244 GLUCOSIDES 



diversity of opinion. They have been described, on insufficient 

 grounds, as direct products of photosynthesis. Many consider 

 them to be of value as food-stuffs on account of the sugar they 

 contain ; the occurrence of certain glucosides in seeds lends 

 some support to this view, for in the case of the bitter almond 

 hydrocyanic acid, in the free state, may be identified when 

 germination starts, also the observations of Treub,* who found 

 that in the case of some plants containing cyanogenetic glu- 

 cosides the amount of the latter decreased if the plant was 

 placed in the dark, in order that photosynthesis could not 

 take place. On the other hand there was an increase in 

 quantity when the plants were exposed to light, and this 

 increase reached a maximum at about midday. 



Weevers f considers that salicin, populin, arbutin and simi- 

 lar glucosides are of the nature of reserve food-materials, for 

 not only do these substances form a suitable means for the 

 storage of sugar on account of their low diffusibility, but 

 the facts of their seasonal or diurnal variation lend support to 

 this opinion. Thus in Vaccinium Vitis-Idcea the arbutin is 

 stored in the leaves, and when the new leaves are formed in 

 the spring it is used up ; it is split by a suitable enzyme, the 

 sugar being used up, and the hydroquinone remains behind and 

 combines with more sugar, so that by the autumn the leaves 

 once more contain arbutin. 



In the case of the willow, salicin is formed day by day, 

 but during the night it is spHt by salicase into sugar and the 

 alcohol saligenin. The glucose is translocated, and the sali- 

 genin remains behind and is converted into salicin by combin- 

 ing with sugar the next day. This process stops in the 

 autumn, by which time there is relatively much sahcin in the 

 cortex of the stem. 



This translocation of glucosides from the leaves of many 

 plants — but not of all, Samhucus and Indigofera being excep- 

 tions — is significant, and so also are the facts relating to the 



* Treub : " Ann. Jard. Bot. Buitenzorg," 1896, 13, i ; 1907, 21, 79, 

 107 ; 1910, 23, 85. 



+ Weevers : " Kon. Akad. Wet. Amsterdam," 1902 ; " Rec. Trav. 

 Bot. Neerl," 1910, 7, i. 



