RHODOSPERME^E. 65 



A red colour, of greater or less purity, now verging to pur- 

 ple, now passing into brown, and fading through all tints of 

 red, green and yellow, into dirty white, is the most obvious 

 characteristic of the plants of this sub-class. Almost all are 

 marine ; the few exceptions exist in the orders Ceramiaceae 

 and Rhodomelacea, the lowest and the highest of the group. 

 The greater part belong to the deeper regions of the zone of 

 Algae, very few growing where they are exposed to air and 

 sunshine, or flourishing when they grow in shallow and open 

 tide-pools. The red tint is never developed in purity in such 

 situations. It requires the shade of a deep, steep-sided pool, 

 or the darkness of a considerable depth of water, to bring out 

 the delicate rosy tint of the frond in perfection. In the for- 

 mer situation we invariably find the best coloured and most 

 luxuriant Rhodosperrns at the side of the pool that faces the 

 north, or in places where overhanging Melanosperms form a 

 protection from the rays of the sun. Such species as habi- 

 tually grow exposed to light assume either the brown tints of 

 the Fuci or else the green and yellow of the Confervas and 

 Ulvae. Some Polysiphoniai, especially P. fastigiata, are in- 

 stances of the first change of colour ; and the Chondri, Lau- 

 rencice, Gracilarice, Ceramia, &c., of the latter. 



This sub-class is by much the most numerous of the class 

 of Algae, so far at least as the vegetation of the sea is con- 

 cerned. In our own Flora they constitute about one half of 

 the whole number of sea-weeds ; and if we take in the Flora 

 of the world generally the proportion will be still greater. 

 Between two and three hundred species of Polysiphonia have 

 been described, and several other genera, though of less ex- 

 tent, contain a large number of species. None attain to the 

 gigantic dimensions of some Melanosperms, although some 

 are of considerable size, having fronds that would cover 

 twenty square feet in expansion. Even on our own coast 

 Nitophyllum punctatum has been seen five feet in length by 

 three in width ; and some of the Irideea of the southern ocean 

 are as large. But red sea-weeds of this size are not common. 

 We do not find many that exceed two feet in length, and 

 great numbers never grow higher than six inches. 



The natural habit is much varied. The simplest are fila- 

 ments, simple or more generally branched, composed of a 

 row of cells strung together : these are exactly analogous to 

 the Ectocarpaceae among Melanosperms. Next, we find a 

 frond consisting of a number of such filaments, bound to- 

 gether by a gelatinous matrix, just as we have in Chordari' 



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