HYDRODICTYON. 1 69 



Family XIILPROTOCOCCACE^E. 



Vegetative cells not provided with cilia, either single, or 

 united, forming coenobia. 



Propagation by copulation of zoogonidia ; or by asexual zoo- 

 spores. Division of vegetative cells does not occur. 



The plants of this Family may be divided into three sections. 



I. Cells which form coeuobia. 

 II. Cells which form pseudo-coenobia,. 

 III. Cells which remain detached. 



I. COENOBIA. Single cells unite into regular, definite forms 

 called coenobia. These are developed primarily from single 

 cells, daughter cells from the same maternal cell. They are 

 separated from multicelliilar plants by the fact, that the single 

 cells are incapable of division and that each may develop the 

 original form. 



Genus 40, PEDIASTRUM, Meyen. 



For the plants of this genus, refer to Desmids of the United 

 States, page 152. 



Genus 41, HYDBODICTYON, Both. 



Coenobia formed by oblong-cylindrical cells united at the ends 

 into a reticulated structure ; primarily saccate, later simple net 

 work; cells all fertile, some producing macrogouidia, which join 

 themselves into a coenobium within the mother cell, others pro- 

 ducing microgonidia, which are furnished with two or four 

 vibratile cilia, and a lateral red spot. These are motile for a 

 time in the maternal cell and then escape through a pore in the 

 cell wall. They continue active for a short time during which 

 they copulate, then rest, assuming a spherical form and a firmer 

 cell membrane ; they lie dormant, may become perfectly dried, 

 but after being restored by moisture they develop 2-5 biciliate 

 zoospores, which in turn, after a short season of mobile life, 

 come to rest, develop into large irregular, somewhat square 

 cells, cuspidate angles, which soon cast off the membrane and 

 yield young saccate coenobia. 



4 



HYDRODICTYON UTRICULATTJM, Eoth. 



Size of the coenobia very variable, differing mainly ac- 

 cording to age ; the younger ones so small that fifty or more 



