10 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



cultivation of barley embryos. This pointed to sucrose as 

 an antecedent of the formation of starch, and the experi- 

 ments on Tropaeolum strengthen the theory. Thus, Brown 

 and Morris regard sucrose as the first sugar to be formed 

 in the green leaf as a product of assimilation. Its removal 

 is effected either by its conversion into starch as a more 

 stable reserve substance, or by its inversion to give the 

 easily respirable hexoses. 



It may here be pointed out that this transformation of 

 sucrose into starch sets a major limit to the osmotic pressure 

 obtainable in the leaf cells ; for whereas sucrose is a crystal- 

 loid, starch is a colloid, and exerts a negligible osmotic 

 pressure. Measurements of the changes in pressure caused 

 by the insolation of plucked leaves were made by Dixon 

 and Atkins (1910) by determining the freezing-point of the 

 expressed sap by the thermo-electric method. The follow- 

 ing table is typical of the results obtained. Under A are 

 recorded the depressions of freezing-point in degrees, under 

 P the osmotic pressures calculated from A, while under M 

 are tabulated the mean molecular weights of the sap 

 solutes. 



TABLE IV. 

 Syringa vulgaris : LEAVES. 



