SEA WEEDS 377 



They usually grow on rocks and stones, from high-water mark to 

 moderately deep water, but some of the smaller species are pseudo- 

 parasitic on other algse. 



Their form is most varied. Some are minute filamentous plants, 

 consisting only of slender jointed threads, and others are mere 

 shapeless masses ; but many of the larger species exhibit a great 

 differentiation of form, having root-like and stem-like structures, 

 and expansions that resemble leaves. The latter, too, often have 

 large vesicles that contain air, sometimes arranged singly along the 

 median line of the frond, or in lateral pairs, or a single vesicle at 

 the base of each segment of the thallus. 



The air vesicles, of course, serve to buoy up the plant when 

 it is submerged, thus enabling the light to penetrate between its 

 fronds to lower portions ; and when the plants have been wrenched 

 from their moorings by the force of the waves, they immediately 

 rise to the surface and are drifted on to the shore or accumulate 

 in the eddies of the surface currents. In this way immense masses 

 of floating weeds are formed, the most remarkable being that of the 

 Sargasso Sea in the North Atlantic. 



Like other algae, the melanospores grow by a continued process 

 of cell-division, and when portions of the thallus are worn away 

 during stormy weather, they are renewed by the same process. 



The cell-walls of many species are very mucilaginous, the 

 gelatinous covering being either the result of the degeneration of 

 the cell-walls themselves, or the secretion of special glands. 



As with the last division, the reproduction of the melanospores 

 may be asexual or sexual. The asexual spores, which are not 

 motile, are formed in some of the surface cells of the thallus. The 

 male and female sexual organs, called respectively the antlieridia 

 and the oogonia, are produced in cavities on special portions of 

 the thallus, both kinds being often formed in the same cavity or 

 depression. The latter contains from one to eight little bodies 

 called oospheres. These escape and float passively away when the 

 wall of the oogonia ruptures. The antheridia are also discharged 

 whole, but the minute fertilising elements (antherozoids), which 

 are eventually set free from them, swarm round the oospheres, 

 being attracted by the latter. Soon one of the antherozoids enters 

 the oosphere, and from that moment all attraction ceases, the 

 remainder of the antherozoids floating passively away ; and the 

 oosphere, previously naked and barren, now develops a cell-wall, 

 and becomes the fertile progenitor of a new plant. 



