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Mr. H. C. Sorby on 



[June 19, 



they are of far less importance iu connexion with my present subject 

 than those which occur in the leaves of the higher or in the fronds of 

 the lower classes of plants. I purpose, therefore, to confine my remarks 

 almost exclusively to these. Even then it is very necessary to distinguish 

 between those which appear to be essential for the healthy growth of each 

 class of plants and those which have the character of accidental products, 

 formed under particular conditions, and often absent, without apparently 

 interfering with the normal development of the plant. There is exactly 

 the same difference in the case of animals. The haemoglobin of the 

 blood and the coloured substances in bile are of the utmost physiological 

 importance, whereas those in the hair and feathers are either of no im- 

 portance whatever, or are of advantage to the animal only in some very 

 indirect manner. Similarly, the colouring-matters belonging' to the 

 chlorophyll and xanthophyll groups appear to be essential, whereas 

 those of the erythrophyll group have all the characters of products vary^ 

 ing with accidental circumstances ; and if they are of any value to the 

 plant, it is only indirectly, as, for instance, in attracting to the petals 

 insects instrumental in causing fertilization. These accidental and un- 

 essential colouring-matters are often so much more conspicuous than 

 those of real importance, that they might easily lead any one to conclude 

 that the study of the colouring of plants could not lead to satisfactory 

 results, since very often the colour is not of value even in distinguishing 

 different species. It is, however, not the mere colour that is of importance, 

 but the nature and relative amount of what may be called the funda- 

 mental colouring-matters, of which there are altogether somewhere between 

 a dozen and a score. For example, we often have varieties of plants 

 which have red leaves instead of green : this is owing to the unusual 

 amount of erythrophyll : and, independent of this, the fundamental con- 

 stituents of the higher classes of plants are present in their normal 

 quantity, or their equilibrium only slightly modified by a corresponding 

 difference in conditions. In those classes of plants where conspicuous 

 accidental colouring-matters are seldom developed (as, for instance, in 

 the different groups of marine Algce), the importance of the colour is 

 universally admitted ; and I find that when studied in accordance with 

 the principles described in this paper, the presence or absence of the 

 different kinds of colouring-matters, or their relative amount in different 

 classes of plants, is in some way or other connected with very important 

 differences in organization and vital energy. 



Description of the Groups of Colouring-matters. 

 In accordance with these principles, I now propose to describe those 

 colouring-matters which appear to be of the greatest physiological im- 

 portance, dividing them into various groups, and in the case of each 

 group arranging them in the order of their optical characters — i.e. com- 

 mencing with those in which the absorption extends nearest to the red 



