BROWN ALGAE 



379 



The product of photo-synthesis appears as semifluid grains of a carbo- 

 hydrate, "fucosan," in place of starch. In the larger forms the 

 tissues are differentiated. For instance, in Fucus the cells of the 

 superficial layer are thin-walled, and divide actively ; they are 

 covered externally by a layer corresponding to cuticle. They are 

 the chief seat of photo-synthesis, and of tissue-formation (Fig. 318). 

 Passing inwards from this layer the 

 mucilaginous cell-walls become more 

 and more swollen, so that the deeper- 

 seated tissues of an old thallus consist 

 of a bulky mucous matrix, in which the 

 cells themselves appear as a complicated 

 network (Fig. 319). Centrally there 

 is a firmer conducting cord, which is 

 well defined in old stalks of the larger 

 Tangles. It contains many tubes with 

 sieve-structure and callus, closely com- 

 parable to the sieve-tubes of Vascular 

 Plants, and serving like them for trans- 

 port. In large stalks an ill-defined 

 cambial activity provides for thicken- 

 ing and increased mechanical strength. 

 This is still further secured by " intru- 

 sive hyphae," which burrow through 

 the softer tissues, and brace them 

 together. In this way they acquire 

 their tough and resistant but yet 

 pliant character. We thus see that 

 both in external form and internal 

 structure the Brown Seaweeds cover 

 a wide range, from the simple to the 

 complex. 



In their sexual propagation they also show an advance, which runs 

 in some degree parallel with their structural progress. Successive 

 steps in differentiation of the sexes may be found. The simplest forms 

 produce isogametes which are motile. More complex forms show 

 differentiation of sexes, the small spermatozoids being motile, but the 

 larger ova are non-motile primordial cells. The Phaeophyceae are 

 thus divided into two orders : the Phaeosporeae, where both gametes 

 are motile, and the Cyclosporeae, where only the spermatozoids are 

 motile. To the first belong the Ectocarpeae, Laminarieae, and 



FIG. 317- 



Laminar la, thallus showing a new frond, 

 intercalated between the stalk and the old 

 frond, which is being thrown off. (Re- 

 duced.) (After Strasburger.) 



