12 INTRODUCTION. IV. 



purplish red, but becomes greenish or whitish when growing in shallow pools. 

 The white colour, therefore, Avhich is preferred in carrigeen by the purchaser of the 

 prepared article, is entirely due to bleaching and repeated rinsing in fresh Avater. 



Many Algse, both of the olive and red series, and in a less perfect manner a few of 

 the grass-green also, reflect prismatic colours when growing under water. In some 

 species of Cystoseira^ particularly in the European C. ericoides and its allies, these 

 colours are so vivid that the dull olive-brown branches appear, as they wave to and 

 fro in the water, to be clothed with the richest metallic greens and blues, changing 

 with every movement, as the beams of light fall in new directions on them. Similar 

 colours, but in a less degree, are seen on Chondrus crispus when growing in deep 

 water; but here the jsrismatic colouring is often confined to the mere tips of the 

 branches, which glitter like sapphires or emeralds among the dark purple leaves. 

 The cause of these changeable colours has not been particularly sought after. The 

 surface may be finely striated, but it does not seem to be more so than in other 

 allied species, where no such iridescence has been observed. In the Chondrus the 

 changeable tints appear to characterize those specimens only which grow in deep 

 water, and which are stronger and more cartilaginous than those which grow in 

 shallow pools. 



Fresh water has generally a very strong action on the colours as well as on 

 the substance of marine Algae which are plunged into it. To many it is a strong 

 poison, rapidly dissolving the gelatine which connects the cells, and dissolving also 

 the walls of the cells themselves ; and that so quickly that in a few minutes one of 

 these delicate plants will be dissolved into a shapeless mass of broken cells and 

 slime. Many species which, when fresh from the sea, resist the action of fresh 

 water, and may be steeped in it without injury for several hours, if again 

 moistened after having once been dried, will almost instantly dissolve and decompose. 

 This is remarkably the case with several species of Gigartina and Iridcea. The first 

 effect of fresh water on the red colours of Algae is to render them brighter and more 

 clear. Thus Dasya coccinea, Gelidium cartilagineum, Plocamium coccineum, and others, 

 are when recent of a very dark and somewhat dull red colour ; but when exposed 

 either to showers and sunshine on the beach, or to fresh water baths in the studio 

 of the botanist, become of various tints of crimson or scarlet, according as the 

 process is continued for a less or greater length of time. At length the colouring 

 matter would be expelled and the fronds bleached white, as occurs among the 

 specimens cast up and exposed to the long continued action of the air ; but if 

 stopped in time and duly regulated, the colours may be greatly heightened by fresh 

 water. Some plants which are dull brown when going into the press, come out a 

 fine crimson ; this is the case with Delesseria sanguinea, though that plant is not 

 always of a dull colour when recent. Others, which are of the most delicate rosy 

 hues when recent, become brown or even black when dried. This is especially the 

 case in the order Rhodomelacece, so named from this tendency of their reds to 

 change to black in drying. The tendency to become black, though it cannot be 

 altogether overcome in these plants, may often be lessened by steeping them in 

 fresh water for some time previous to drying. Hot water generally changes the 

 colours of all Algo3 to green, and if heat be applied during the drying process, an 



