CRYPTOGAMS 



361 



it produces green thalloid segments which vary in shape in the different species. 

 In Caulerpa prolifera (Fig. 295) these outgrowths are leaf-like and are frequently 

 proliferous. In other species they are pinnately lobed or branched. The whole 

 thallus, however branched and segmented it may be, encloses but one cell -cavity, 

 which is, however, often traversed by a network of cross-supports or trabeculaj. 

 Starch-forming leucoplasts are present in the colourless parts of the thallus. 



The genus Bryopsis, on the other hand, has a delicate, pinnately branched 

 thallus. The thallus of Halimeda, the species of which occur in the wanner seas, 

 is composed of flattened segments, and resembles an Opuntia on a small scale. By 

 incrustation with lime it at- 

 tains a hard, coral-like texture. 

 The segments are formed of 

 branched tubular filaments. 



In Jji'i/opsisthe conjugating 

 gametes are differentiated into 

 a larger female cell and a 

 smaller male cell ; in Vau- 

 rlii'i-in and Dichotomosiphon 

 oogamous reproduction is well 

 marked ( as ). The latter Algae 

 occur in fresh water or on damp 

 soil. The thallus consists of 

 a single branched, filamentous 

 cell attached to the substratum 

 by means of colourless rhizoids 

 (Fig. 296 Z>)- 



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. 296). The whole con- 

 tents of such a sporangium be- 

 come converted into a single, 

 green swarm-spore. The wall 

 of the sporangium then rup- 

 tures at the apex, and the swarm-spore, rotating on its longitudinal axis, forces 

 its way through the opening. The swarm-spore is so large as to be visible to 

 the naked eye, and contains numerous nuclei embedded in a peripheral layer 

 of colourless protoplasm. It is entirely surrounded with a fringe of cilia, which 

 protrude in pairs, one pair opposite each nucleus. Morphologically the swarm- 

 spores of Vnuchcria 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 

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

 short lateral branches, and become separated by means of septa from the rest of 

 the thallus (Fig. 297 o, a). At first, according to OLTMANNS, the rudiment of an 



FIG. 205. Caulerpa prollfen'. The shaded lines on the 

 tliiillus leaves indicate the currents of protoplasmic move- 

 ment ; a, growing apex of the thallus axis ; b, b, young 

 thallus lobes ; r, rhizotds. (A nat. sizi>.) 



