156 FRESH-WATER ALG^ OF THE UNITED STATES. 



developing in the radicles, matured and ready to escape ; 

 fig. 14, radicle cells, more highly magnified, producing 

 zoospores under culture in water ; figs. 15, 16, zoospores 4-6 

 days old ; figs. 17, 18, seven to eight days old ; fig. 19, zoo- 

 spores in radicles germinating, direct, on soil ; fig. 20, 

 zoospores developing into vegetative plants; figs. 21, 22, 

 zoospores enlarging and producing sexual zoospores ; figs. 

 23, 24, zoospores copulating; fig. 25, isospores, the result of 

 the copulation of zoospores ; fig. 26, after a few days the 

 same become stellate. 



Order IV. PEOTOCOCCOIDE^E. 



Strictly unicellular algae, green, without terminal growth or 

 ramification and without a vegetative generation of cells. Either 

 single, segregate, or associated in families. Cells of the families 

 either indefinitely increasing in number, then forming daughter 

 families ; or of a definite number, then forming a coenoMum, some- 

 times parenchymatously united, yet unicellular because each cell 

 is possessed of power of reproduction. 



Propagation sexual by means of oogonia and antheridia, or by 

 copulation of zoospores, bi-ciliate macrospores, or microspores. 

 The life-history of many forms belonging to this Order of plants 

 is not yet fully understood, but as far as known they develop 

 zoospores of one or the other kind, or both, and are thereby 

 separated from the CONJUGATE. 



Family XILVOLVOCACE^l. 



Coenobia mobile during the entire period of life, each cell 

 being furnished with two cilia. Cells green ; common tegument 

 of the coenobium more or less ample, usually hyaline. 



Propagation sexual or asexual. 



SEXUAL FORMS. 



Coenobia spherical which by means of the cilia of the indi- 

 vidual cells are mobile with more or less of a revolving motion. 



Sexual propagation by means of oogonia and antheridia. The 

 latter are developed from a vegetative cell of the coenobium ; 

 this enlarges, the contents of the cell divide and form a cluster 

 of small cells developing spermatozoids ; the cell-membrane 

 breaks and these escape. The oogoniuni in like manner is de- 



