The RHODOSPERMEÆ, Florideæ, or Rosetangles, are the most beautiful of the marine vegetation. No sea plant surpasses them in delicacy and grace of form or richness of colouring, but the most beautiful are seldom seen, because they grow below the line of ebb tides, or under the shelter of other sea weeds in the rock pools left at low water, their crimson tints being deepest when sheltered from strong light. The Rhodosperms, which have representatives in every sea, are much more numerous than the green Algæ both in genera and species. Thirteen orders, comprising sixty-seven genera, inhabit the British coasts. Many are exceedingly minute, forming patches and velvety cushions on rocks and other Algæ; a vast number have jointed filamentous fronds, while others consist of tubular filaments, and many exhibit a shrub-like collection of firm branches; some are flat and foliaceous expansions without a midrib, either thin and delicate, or thick and strong, while a very brilliant group of both narrow and spreading fronds possess a midrib as a distinguishing character. The structure of the frond varies from a simple membranous to a cartilaginous or even horny substance, caused by a greater development of the cellular tissue, which in the higher kinds of Florideæ divides the epidermal layer or skin from the parenchyme or spongy matter within. The mode of reproduction by tetraspores, as well as by simple spores, distinguishes the Rhodosperms from the other two great divisions of the marine Algæ. These bodies are produced by the division of the red or crimson endochrome into four parts, which remain in the cells till they acquire an envelope; their form, which is much varied, depends upon that of the endochrome. Some are produced by the breaking up of a globe of endochrome from the centre into four pyramidal segments; or should the endochrome be elliptical, by dividing it into four by three parallel segments, or a mass may be divided into four by horizontal and vertical sections. Some of these are represented, greatly magnified, in fig. 23. The tetraspores are lodged in wart-like excrescences, immersed either partially or wholly in some part of the frond. [Illustration: Fig. 23. A, Polyides rotundus:—_a_, thin slice showing the wedge-shaped spores; _b_, tetraspores. B, Furcellaria fastigiata:—_c_, thin slice showing a nucleus with the dividing spores; _d_, one of the large cells; _e_, a tetraspore. ]