and Genera 0/ Chlorospermous Algae. 419 



developing within itself a similar aggregate group of minute 

 zoospores, which enlarge and afterwards become free. 



Fam. 1. Hydrodictyeae. 



Frond green. Zoospores naked, oblong, united at the ends 

 into a saccate net with polygonal meshes, each side of the mesh 

 being formed of a single zoospore. 



1. Hydrodichjon. 



" The granular mass gives rise, at a certain period of growth, 

 to a number of elliptical grains endowed with active motion ; 

 these become attached to each other by their extremities so as 

 to form a network; union takes place between the several 

 bodies, and in process of time a new individual is formed, which 

 becomes free by the absorption of the external wall." (Berkeley, 

 p. 238.) 



Fam. 2. Pediastreaa. 



Frond green. Zoospore naked, free, oblong or angular, 

 united side by side into an expanded frond. 



The passage from the family Hydrodictyeae to the Pedias- 

 treae is easy by the genus Serastrum, in which the cells form 

 a small oblong sphere. Indeed the resemblance is so great, 

 that it has been suggested that the genus Hydrodictyon should 

 be removed to the group of Desmidieae to be near that genus. 



* Cells fusiform or elliptical. 



1. Scenedesmus. (S. quadricauda, Ralfs, t. 31. f. 12.) 



** Cells angular , forming aflat disk. 



2. Pediastrum. (P. ellipticum, Ralfs, t. 31. f. 10.) 



3. Ccelastrum. 



*** Cells angular, forming a hollotv sphere. 



4. Serastrum. 



Fam. 3. Volvocineae. 



Frond green. Zoospores circular or square, enclosed in a 

 membranous or gelatinous cyst of definite form. 



" Propagating by the repeated segmentation of the contents of 

 the old cells into a definite number of portions or 'gonidia/ which 

 ai'e either still or for a time mobile, and which are either arranged 

 according to the typical plan within the parent-cell, and by its 

 bursting set free as a new frond or family, or become so arranged 

 without the parent-cell, but still involved in its inner membrane, 

 the whole having emerged by a transverse fissure." (Pritchard, 

 Diatom, p. 753.) 



The number of cells is always constant in young fronds 



' 27* 



