328 



BOTANY 



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 trabeculae. 



The thallus of Codium, also a marine form, consists at first of a single cell, but 

 in time develops lateral outgrowths which become thickly intertwined and cut off 

 by transverse walls. In the case of Codium Bursa, the vegetative body thus 

 formed has the shape of a hollow sphere, while the thallus of Codium tomentosum 

 is cylindrical and dichotomously branched. The genus Bryopsis, on the other hand, 

 has a delicate, pinnately -branched thallus. Although originally unicellular, the 



Fig. 250. — Ccmlerpa prolifera. The shaded lines on the Fig. 251.— Acetahularia medi- 

 thallus leaves indicate the currents of protoplasmic move- terranea. (Nat. size.) 



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

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



thallus develops lateral tubular branches that eventually become septated from it 

 by the formation of transverse^walls. 



Other marine Siphoneae become encrusted with calcium oxalate and calcium 

 carbonate, and bear a resemblance to coral, e.g. Ralimeda Opuntia, which resembles 

 Opuntia on a small scale. Acetahularia mediterranea, also one of the calcareous 

 Siphoneae, -has a stalked umbrella-like thallus (Fig. 251) attached firmly to the 

 substratum by means of rhizoids. The disc consists of a number of closely -crowded 

 tubular outgrowths radiating from the tip of the stalks, in which are developed the 

 non-motile spores, the so-called aplanospores. These are liberated when the disc 

 falls to pieces, and form gametangia (as in Botrydium) ; and in the latter piano- 

 gametes are developed, which conjugate in pairs. 



