THE FRESH-WATER ALGAE 173 



Leaflets and interaodes of both axis and leaf are each of but a single cell, the 

 walls of which are lined with chloroplasts and in the center of which is a large 

 sap cavity. In Cham the internodal cell is more or less completely covered by 

 a layer of cortical cells of the same structure. A swamp-like odor is usually 

 present. Reproduction sexual only; plants either dioecious or monoecious, but 

 in the latter case the antheridia mature before the oogonia. The antheridium 

 is spherical, its wall composed of eight "shields" which contain red chromo- 

 plasts on their inner surfaces. Attached to the middle of each shield and pro- 

 jecting inward is a club-shaped cell, the manubrium, which in turn bears a 

 short cell, the capitulum. To the capitula are attached secondary capitula 

 bearing four long, slender filaments made up of many cells, each containing 

 an antherozoid; the antherozoids are spiral in form and have two cilia at their 

 anterior ends; the oogonia are egg-shaped and are covered by five spiral 

 cells, the tips of which are divided, once in Chara and twice in Nitella, to form 

 the "crown." The term sporophydium has been suggested for the structure 

 including the oospore, its basal cell, and enveloping cells. Below the crown 

 cells the antherozoids penetrate to effect fertilization. Oospores are brown or 

 yellowish; on germination they produce first a simple row of cells, the pro- 

 embryo, on which the new individual arises. 



254 (257) Points of the crown of the oogonium two-celled. 



Subfamily NITELLEAE . .255 



2 55 (256) Leaflets projecting beyond the tips of the leaves, giving the appear- 

 ance of forked leaves Nitella Agardh. 



Axis and leaves never with a cortical covering and seldom encrusted 

 with lime. Leaves with but one whorl of leaflets, but these in turn 

 may bear whorls of leaflets, those of the last order always projecting 

 beyond the leaves, giving them a divided appearance. The antheridium 

 always terminal on the middle leaf or leaflet. Oogonia either single or 

 several together, in the place of lateral leaflets. 



FIG. 245. Nitella sp. Natural size. (Original.) 



256 (255) Leaflets not projecting beyond the tips of the leaves, or not present. 



Tolypella A. Braun. 



Stem and leaves never with a cortical covering. Leaves with one to 

 three whorls of leaflets, which in turn may bear other whorls of leaflets, 

 much smaller than the first. Antheridia single or several together, which 

 arise from the basal or the first node of a leaf. Oogonia several, sur- 

 rounding the antheridia. Plants usually monoecious. 



FIG. 246. Tolypella nidifica v. Leonh. Three-fourths natural size. (Portion 

 of figure after Wille.) 



