368 



BOTANY 



the other Brown Seaweeds, and even all other Algae, in the segmentation of its 

 thallus. 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). 



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

 contain a larger or smaller number of simple or lobed, disc-shaped 

 chromatophores, which contain a brown pigment (phaeophyll, p. 

 62), -giving to the algae a yellowish-brown or dark brown colour. 

 Numerous grains of a semi-fluid substance, to which the name 

 fucosan has been given, appear as the product of assimilation. 

 This substance appears to be a carbohydrate, and, according to 

 HANSTEEN ( 39 ), originates in the chromatophores. Among the more 

 highly developed forms the thallus exhibits a highly differentiated 

 anatomical structure. The quter cell layers, as a rule, function as 

 an assimilatory tissue, the inner cells as storage reservoirs. In 

 some species the axial cells of the thallus are arranged in strands 

 containing sieve-tubes ( 40 ). 



Two orders of Phaeophyceae may be distinguished. The Phaeo- 

 sporeae are vegetatively reproduced by means of zoospores and 

 sexually by ciliated gametes ; they thus resemble Ulothrix among the 



Green Algae. The Cyclosporeae show a 

 marked differentiation of their sexual cells 

 into large, naked, non-motile, spherical 

 oospheres and small, ciliated spermatozoids; 

 some forms are also asexually reproduced by 

 means of naked, non-motile spores. 



Order l t . Phaeosporeae ( 41 ) 



In this order are included the Laminarias, as well 

 as the majority of the other Phaeophyceae. Asexual 

 multiplication is effected by means of swarm-spores, 

 which are produced in large numbers in simple, 

 so-called unilocular sporangia ; they have a red eye- 

 spot, a chromatophore, and two LATERALLY ix- 

 si.i:n:i) cilia (Fig. 303J, one directed forwards and 

 the other backwards. Only this asexual reproduc- 

 tion is yet known in the Laminariaceae. 



In addition to unilocular sporangia, multilocular 

 sporangia are produced in the Phaeosporeae (Fig. 

 304). Each cell of the latter produces a single 

 swarm -spore, rarely several. The conjugation of 



BAHN.) }}, Chorda Jttum. Zoo- these swarm-spores has been observed in some genera. 



spores. (After REINKE.) (From Qn tw account these swarm . spores mus t be termed 



OLTMANNS Algae.) 



gametes, and the corresponding sporangia gamet- 



angia. The degree of sexual differentiation varies, and in some cases the swarm- 



FIG. 303. A, Pleuroeladia lacmtris. 

 Unilocular sporangium with its 

 contents divided up into the 

 zoospores ; a, eye - spot ; ehr, 

 chromatophore. .(After KLE- 



