294 PART IV. VEGETABLE TAXONOMY. 



sexually by means of oogonia and antheridia. The former are 

 always modified terminal cells of a branch. The cell becomes 

 swollen at the base and elongated into a tube at its apex, and 

 when ready for fertilization, the tube opens and emits a colorless 

 mucilage. The antheridia are small flask-shaped bodies, borne 

 singly, or two or more together, at the ends of other branches, 

 or on adjacent cells. Each of these when ripe emits a single 

 antherozoid. The latter moves about by means of two cilia, and 

 probably finds its way through the mucilage down the tube of the 

 oogonium to the oSsphere at its base and fertilizes it. Tn the 

 course of the ripening of the oogonium, there grows up about it 

 from adjacent cells a cellular rind or protecting sheath. See 

 Fig. 490, A and B. The ripe oospore does not immediately 

 develop into the mature form of the plant, but, after a period of 

 rest, first forms swarm-spores, from which the filaments are finally 

 produced. 



Fig. 491. — Spirogyra maxima in incipient stages of conjugation, a, cell-nucleus, fusi- 

 form in shape and suspended in the interior of the cell by delicate protoplasmic threads. 

 Magnified about 100 diameters. 



The Conjugatae. This group differs from all other algae in 

 the peculiarly complex structure of their chlorophyll-bodies ; 

 from all, except some of the Diatomaceae, in their mode of sexual 

 reproduction, which consists in the direct conjugation of ordinary 

 vegetative cells, and from many in not producing swarm-spores. 

 In some of the species the chlorophyll-bodies form plates of 

 definite form and arrangement, in others, star-shaped masses, 

 and in still others, spiral bands. A few of the forms are uni- 

 cellular, but most of them consist of unbranching filaments. To 

 this group belong a number of genera represented by species 

 common in our fresh waters, most important of which are Meso- 

 carpus, the Spirogyras, the Zygnemas and the Desmids. 



