V] CARBOHYDRATES 73 



the view that glucose is the first-formed sugar, and bring forward 

 evidence to this effect. There is however no reason why hexoses should 

 not be formed first and then converted into cane-sugar and temporarily 

 stored as such, being again reinverted into hexoses for translocation. 

 Nor is there any reason for supposing that the first formed sugar is always 

 the same in every plant. 



There appears to be very little doubt that maltose is formed in 

 the hydrolysis of starch, and also that starch is a temporary reserve 

 material in the leaves, but whether formed direct from sucrose or fi-om 

 hexoses cannot be stated. 



There is some evidence in favour of the view that glucose is more 

 readily used in respiration than laevulose, for under circumstances when 

 neither can be increased, the glucose tends to disappear. 



From the leaf the various sugars are translocated to other organs of 

 the plant, e.g. root, stem, flower, fruit and seed. In some cases starch 

 is synthesized from the sugars and "stored" in roots, tubers, tuberous 

 stems, fruits and seeds. In other cases the sugars themselves may be 

 "stored," as, for instance, in the root of the Beet (Beta vulgaris), or they 

 may have a biological significance, as in sweet fruits. It must also be 

 borne in mind that sugars are employed throughout the plant in re- 

 spiration and in the synthesis of more complex substances, i.e. cellulose, 

 gums, pentosans, mucilage, aromatic substances, fats and to a certain 

 extent proteins: in fact they or their precursors constitute the basis from 

 which all organic compounds are synthesized. 



The following experiments can be performed with either the Garden 

 Beet or the Mangold Wurzel, both of which are varieties of Beta vulgaris, 

 the Common Beetroot. The sugars in the leaves and petioles of the 

 Mangold have been investigated (Davis, Daish and Sawyer, 17) and 

 sucrose, laevulose and glucose have been found. Starch is absent in 

 the adult plant and also maltose. The opinion is held that sucrose is 

 the first-formed sugar of photosynthesis and that this is hydrolyzed for , 

 translocation on account of the greater rate of diffusion of the smaller 

 molecules of glucose and laevulose. These are again synthesized in the 

 root to form sucrose where the latter is stored, and hexoses are almost 

 absent from this organ. Though the facts concerning the distribution 

 of the sugars stated above are reliable, it is not certain that the deduc- 

 tions are permissible. The leaf contains the enzymes, invertase, maltase 

 and diastase (Robertson, Irvine and Dobson, 28). 



In connexion with the occurrence of various sugars in leaves it is of 

 interest to note that glucose, fructose and mannose can pass over into 



