FUCACEJ3. . 265 



cases there is a differentiation of the thallus into parts which 

 have a considerable resemblance to roots, stems, and leaves ; 

 and in size they approach, and, in some cases, equal or exceed 

 the larger Phanerogams. Their tissues, too, show a much 

 higher degree of differentiation thun is common in Thallo- 

 phytes ; the cells are arranged in cell-masses, and these are 

 differentiated into several varieties of parenchyma, approach- 

 ing, in some instances, to the condition which prevails in 

 the Bryophytes ; the outer tissues are composed of small and 

 closely crowded cells, which form a dense, and, in some cases, 

 a, hard mass ; the interior tissues are generally looser, and 

 are for the most part composed of elongated cells so joined 

 a& to leave large intercellular spaces. 



351. With the foregoing there is found in the higher 

 genera a marked differentiation of portions of the plant- 

 body into general reproductive organs, analogous to the 

 floral branches of higher plants. The sexual organs are 

 iound upon modified branches, which differ more or less in 

 shape and appearance from the ordinary ones. This differ- 

 entiation into vegetative and reproductive parts is an impor- 

 tant and significant feature in the plant-body, indicating a 

 decided advance over all the previous groups of Thallo- 

 phytes. 



In their greater duration many of the Fucaeese are in 

 marked contrast to other Thallophytes, which are generally 

 short-lived. They are, for the most part, of considerable 

 size, rivalling, in some cases, even the larger Phanerogams. 

 They grow principally between and a little beyond the tide- 

 marks, and furnish the great bulk of the shore vegetation. 



352. The reproduction of the higher Fucacese is sexual 

 only ; but in some algae which appear to be nearly allied 

 (Phseosporeae) asexual zoospores are known. In Fucus 

 the sexual organs are found in the thickened ends of the 

 lateral branches of the thallus (A, Fig. 181). They occur 

 on the walls of hollows termed conceptacles, which are 

 spherical, with a small opening at the top (B, Fig. 181). 

 The conceptacles are at first portions of the general surface, 

 which afterward become depressions which are walled in 

 and overgrown by the surrounding tissues ; they are thus to 



