THE BROWN AI/I.E 485 



grows into a sexual plant like the one already described. Thus in 

 the life-history of Coleochajte a sexual form producing gametes, 

 alternates with a form of plant which produces only non-sexual 

 spores. That which bears gametes is termed the gametophyte,'- 

 while the merely spore-bearing one is the sporophyte.- Each repre- 

 sents a generation; hence the plants whose life-histor}' is thus di- 

 vided are said to exhibit an alternation of generations. 



175. The brown algae (Class Phaeophyceae) are charac- 

 terized in general by a hroion coloring matter, phycophcein,' 

 masking the chlorophyll. They are almost entirely marine. 

 Besides many comparatively simple forms there are some 

 showing a remarkably high development of the vegetative 

 system. 



In their methods of reproduction the brown algaj present rather 

 close parallels to \'arious chlorophyceous types, very rarely, however, 

 exhibiting an alternation of generations. 



One of the commonest genera is Laminaria (Fig. 316) 

 which includes the familiar leathery "sea-tangles," "kelps," 

 or "Devil's aprons" often cast upon beaches after a storm. 

 The thallus consists of a flat, more or less leaf-like part 

 {pseudo-leaf) attached to a stalk {pseudo-stem) at the base of 

 which is a hold-fast {pseudo-root), often much branched, which 

 clings to stones or other means of anchorage on the bottom. 

 This thallus which may be yards in length consists of an exceed- 

 ingly large number of cells among which a considerable differ- 

 entiation may be observed. Thus in the stalk as shown in 

 Fig. 317 we have an outer group of cells forming a sort of rind 

 (r, r) which is comparatively tough and thus protective, while 

 at the same time it serves as a food-making part since the 

 cells are rich in chlorophyll. Those inclosed by the rind, 

 {p, p) form the chief bulk of the stalk, are pale in color, and 

 serve largely for the storage of food-materials elaborated 

 by the outer cells. In the rind occur numerous cavities 

 ig, g) filled with a mucilaginous material. The pseudo-leaf 

 shows a differentiation of cells similar to that of the stalk. 

 For the most part as soon as they are formed the cells lose 

 the power of dividing; but in the region where the pseudo- 



1 Gam'et-o-phyte < Gr. gametes, spouse; phyton, plant. 



' Spor'o-phyte < Gr. .ipora, spore. 



■' Phy"co-phii''in < C!r. phyco-i, seaweed; phaios, bro-rni. 



