INTRODUCTORY DISCUSSION. 23 



readily dissociated and drawn into the stream of catabolism. It is only 

 under conditions of stress that the plant uses the disaccharides or poly- 

 saccharides with the result of the formation of pentosans and pentoses. 

 These actions will be discussed under the section on pentose sugars. With 

 ample water-supply and oxygen the hexoses are burned in such a manner 

 that there is not an accumulation of end-products which are deleterious to 

 the plant. Another feature of such complete burning is the influence of the 

 products on the water-absorbing capacity of the colloidal substratum of the 

 cell, and hence on the growth of the organism. The influence of the various 

 intermediate products of metabolism on imbibition and the manner in which 

 this is correlated with growth is being extensively studied at the Desert 

 Laboratory. 



Early in the course of the investigations on the metabolism of succulent 

 plants it was recognized that the organic acids found therein are the accumu- 

 lating intermediate or end-products of a modified form of catabolism due to 

 morphological peculiarities of the plant The chemical principle which 

 underlies the acidification is the formation of acids by the oxidation of 

 sugars ; the manner in which this occurs and the products formed have 

 already been indicated. One of the principal agents of deacidification in 

 plants is sunlight ; organic acids, such as oxalic, malic, tartaric, etc., break 

 down very readily in the light in that the carboxyl group, COOH, splits off 

 COj. 1 It is a noteworthy fact, discussed by Richards, that cacti rich in acid 

 exhale carbon dioxid abundantly in the sunlight, even at moderately low 

 temperatures. This fact also finds application in the formation of pentose 

 sugars, as will be discussed later. 



The total carbohydrate content or of food-supply in general is of little 

 significance or value in studying the various functions of a plant It is 

 rather the nature of the sugars, or the degree of general chemical inversion, 

 that determines the supply of material available for respiration or growth. 

 The relative amounts and proportion of the carbohydrates in a leaf, for 

 example, are dependent upon several factors, primarily water-content and 

 temperature. Also it is essential to realize that the transformation of the 

 various groups of carbohydrates represents thus an equilibrium system 

 subject to change by several factors. It is evident, therefore, that only 

 after determining the influence of these factors and taking them into con- 

 sideration can any safe conclusion be drawn as to the significance of the 

 presence of any one of the various groups of sugars. This fact finds 

 immediate application in the problem of photosynthesis. There has been a 

 great deal of dispute over the first sugar formed in the process of photo- 

 synthesis. Various workers have arrived at conclusions which are still very 

 contradictory. Unfortunately, in most of the work the factors already 

 referred to were not determined, so that it is impossible to explain the 

 variable results on this basis. 



1 RICHARDS, H. M. L. c., 83. 



SPOEHB, H. A. Photochemische Vorgaenge bei der Diurnalen entsaeurung der 

 Succulenten, Biochem. Zeitschr., 57, 95-111, 1913. 



