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 lightand 
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 
