THE RED GROUPS OF ALG2B. 3;' 



genera and species; and much remains to be learned respecting 

 the cause of this variation. There appears to be no doubt that 



the age of the plant and the circumstances in which it grows have 



influence on their amount and relative proportion ; but at the same 

 time there is manifestly a most intimate connexion between them 

 and the general organization ; so that there is a relation between 

 the particular colouring-matters and the position of the plants in 

 the natural system of classification. Premising, then, that very 

 much remains to be learned, I will now proceed to give an outline 

 of some of the principal facts which I have so far been able to ob- 

 serve in a more or less satisfactory manner. 



In the first place, I must say that in this paper I shall describe 

 the position of the absorption-bands seen in the spectra by giving 

 the wave-lengths of the light corresponding to their centres in 

 millionths of a millimetre, as explained in my late paper to the 

 Royal Microscopical Society*, since I am more and more con- 

 vinced that it is the true method. I shall also express the width 

 of the bands when seen to the greatest advantage by giving the 

 difference between the wave-lengths of their two extreme edges ; 

 but since my chief object is to point out the bearing of my sub- 

 ject on botany, I shall enter as little as possible into mere physics 

 and chemistry. I must, however, to some extent ; or else the evi- 

 dence on which my conclusions are founded could not be 

 understood. 



As far as my present knowledge extends, red Algae contain at 

 least six different characteristic colouring-matters soluble in water. 

 Various mixtures of these have been called Phycocyan and Phyco- 

 erythrin ; and I do not think that one can do better than adopt 

 these terms generically, and express the difference between the in - 

 dividual species by the addition of words indicating their colour. 

 Thus, for example, the dark species of Oscillatoria, so common in 

 clear still water (O. nigra) , yields a splendid purple solution, 

 which has been called phycocyan ; but, as I have long ago shown 

 in various papersf, this is chiefly a mixture of two distinct sub- 

 stances — one a purple, having a well-marked absorption-band 

 whose centre is at wave-length 621 millionths of a millimetre, and 

 the other a pink, having an absorption-band at 567 millionths of 

 a millimetre. They may therefore be called respectively purple 

 phycocyan and pink phycocyan. Solutions of such substances in 



Monthly Mic. Jouni. 187~>, vol. xiii. p. 198 



t Tb. 1870, vol. iii. p. 229, &c. 



13 2 



