. Botany of the living plant. Botany; Plants. 38o BOTANY OF THE LIVING PLANT Laminaria, where a new frond is formed each season between the old one, which is thrown off, and the persistent stalk (Fig. 281). In simple forms the cells are all alike. Soft cell-walls surround a uni-nucleate protoplast, which includes simple brown chromatophores. The plastid pigments are chlorophyll, xanthophyll and carotin, to- gether with fucoxanthin which masks the others and is peculiar to the group. The products of photosynthesis are soluble carbohydrates like laminarin or mannitol. In larger forms the tissues
. Botany of the living plant. Botany; Plants. 38o BOTANY OF THE LIVING PLANT Laminaria, where a new frond is formed each season between the old one, which is thrown off, and the persistent stalk (Fig. 281). In simple forms the cells are all alike. Soft cell-walls surround a uni-nucleate protoplast, which includes simple brown chromatophores. The plastid pigments are chlorophyll, xanthophyll and carotin, to- gether with fucoxanthin which masks the others and is peculiar to the group. The products of photosynthesis are soluble carbohydrates like laminarin or mannitol. In larger forms the tissues are differentiated. For in- stance, in Fucus the cells of the super- ficial layer are thin-walled, and divide actively; they are covered externally by a layer corresponding to cuticle. They are the chief seat of photosyn- thesis, and of tissue-formation. Pass- ing inwards from this layer the mucilaginous cell-walls become more and more swollen, so that the deeper- seated tissues of an old thallus con- sist of a bulky mucous matrix, in which the cells themselves appear as a complicated network (Fig. 282). Centrally there is a firmer conducting cord, which is well defined in old stalks of the larger Tangles. It contains manv tubes with sieve- structure and callus, closely com- parable to the sieve-tubes of Vascular Plants, and serving like them for transport. In large stalks an ill-defined cambial activity provides for thickening and increased mechanical strength. This is still further secured by " intrusive 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 Sea- weeds 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. The propagative cells are produced in sporangia and gametang
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