'CHAP. II. FUNCTION OF NUTRITION. 201 



of immediate reference whenever we wish to describe 

 any colour. The annexed figure (156.) may be taken as 

 a representation of one of its sectors, containing the three 

 shades of grey (a 6), and those of the " very impure" 

 (b c), " impure" (c d), and " pure" (d e) blues. If 

 the other eleven basial colours were similarly disposed 

 round the same centre the chromatometer would be 

 complete. 



It seems unnecessary to include in this scale the 

 different tinges commonly ascribed to white, black, and 

 grey ; as these after all are only very faint or dark 

 shades of some defined colour, and may be recognised 

 by comparison with the nearest shades expressed in the 

 chromatometer. 



(187.) Limitation of Colour. It has often been 

 observed by horticulturists, that among different va- 

 rieties of the same species a limited number of colours 

 is found, among which are not more than two out of 

 three of the basial colours similarly disposed upon the 

 chromatometer. Thus there are blue and red hyacinths, 

 but none that are pure yellow; there are yellow and 

 red dahlias, but none that are blue. The rule is not 

 free from exceptions, still less does it apply to those 

 flowers which have different bands of colour on their 

 corolla. It has been conjectured that those colours 

 which pass from green through yellow to red arise from 

 combinations of oxygen with the chromule in its 

 green or neutral state ; whilst those which pass from 

 green through blue to red contain a less proportion of 

 oxygen than the green chromule itself. But as these 

 two series meet in the same colours at both ends of 

 such a scale it is not easy to understand how this can 

 be the case, since the red would equally result from a 

 union of the chromule with a maximum and with a 

 minimum of oxygen. 



(188.) Results of Vegetable Respiration. From 

 what has been said it seems necessary to conclude 

 that carbon, in order to be fixed in vegetation must be 

 presented to a plant in the form of carbonic acid ; and 



