i8 93 . PLANT NUTRITION. 25 



latory process, and at the same time establishes almost beyond doubt 

 that the starch dissolution in the leaf is mainly brought about by 

 the action of the diastase which they have proved is so generally 

 present. 



It is in the determination of the nature and variation in the 

 amount of the different sugars present in the leaves of plants that 

 Messrs. Brown and Morris have laid the foundation of a more com- 

 plete and extended knowledge of the complex chemical changes 

 occurring in plant carbon-assimilation, and their work may be charac- 

 terised as the first which has placed the subject on more than a 

 hypothetical basis. 



When diastase acts upon starch the result is the formation of a 

 sugar — maltose — and dextrins, which latter on further action are also 

 changed into maltose. If, therefore, this ferment is concerned in the 

 dissolution of starch in leaves it ought to be possible to prove the 

 existence of maltose in these organs. This they have satisfactorily 

 accomplished by processes which are too technical to be introduced 

 here, but about which there can be no doubt. The only other sugars 

 found to be present in Tropaeolum leaves were cane-sugar, dextrose, 

 and levulose. Simpler pentoses were looked for but none discovered. 

 Experiments were afterwards directed towards the quantitative deter- 

 mination of these sugars in the leaves at different times of the day 

 and night, in the hope that some light would be thrown on their func- 

 tion, translocation, and genetic relation to each other, and to the 

 carbohydrate starch, and to ascertain something about the sugars 

 which stand between the first products of assimilation and starch 

 and those which are to be looked upon as derived from the latter by 

 chemical transformation. 



A considerable number of leaves of Tropaeolum were picked at 

 5 a.m. and separated into two batches. One set (a) was dried at 

 once and the carbohydrates estimated, and the leaves of the other 

 set (b) were placed with their petioles in water and exposed to sun- 

 light until 5 p.m., and then a similar series of leaves (c) was picked 

 after exposure to sunlight during the same period, but still attached 

 to the plant. The starch and sugars of these three samples were 

 determined with the following results :— 



Starch 

 Sugars — 



Cane-sugar 



Dextrose 



Levulose 



Maltose 



Total sugar percentage on dry leaf 979 1718 9-58 



From (b) and (c) it is seen that the production of starch in cut 

 leaves does not go on so rapidly as in leaves which are allowed to 

 remain attached to the plant. 



