PROTEINS OR ALBUMINOUS SUBSTANCES 433 



while the other will not. But though the chemist can in the 

 laboratory build up from the elements carbon, hydrogen, and 

 oxygen, compounds identical with those which are provided for 

 us by nature in the plant, the physiologist cannot as yet form 

 more than rude conjectures as to how they are generated in the 

 plant. 



The first step in the construction of the tissues of the higher 

 plants consists in the decomposition of carbon dioxide derived 

 from the air in the green parts of leaves and stems under the 

 stimulus of the sun's rays. -But how this comes about no one 

 knows, and there is difference of opinion even as to the nature 

 of the first product of the fixation of the carbon and rejection of 

 the oxygen which is known to go on. It is probably a sugar, and 

 the preponderance of evidence appears to be in favour of the 

 view that it is the comparatively complex disaccharose, common 

 sugar. The case is still more difficult when an attempt is made 

 to conceive how protein matters are formed which require the 

 co-operation of nitrogen, sulphur, and phosphorus derived from 

 mineral compounds of these elements brought up from the soil. 

 It is the first step which causes the chief difficulty ; once a 

 molecule is formed of the kind which has been described as a 

 protein, even of the very simplest kind, the attachment of 

 carbon dioxide to its amino-constituent or of ammonia to its 

 carboxylic constituent may conceivably lead to accumulations 

 of carbon and of nitrogen which may lead to the formation of the 

 complex albuminous matters which are so intimately concerned 

 in the affairs of the living tissue. 



Beside the sugars fats also play an important part even in the 

 vegetable economy. Nearly all the oils used for food or other 

 practical purposes are derived from plants, and more especially 

 from the fruits (palm and olive oils for example) and seeds (coco- 

 nut, cotton-seed, linseed, etc.). The composition and constitu- 

 tion of fixed fats and oils is well known, but the problem of their 

 production in nature is far from being solved. 



Carbohydrates furnish the raw material in all probability, but 

 how they are converted into fatty acids is quite unknown, 

 though the resolution of a sugar into glycerin is readily con- 

 ceivable. On the other hand the digestion of fats in the stomach 

 is a subject on which physiologists are not agreed. And to refer 

 these changes to the action of enzymes is only to substitute one 

 kind of mystery for another, inasmuch as the precise nature of 

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