 in the plate type of colony, whereas in Hydrodictyon several 

 hundreds of cells are involved, always in multiples of 2. The 

 chloroplast is parietal, either a continuous or perforate sheet, with 

 1 to many pyrenoids. 



Like other families in this order, vegetative reproduction by cell 

 division does not occur. The most common method of reproduction 

 is by the formation of daughter colonies within the parent cell, 

 these developing from retained zoospores. Sexual reproduction is 

 by biflagellate isogametes. 



Key to the Genera 



1. Thallus a cylindrical closed reticulum of cylindrical cells 



which form 5- or 6-sided meshes Hydrodictyon 



1. Thallus not a cylindrical net 2 



2. Thallus composed of 2 triangular or trapezoidal cells with 



their bases adjoined - .Euastropsis 



2. Thallus composed of more than 2 cells 3 



3. Thallus a Hat, circular plate of polygonal cells Pediastrum 



3. Thallus a spherical colony of spine-bearing cells on stalks 



radiating from a common center Sorastrum 



HYDRODICTYON Roth 1800, p. 531 



Thallus macroscopic, composed of cylindrical cells which are 

 adjoined at their ends to form a cylindrical net with 5- or 6-sided 

 meshes; chloroplast at first a parietal plate with a single pyrenoid, 

 later becoming a reticulum covering the entire wall and containing 

 many pyrenoids; cells multinucleate. 



Hydrodictyon reticulatum ( L. ) Lagerheim 1883, p. 71 



PI. 47, Fig. 1 



Characteristics as described for the genus; cells up to 200/a in 

 diameter, as much as 1 cm. long when fully enlarged, forming a 

 net up to 2 dm. in length; chloroplast a much diffused reticulum, 

 light yellow-green color in the plant mass, especially at maturity. 



This is a plant which prefers quiet water and is found in lakes 

 where there is little wave action, in pooled streams, and in the 



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