70 



Chlorophycew 



forms hemispherical cushions commonly 2 4 mm. in diameter, but occasion- 

 ally greatly exceeding these dimensions. The filaments are erect and radiating 

 and the cells are 1 3 times longer than their diameter, which is from 20 50 p. 

 (fig. 17). This species is more frequently observed with sexual organs than 

 any of the others. C. irregularis Pringsh. possesses a more or less parenchy- 

 rnatous thallus in which the branching is very irregular. All the species are 

 readily eaten by pond-snails of the genera Limnwi and Planorbis. 



Family 2. HERPOSTEIRACE^E. 



This is a small family including only the genus Herposteiron. 

 The plants are epiphytic on larger Algse and on other water-plants, 

 and occur as short irregular filaments which are little branched. 

 Most of the cells of the filament possess one or more bristle-like 

 seta? or hairs, cut off from the cell which bears them by a basal 





Fig. 18. Herposteiron confervicola Nag. (=Aphanochcete repens A. Br.). oo, 

 oogonium ; os, oospbere ; a, antheridium ; an, spermatozoid. (After Huber.) 



septum. Chodat has found that in cultures the setae are sometimes 

 replaced by branches, showing the relationship between this genus 

 and the Chsetophoracese. 



Asexual reproduction takes place by zoogonidia, one to four 

 being produced from the mother-cell, the wall of which ruptures 

 and sets them free. They vary much in size, possess four cilia, 

 and usually a red pigment-spot. On coming to rest they generally 

 develop unilaterally into a new plant. Sometimes aplanospores 

 are formed (fig. 19 Ca). 



The sexual reproduction of Herposteiron is of special interest. 

 The oogonia are differentiated from certain of the central cells of 



