INTRODUCTION. XIX 



which the specimen has been removed. No doubt a fine 

 lake could be prepared either from G. setacea or multifida, 

 could they be procured in sufficient quantity. Paper 

 stained by them retains its colour after many years in the 

 herbarium. At the Cape of Good Hope there is a species 

 of Callithamnion (C. purpuriferum), which, when grow- 

 ing, is of a dull, deep, grayish brown, with but a slightly 

 reddish hue ; but the moment it is placed in fresh water 

 it discharges an abundance of fine, brilliant, purple pow- 

 der, and almost immediately becomes flaccid and putrid. 

 The beautiful Thorea ramosissima, lately discovered by 

 Mr. Mc'Ivor in such abundance in the Thames, at Walton 

 Bridge, is at first of a dark olive, but gradually acquires, 

 after it has been dried, a purple tint. Many of the Polysi- 

 phonice also, which are, when growing, of a brown colour, 

 become, in fresh water, purple or pink ; while some of the 

 same group, as Rytiphlaa complanata, &c., which are at 

 first red, give out, in fresh water, quantities of blackish 

 brown juice, and would become wholly black if dried with- 

 out long previous steeping. Heat converts the colour of 

 most species to green. If any of the Fucacew be plunged 

 in boiling water they rapidly assume a bright green, but, 

 on removal, revert to their original olive, and finally to 

 black. The colours of the Rhodosperms may be more per- 

 manently changed, and also to green, by similar treatment. 

 Dictyotaceae perhaps are less affected by fresh water, either 

 cold or hot, than any others. Some of them are nearly un- 

 changed ; others assume more or less of a green shade. 



Most Algae are, at some period of their growth, found 

 attached to other substances by means of a root, or at least 

 a hold-fast. It has been doubted whether, as no distinct 

 vessels of absorption have been discovered, they receive 

 any nourishment through this organ, but the question is by 

 no means settled. Thus much is at least certain : some 



