THE FRESH-WATER ALGAE 



175 



Only one genus listed here. Plant upright, many centimeters long, differ- 

 entiated into a pseudo-parenchymatous principal axis and branches, covered 

 with short, unbranched hairs. Color an olive brown Thorea Bory. 



Reproduction asexual only, con- 

 sisting in the formation of sporangia 

 on the outer surface of the axis, 

 each containing but a single spore, 

 without cilia and without membrane. 

 The position of this alga in the sys- 

 tem of classification is very doubtful, 

 but for convenience it is placed with 

 the Phaeophyceae. 



Fig. 248. Thorea ramosissima Bory. 

 Portion of a longitudinal radial section. 

 X about ISO. (After Hedgecock & 

 Hunter.) 



Class IV. Rhodophyceae 



Color red, or a dull, purplish green; plant sometimes complex in structure; 

 reproduction sexual and in most cases asexual also. 

 Only one order Florideae. 



Plants mostly inhabitants of salt water, but represented in fresh water by several genera. 

 The structure of the different fresh-water genera varies, but the sexual form of reproduction is 

 essentially the same in all. The male reproductive organs are borne on the ends of filamentous 

 branches, the contents of each of which produce a single spermatium. The female organ is 

 flask -shaped, in the larger portion of which, the carpogonium, Hes the oosphere; through the 

 long neck, the trichogyne, the spermatium is conducted to the oosphere at the base, it having 

 been previously carried by the water to the projecting tip of the trichogyne. As a result of 

 fertilization, densely branched filaments arise from the base of the carpogonium, on the ends 

 of which are borne carpospores; these spore-bearing branches, and the sterile branches which 

 usually surround them, together form the cystocarp. In Chanlransia and in many salt- 

 water species tetraspores are also formed. 



1 (8) Plant branched 2 



2 (s) Branches simple and not in whorls 3 



3 (4) Plants coarse, of simple or occasionally branched, hollow, tapering 



bristles with node-like swellings; brownish or dark bluish- 

 green in color Lemanea Bory. 



Bristles attached to a fine, filamentous structure which is furnished 

 with rhizoids. Bristles hollow, with a single row of cells through the 

 center, supported at intervals by transversely placed cells. Anther- 

 idia borne in great numbers on the surface of the node-like swellings, 

 a single spermatium in each. Carpogonia imbedded in the outer 

 wall of the bristles, the tip of the trichogyne only projecting. Chains 

 of carpospores project toward the center. 



Fig. 249. Lemanea torulosa Agardh. One-half natural size. (After 

 Kirchner.) 



