160 VEGETABLE PHYSIOLOGY 



from the simple materials absorbed, but appears as'a secretion 

 product of the chloroplast. 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. Kegarded as 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 



