1 873-] Comparative Vegetable Chromatology. 457 



eliminate the effects due to special conditions. The facts 

 described above clearly show that, if we wish to ascertain 

 what changes depend on a difference in organisation, it 

 is necessary to compare normal specimens of each class, 

 growing as nearly as possible in similar circumstances ; 

 though, at the same time, it is very desirable to determine 

 what is the effect of different conditions. 



In studying comparative vegetable chromatology, it is re- 

 quisite to distinguish between fundamental and accidental 

 colouring matters. There is the same sort of difference in 

 the case of animals. The haemoglobin of the blood, and 

 the colouring matters in the bile are, as is well known, of 

 such great physiological importance, that they are essential 

 to the healthy life of the higher classes of animals, whereas 

 the colouring matters in the hair or feathers are of only 

 very indirect utility. In a similar manner the higher classes 

 of plants cannot permanently grow without the colouring 

 matters belonging to the chlorophyll and xanthophyll groups, 

 whilst the various red and blue substances belonging to the 

 erythrophyll group may be present or absent without 

 materially interfering with the growth of the plant, and are 

 either of no use, or only very indirectly advantageous, as for 

 example, in attracting to the flowers the insects instrumental 

 in causing fertilisation. At present it is impossible to 

 decide whether certain kinds of colouring matters are or 

 are not essential to the growth of particular plants, or 

 whether they may not be necessary for some classes, and 

 present in others like those organs of animals which, though 

 requisite for some classes, are only rudimentary and of no 

 use to others. Some, also, may be only constant products. 

 The whole subject is, indeed, only in its infancy ; many funda- 

 mental questions remain to be decided, and for the present 

 we must be content with having obtained a clue to a kind of 

 research which promises to throw a new light on such 

 inquiries. 



It is very common to find that accidental colouring 

 matters are much more conspicuous than some that are 

 probably of great importance. Thus, for example, the 

 crimson-coloured substance which is developed in the leaves 

 of certain varieties of the beech, is so very conspicuous, 

 and disguises the other colouring matters so much, that 

 perhaps few persons would imagine that the normal amount 

 of chlorophyll is present, and yet this is easily proved by 

 comparing the spectrum of a very red leaf, growing where 

 much exposed to the sun, with that of a green leaf, growing 

 in a very shady place on the same tree, the absorption-band 



