128 



BOTANY 



Tissues of Laminariaceae. The tissues of the Laininariaceae are well 

 developed. The outer tissues are composed of small, closely set cells, which 

 contain numerous chromatophores, and constitute the assimilative tissue. 

 Within this layer the cells are very much larger, and more delicate, and contain 

 but few chromatophores. The central part forms a sort of pith with loosely 

 woven elongated cells, some of which are elongated elements, closely resembling 

 the sieve-tubes of the higher plants, and probably serving much the same purpose. 

 The outer tissues are generally tough and flexible, so that the plants can endure 

 the beating of the surf without being injured (Fig. 99). 



The formation of the leaves is in many cases the result of a split- 

 ting of an originally entire lamina. This is very clearly shown in 

 Macrocystis (Fig. 98, B). The sickle-shaped terminal piece of each 

 branch consists largely of actively growing tissues. Short slits 

 appear at regular intervals, which extend until they reach the mar- 

 gin and the young leaf is separated. The young leaf shows a slight 

 enlargement at its base, which is at first solid, but later develops into 

 the pear-shaped float which is found at the base of the older leaf. 



par 



FIG. 99. A, Nereocystis Lutkeana, transverse section of leaf, showing sporangia, xp. 

 B, outer tissue of leaf, more highly magnified. C, sporangia and paraphyses, par 

 (X500). D, Chorda filum, zoospores (X 1000). E, Ec/regia Menziesii, trumpet- 

 hyphae (x 500). F, Postelsia palmseformis, sieve-tube (x 500). (D, after 

 REIXKK.) 



Reproduction 



Most of the Phaeosporeae give rise to biciliate zoospores, which 

 are formed in the so-called unilocular sporangia (Fig. 99, C). The 

 nucleus of the sporangium divides repeatedly, and this is followed by 

 the division of the cytoplasm, but no cell-walls are formed. The 

 zoospores are usually oval, or kidney-shaped, and have two lateral 



