THE PLANT AND ITS FOOD. ii 



causes are at work to bring about the inhibition 

 of the carbon-assimilation : first, the chlorophyll- 

 corpuscles become filled to excess with starch, 

 which cannot get away because all the passages 

 are full and the products are inhibiting the further 

 action of the enzymes which should dissolve the 

 solid granules ; secondly, the leaf being detached 

 from the plant explains why the soluble products 

 cannot get away, for this makes a great difference in 

 the rate of exhaustion of the leaf; and, thirdly, the 

 same fact involves that the leaf can obtain no further 

 supply of salts of potassium, etc., without which 

 elements the processes in question cannot go on. 



These and numerous other deeper insights into 

 the process of assimilation, obviously strengthen 

 the force of Sachs' discovery ; though it by no 

 means necessarily follows that starch-grains are 

 always the resting form of the products of 

 assimilation, and we now know that such is often 

 not the case : we now have much deeper glimpses 

 into the initial products of carbon-assimilation 

 than Sachs had in i860, but this enhances rather 

 than detracts from the importance of his splendidly 

 worked-out discovery. Put more generally, we 

 may now say that the process of carbon-dioxide 

 assimilation in green leaves under the influence 

 of light is a process of synthesis photo-synthesis 

 resulting in the building up of a carbohydrate 

 such as sugar, inulin or starch from the elements 

 carbon, hydrogen and oxygen. 



But it must not be supposed that the import- 

 ance of Sachs' discovery, and the rapid consequent 



