156 VEGETABLE PHYSIOLOGY 



mand, the construction of sugar and not starch being the 

 completion of the photosynthetic process of the chlorophyll 

 apparatus. Though starch is a very general accompaniment 

 to this process, it never appears till a certain amount of sugar 

 has been formed, and in many plants, particularly the 

 onion and certain other Monocotyledons, it is not produced 

 at all, however active photosynthesis may be. To this 

 point we shall return in a subsequent chapter. 



The most recent hypothesis of carbohydrate formation 

 was put forward in 1906 by Usher and Priestley. They 

 claim to have found that the interaction of carbon dioxide 

 and water leads to a coincident formation of formaldehyde 

 and hydrogen peroxide. The latter is stated to be at once 

 decomposed by an enzyme into water and oxygen. The 

 first decompositions are held to be effected by the light and 

 the colouring matter, the body of the plastid taking part 

 only in the subsequent constructive processes. 



Though the production of starch is apparently not the 

 ultimate aim of the photosynthetic processes, its ready 

 occurrence affords us an easy method of demonstrating the 

 activity of the chlorophyll apparatus. If a leaf is partially 

 covered by a piece of opaque material, and is then exposed 

 to the light, starch rapidly appears in the illuminated 

 portion, as can be shown by bleaching the leaf with boiling 

 alcohol, and then immersing it in iodine, which forms a 

 blue colour with starch. The blue tint only appears where 

 the light has reached the chlorophyll apparatus. 



These processes are carried out by the chlorophyll 

 apparatus under the conditions set forth. It is evident 

 that such changes as have been described cannot be accom- 

 plished without the expenditure of a considerable amount 

 of energy. In this need we have the explanation of the 

 composite nature of the chloroplast. The chlorophyll 

 absorbs certain rays of light which fall upon it, and the 

 energy which is liberated by the extinction of their vibrations 

 is taken up by the protoplasm of the plastid and applied 

 by it to effect the decompositions that take place. If the 



