388 



BOTANY OF THE LIVING PLANT 



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

 Irom pink to purple, or reddish brown. This is due to cLromatophores 

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

 relation to light. The greatest activity of photo-synthesis 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 

 Rhodophvceae are not red. It is significant that Lenianea, which is exposed 

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



I'"ic. 327. 

 Ncmahini nni!!i/ii/iim. i, Branch bearing antheridia (o the left and a carpn^oiiiuni 

 to the right, witli speniiatia, some of which adhere to the trichogyne. j-5 are 

 sttccessive stages of development of the very simple frnit. {."^fter Kny.) 



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

 which in form and colour are among the inost beautiful, and therefore arc 

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

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

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

 sometimes linre-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-molile spermaliinn (Fig. 327, i). The female organ is a 

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

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

 .and then shrivels : the cavpospores arise directly or indirectlv fronr the enlarged 

 base. Ferlilisaiion is indirecl, in the sense that the nucleus of the spennatium 

 received by the (richogync is passed down to the base, where fusion anil 



