THE METABOLISM OF PLANTS. 243 



concerned ; but indirectly they are, in many cases, of great 

 importance. The fact that chlorophyll is essential to the 

 process of the formation of organic substance from carbon 

 dioxide and water has been already dwelt upon at length 

 (p. 151). The colours of flowers play an important part in 

 attracting insects to visit the flower, and by this means cross- 

 fertilization is ensured. 



In addition to the substances already enumerated, certain 

 others, known as bitter principles, are frequently present in 

 plants. It has been ascertained that some of these are glu- 

 cosides, and some alkaloids, but the chemical nature of many 

 of them is still undetermined. Such are Santonin (C 15 H 18 O 3 ), 

 AloTn (C 15 H 16 O 7 ), Quassiin (C 10 H 12 O 3 ). It is of course im- 

 possible to say anything as the possible mode of their origin 

 or as to their physiological significance in the plant. 



We have, finally, to consider certain fatty bodies. It has 

 been already pointed out (p. 216) that the ordinary fats are 

 plastic products, but there are certain fatty bodies of which 

 we cannot make this statement : such are cholesterin, lecithin, 

 and wax. We do not know how these substances are formed, 

 though we may regard it as probable that they, like the 

 ordinary fats (glycerides), are derived from protoplasm : this 

 view is especially probable in the case of lecithin, which is a 

 nitrogenous and phosphorised fat. Nor do we know much 

 about their fate in the plant. It is possible that cholesterin 

 and lecithin may be used in the constructive metabolism of 

 the plant, but wax is to be regarded as a waste-product. 

 Wax occurs especially, and perhaps exclusively, in the 

 external cell-walls or on the surface of those parts of plants 

 which have a cuticularised epidermis. The " bloom " on 

 fruits, for example, is a layer of wax, and it is in the presence 

 of such a layer that the glaucous appearance of many suc- 

 culent leaves and stems is due. In some cases, more par- 

 ticularly in Palms, the layer of wax is so thick that it is 

 collected for commercial purposes : the Palms which espe- 

 cially yield wax are the Carnauba Palm of Brazil (Copernicia 

 cerifera], and the Wax-Palm of New Granada (Ceroxylon 

 Andicola). 



162 



