INTRODUCTION. XV 



but those from the extreme depths of the sea are of the 

 olive series in its darkest form. For the colours of these 

 last it has puzzled botanists not a little to account. It is 

 well known that liglit is absolutely necessary to the growth 

 of land-plants, and that the green colour of their foliage 

 altogether depends upon its supply : if placed in even par- 

 tial darkness they quickly acquire a sickly yellowish hue, 

 and finally become white. But with Algae it is different. 

 At depths to which the luminous rays, it is known, do not 

 penetrate, species exist as fully coloured as those along the 

 shore. They therefore, in this respect, either differ from 

 all other plants (Fungi included), or perhaps, what are 

 called the chemical rays, in which seem to reside the most 

 active principles of solar light, may be those which cause 

 colour among these vegetables, and may penetrate to depths 

 to which luminous rays do not reach. But this is mere 

 supposition. However this may be, it is worth remarking 

 that this property among Algse, of producing vigorous 

 growth and strong colour without the agency of light, af- 

 fords another link between them and the animal kingdom, 

 among the lower tribes of w^hich light is by no means es- 

 sential to growth and the most brilliant colour. 



There is this difference also in the distribution of colours 

 among Algae to what obtains among other plants. Among 

 plants in general, nothing is so variable or uncertain as 

 colour : far from serving as a mark to distinguish groups 

 or genera, colour does not even aspire to the rank of a spe- 

 cific character, and the utmost to which it can pretend is 

 to separate one variety of a species from another. Among 

 Algae, on the contrary, it has been ascertained that the 

 classes of colour enumerated above, are, to a great extent, 

 indicative of structure, and consequently of natural affinity. 

 Thus, the green species are of the simplest structure, and 



