228 RHODOSPERMS. PAKT n. 



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 Algae 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 

 Algae; 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 Morideae 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 Algae. 

 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 endo- 

 chrome 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 di- 

 vided into four by horizontal and vertical sections. 

 Some of these are represented, greatly magnified, in 

 fig. 23. The tetraspores are lodged in wart-like excres- 



