160 VEGETABLE PHYSIOLOGY 
from the simple materials absorbed, but appears as a secretion 
product of thechloroplast. He suggested that a body possibly 
allied to formaldehyde is first formed according to Baeyer’s 
theory, and that this is used in the construction of protein, 
by combining with the nitrogen and sulphur absorbed in the 
form of salts from the soil, or with nitrogenous substances 
derived from previous decompositions of protein. This 
protein then is assimilated by the protoplasm of the chloro- 
plast, and from the latter the carbohydrate (starch) is 
secreted. 
This view, while no doubt, in the main, accurate as far 
as the mode of formation of starch is concerned, cannot be 
regarded as explaining the formation of carbohydrates from 
the simple compounds absorbed. The leucoplast of the 
tuber, as well as the chloroplast itself under certain con- 
ditions, can form starch grains when supplied with sugar 
in the absence of carbon dioxide, and in all probability the 
appearance of the starch is the result of the presence of an 
excess of sugar in the leaf-cells. Regarded ag an explanation 
of the photosynthesis of carbohydrates, it, like the others, 
must remain hypothetical. Moreover it is based upon the 
assumption that starch is the highest term reached in the 
plant in the series of carbohydrate bodies. This assump- 
tion, however, is not supported by the evidence at our com- 
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. 
Another hypothesis of carbohydrate formation was put 
forward in 1906 by Usher and Priestley. They claimed 
to have found that the interaction of carbon dioxide and 
water leads to a coincident formation of formaldehyde 
and hydrogen peroxide. The latter was stated to be at once 
