26 NATURAL SCIENCE. July, 



Sachs first noticed that the dry weight of a plucked leaf increases 

 considerably when isolated and its petiole placed in water, and this 

 increase is seen to be due mainly to the accumulation of sugars, not 

 starch. The total sugars in (b) have been nearly doubled, and this is 

 mainly due to the increase in cane-sugar, a result which militates 

 against the generally received opinion that a glucose is the principal 

 " up-grade " sugar first formed by the leaf. 



The changes which the sugars undergo when assimilation is 

 prevented are well brought out by the following experiment. A 

 considerable number of leaves were plucked in the afternoon of a 

 fairly sunny day, and dried immediately (a). Another similar set (b) 

 were taken and placed in the dark for twenty-four hours, their stems 

 being dipped in water. The result of the analysis gave : — 



Starch 

 Sugars — 



Cane sugar . . 



Dextrose 



Levulose 



Maltose 



Total sugar percentage in dry leaf i3'64 9 39 



Total loss of sugars and starch in (b) = 4-96 per cent. 



The loss or destruction of starch and cane-sugar is due to respi- 

 ration. The disappearance of the cane-sugar has been accompanied 

 by increase in levulose. Seeing that the conditions of the experiment 

 preclude the possibility of formation of levulose by assimilation, 

 this fact points to the conclusion that the cane-sugar is first 

 inverted, and the resulting dextrose and levulose are used up at uneven 

 rates by the respiratory processes, the former, together with a little 

 maltose, contributing chiefly for this purpose. The experiment, at 

 the same time, renders it almost certain that the increase of levulose 

 in (b), previously described, is derived from cane-sugar inversion, and 

 not from direct assimilation. 



Looked at from all points of view, Messrs. Brown and Morris's 

 results undoubtedly point to the conclusion, so far as Tropaeolum is 

 concerned, that cane-sugar is the first sugar to be synthesised as a 

 result of the carbon-assimilation by the leaf, and that only after a 

 certain concentration of this substance in the cell-sap is starch 

 formed as a more stable reserve material. In the leaf, cane-sugar 

 suffers inversion, and ultimately passes from cell to cell and out into 

 the stem as dextrose and levulose, the starch being at the same time 

 translocated as maltose. 



The whole work is full of suggestion, and is a model of con- 

 summate skill and accuracy. Plant physiologists have reason to 

 be grateful for the improved methods of research devised by these 

 two workers, and the foundation which has been laid for further 

 investigation, as well as for the results obtained. Taken in conjunc- 

 tion with the extraordinary haziness manifested in the chemistry of 



