388 



BOTANY OF THE LIVING PLANT 



their limit at about 150 feet below low-water mark. Their colour varies 

 from pink to purple, or reddish brown. This is due to chromatophores 

 containing red pigment which masks the chlorophyll. The colouring has its 

 relation to light. The greatest activity of photo-syn thesis is in light comple- 

 mentary to the colour of the plant. Ordinary plants make special use of 

 the rays at the red end of the spectrum ; but for Red Algae rays further 

 along the spectrum are effective, and it is the rays towards the blue end of 

 the spectrum which penetrate into the depths of sea water. But all of the 

 Rhodophyceae are not red. It is significant that Lemanea, which is exposed 

 to ordinary sunlight in shallow fresh-water streams, is green. 



FIG. 327. 



Nentalion multifidum. i, Branch bearing antheridia to the left and a carpogonium 

 to the right, with spermatia, some of which adhere to the trichogyne. 2-5 are 

 successive stages of development of the very simple fruit. (After Kny.) 



In form the Red Algae are various, but never large. They include plants 

 which in form and colour are among the most beautiful, and therefore are 

 prized by collectors. They may consist merely of branched septate filaments : 

 or fronds, variously thickened and flattened, may be formed by matting 

 and webbing of many filaments together. Often they are fan-shaped and 

 sometimes lime-encrusted. The feature they have in common is their method 

 of sexuality. The male organs are unicellular, the whole content of each cell 

 escaping as a naked, non-motile spermatium (Fig. 327, i). The female organ is a 

 carpogonium, consisting of a cell with an enlarged base, and elongated upwards 

 into a filamentous trichogyne. This receives the non-motile spermatium, 

 and then shrivels : the carpospores arise directly or indirectly from the enlarged 

 base. Fertilisation is indirect, in the sense that the nucleus of the spermatium 

 received by the trichogyne is passed down to the base, where fusion and 



