. Botany of the living plant. Botany. 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 arc covered externally by a layer corresponding to cuticle. They arc 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
. Botany of the living plant. Botany. 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 arc covered externally by a layer corresponding to cuticle. They arc 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 spennatozoids being motile, but the larger ova are non-motile primordial cells. The Phacoph\ thus divided into two orders: the Phaeosporeae, where bolli are motile, and the Cyclosporeae, where only the spcrmatozoids arc motile. To the first belong the Ectocarpeae, La
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1919
