186 PHOTOSYNTHESIS 



ceedingly arduous. The problem is, in fact, very similar to that which 

 Osborne and his collaborators have pursued with such success on the 

 proteins in plants. As yet we have reliable evidence regarding only 

 those carbohydrates which are present in relatively large quantities. Those 

 which may be of special importance in connection with the mechanism 

 of photosynthesis and to the physiology of the leaf in general may be 

 present in only very low concentration. It does not follow that just 

 because a substance is present in large quantities it plays a proportion- 

 ately important role in the chemical changes taking place in the cell. In 

 fact it is probably the more stable and less reactive substance which 

 accumulates and is thus detectable ; the highly reactive molecules prob- 

 ably determine the course of the reactions, though they are naturally 

 much more difficult to obtain. Few attempts have been made to obtain 

 evidence of the existence of such transitory substances, with the possible 

 exception of formaldehyde. 



A larger number of carbohydrates have been obtained in pure form 

 from plants. In most cases, however, these have been obtained not from 

 organs in which photosynthesis was taking place, such as leaves, but 

 in more specialized organs in which the carbohydrates were deposited 

 in large quantities. We are here more particularly interested in the 

 immediate products of photosynthesis, while the carbohydrates in storage 

 organs such as fruits and tubers may well represent conversion products 

 of the substances formed in photosynthesis. As a matter of fact only 

 few of the carbohydrates have been isolated in pure form from leaves 

 and the evidence of their existence is based upon indirect proof, such 

 as reducing power and optical rotation. The experimental difficulties 

 of determining the first product of photosynthesis and, in fact, of any 

 of the components in leaves have already been alluded to. These diffi- 

 culties are, first of all, of an analytical nature, that is, of devising methods 

 by means of which the great mixture of substances can be quantitatively 

 determined ; and secondly, of ascertaining the nature of the transforma- 

 tions which the carbohydrates in the leaf undergo quite independently 

 of photosynthesis. The green leaf is not only an organ for the manu- 

 facture of carbohydrates which are used and transformed in other por- 

 tions of the plant. It is a living organism, in it the carbohydrates 

 undergo a great variety of changes yielding scores of products. To 

 separate these various chemical reactions and endeavor to follow any one 

 of them is, needless to say, a most difficult task. 



For the purpose of orientation we shall discuss briefly the carbo- 

 hydrates which are of special importance in relation to the photosyn- 

 thetic process and the carbohydrate transformations in the leaf. For a 

 more exhaustive discussion of the chemistry of the carbohydrates refer- 

 ence must be made to special works on this subject.^ When regarded 



'Lippmann, E. O. von, "Die Chemie der Zuckerarten," 1904. Armstrong, E. E., 

 "The Simple Carbohydrates and the Glucosides," 1924. Maquenne, L., "Les Sucres 

 et Principaux Derives," 1900. Czapek, F., "Biochemie der Pflanzen," Vol. I, 1913. 



