204 



RED ALGM (RHODOPHYCE^) 



In many the thallus is branched and thread-like (e.g. Calli- 

 thamnion, Ceramium, Fig. in, C), whilst in others it is flattened 

 (Plocamium, Chondrus, Fig. in, A). In the former the larger 

 branches usually consist of several rows of cells, whilst in the latter 

 a number of distinct tissue-regions can often be distinguished. 

 An interesting form, often very abundant in the rock-pools, is 

 Corallina (Fig. in, B), whose branched thallus is composed of 

 numerous pinkish- white segments loosely jointed to one another, 



and densely encrusted 

 with carbonate of lime. 

 Similar calcareous 

 Algce, in part of larger 

 dimensions, are com- 

 mon in tropical seas, 

 where they often play 

 an important part in 

 the production of 

 coral-reefs, and com- 

 parable forms are 

 known to have contri- 

 buted largely to the 

 formation of certain 

 limestone rocks. Di- 

 verse tropical Red 

 Algae are the source 

 of agar-agar, a sub- 

 stitute for gelatine 

 extensively used for 

 bacterial cultures, 

 whilst Carrageen 

 (Chondrus crispus, Fig. in, A) furnishes a valuable invalid diet. 

 There is still another large group of Algae (Cyanophycece] 

 named after the prevailing colour, which in this case is blue-green. 

 The forms in question, though relatively rare in the sea, are com- 

 mon in fresh-water, and often particularly characteristic of damp 

 terrestrial habitats, such as rocks exposed to a constant trickle 

 and the muddy sides of watercourses. Their great adaptability 

 to varied conditions is also shown by their playing the leading 

 role alike in the vegetation of hot springs and in the frigid lakes 



FIG. no. Part of a plant of Sargassum 

 bacciferum, about natural size, showing 

 the numerous air-bladders to which the 

 specific name is due. 



