NATURAL COLOUKS 443 



and vessels are composed, and in wood these are thickened and 

 strengthened by encrusting deposits of similar material. 



Cellulose is a carbohydrate and may conceivably be formed 

 by condensation from other carbohydrates, the sugars and 

 starch, which are among the first, if not the very first, com- 

 pounds built up from the elements of water and carbon taken 

 from the carbon dioxide of the air. But these synthetic opera- 

 tions are only affected through the agency of the complex mix- 

 ture of nitrogenous substances contained in the protoplasm, 

 which are themselves constantly in process of formation and 

 decomposition. 



In these highly complicated chemical changes by-products 

 are doubtless formed, and unless they are utilised for some pur- 

 pose in the plant itself they may accumulate in the tissues and 

 may even obstruct the processes of growth. Such deposits are 

 sometimes mineral, as in the deposits of phosphate of lime in 

 teak, sometimes resinous, as in the heart wood of many trees, 

 sometimes in the form of alkaloids, such as quinine, which are 

 found in the bark. These by-products may be compared to the 

 chips and shavings which collect in a carpenter's shop, but which 

 have no relation to the form or purpose of the object which 

 occupies his labour. 



The blue, violet, and red pigments which are extracted from 

 fruits, flowers, and many leaves are called anthocyanins. These 

 exist in the flowers in the form of glucosides, and probably, 

 therefore, serve to some extent as store of nutriment available 

 during the process of fertilisation and development of the ovary. 

 The colouring component, which is associated with the glucose 

 or other sugar, is called an anthocyanidin, and it exhibits a certain 

 degree of basic character as it combines with the elements of 

 acids. These colouring matters contain no nitrogen, but only 

 carbon, hydrogen, and oxygen, and, unlike chlorophyll, no 

 magnesium or other metal. The anthocyan pigments are fre- 

 quently mixed with a yellow substance, and as a result of this 

 mixture some of the brown effects seen in such flowers as the 

 dark wallflower are due. This is easily reconcilable with the 

 existence of pure yellow varieties and a purple kind of wall- 

 flower in which the pigments are separate. The reds, purples, 

 and blues of flowers seem in most cases to be attributable to the 

 same substance, the tint varying according to the degree of 

 acidity of the plant juice, acidity producing from the blue a red 



