364 



BOTANY 



PART II 



it produces green thalloid segments, whicli vaiy in .sliape in tlie different species. 

 In Caulcrpa prolifera (Fig. 286), which occurs in the MediteiTauean, these 

 outgrowths are leaf-like and are frequently proliferous. In other species they 

 are pinnately lohed or branched. The whole thallus, liowever branched and 

 segmented it may be, encloses but one cell-cavity, which is, however, often 

 traversed by a network of cross-supports or trabeculae. Starch-forming leuco- 

 plasts are present in tlie colourless parts of the thallus. 



The genus Bryopsis (■*"), on the other liand, has a delicate, pinnately branched 

 thallus. The thallus oi I[aUineda, the specie's of which occur in the warmer seas, 



is composed of flattened seg- 

 / i ments, and resembles an Op)- 



untia on a small .scale. By 

 incrustation with lime it at- 

 tains a hard, coral-like texture. 

 Tlie segments are formed of 

 branched tubular filaments. 



In Bryopsis the biciliate, 

 pear-shajied, conjugating gam- 

 etes are differentiated into a 

 larger female cell with a green 

 chromatophore and a smaller 

 male cell, three times smaller 

 than the female and with a 

 single yellow chromatophore ; 

 in Vaucheria and Dichotomo- 

 sipJion oogamous reproduction 

 is well marked (""). The latter 

 Algae occur in fresh water or on 

 damp soil. The thallus consists 

 of a single branched, filament- 

 ous cell attached to the sub- 

 stratum by meaus of colourless 

 rhizoids (Fig. 287, D). 



The swarm-spores of Vau- 

 cheria, which differ from those 

 of the other Siphonales, are 

 developed in special sporangia, 

 cut off from the swollen ex- 

 tremities of lateral branches by means of transverse walls (Fig. 287). The whole 

 contents of such a sporangium become converted into a single, green swarm-spore. 

 The wall of the sporangium then ruptures at the apex, and the swarm-spore, 

 rotating on its longitudinal axis, forces its way through tlie oi)ening. The swarm- 

 spore is so large as to be visible to the naked eye, and contains numerous nuclei 

 embedded in a i)eripheral layer of colourless protoplasm. It is entirely surrounded 

 with a fringe of cilia, which protrude in pairs, one pair opposite each nucleus. 

 Morjihologically the swarm -spores of Vaucheria correspond to the collective, 

 individual zoospores of an ordinary sporangium. 



The sexual reproduction of Vaucheria is not effected, like that of the other 

 Siphoneae, by the conjugation of motile gametes, from which, however, as the 

 earlier form of reproduction, it may be considered to have been derived. The 



Fig. 2SG. — Caulerpa prolifera. The shaded lines on the 

 thalhis leaves indicate the currents of protoplasinio move- 

 ment ; fi, growing apex of the thallus axis ; h, b, young 

 thallus lobes ; r, rhizoids. (A nat. size.) 



oogonia and antheridia first appear as small protuberances, which grow out into 



