330 



BOTANY 



The greatest dimensions attained by any of the PJiaeophyceae are 1 

 exhibited by certain of the Antarctic Laminariaceae. Of these, 

 Macrocystis pyrifera is noted for its gigantic size ; rising obliquely 

 upwards to the surface of the water from the sloping sides of 

 elevations in the ocean bed ; its floating thallus has a length of 200 

 to 300 m. With the exception of a naked lower portion this bears 

 numerous long pendent lobes, each of which is provided at the base with 

 a large bladder-like float filled with air. Even more remarkable, on 

 account of their tree-like character, are the Antarctic species of Lessonia, 

 in which the main axis is as thick as a man's arm ; from it are given 

 off lateral branches with hanging leaf-like segments. The plant attains 

 a height of several metres, and has a tree-like habit of growth. 



The Fucaceae, although relatively large, do not compare with the 

 Laminariaceae in size. As examples of well-known forms of this 



order may be cited Fucus vesiculosus 

 (Pig. 253, b), which has a band-shaped, 

 dichotomously branching thallus with 

 air-bladders, and Fucus platycarpus 

 without bladders. Both species are 

 fastened to the substratum by discoid 

 hold -fasts, and growing sometimes 

 over 1 metre long, are found cover- 

 ing extended areas of the littoral 

 region of the sea-coast. Sargassum, 

 a related genus chiefly inhabiting 

 tropical oceans, surpasses the other 

 brown sea-weeds and even all other 

 Algae in the segmentation of its 

 thallus, and in this respect it bears 

 a close resemblance to the higher 

 plants. The thallus of Sargassum 

 shows in fact a distinction into slender 

 branched cylindrical axes with lateral 

 outgrowths, which, according to their 

 function, are differentiated as foliage, bracteal, or fertile segments or 

 as air-bladders. Various species of Sargassum which have been swept 

 away from the coast by currents, finally collect in large floating masses 

 in quiet regions of the ocean (Sargasso Sea). Sargassum bacciferwm is 

 carried even to the coast of Europe. 



The cells of the PJiaeophyceae have usually but one nucleus. They 

 are supplied with a larger or smaller number of chromatophores, 

 which, in addition to chlorophyll, contain a brown pigment, PHYCO- 

 PHjEIN, which imparts to the Algae a yellowish -brown or dark 

 brown colour. Many Phaeophyceae produce and store up a fatty sub- 

 stance in the place of starch. Among the more highly-developed forms 

 the thallus exhibits a fairly highly differentiated anatomical structure. 



Fig. 253. — Fucus vesiculosus. 6, Air-bladder ; 

 /, receptacles. (Reduced J.) 



