12 
INTRODUCTION. 
IV. 
purplish red, but becomes greenish or whitish when growing in shallow pools. 
The white colour, therefore, which is preferred in carrigeen by the purchaser of the 
prepared article, is entirely due to bleaching and repeated rinsing in fresh water. 
Many Algae, 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 prismatic 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 JDelesseria 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 Algae to green, and if heat be applied during the drying process, an 
