164 LECTURE X. 



substance is sugar. By the researches of Kossmann and of 

 Baranetzky it has been ascertained that leaves and shoots 

 contain a substance, an unorganised ferment, which possesses 

 the property of converting starch into sugar, and Sachs has 

 found that sugar is very commonly present in the parenchyma 

 surrounding the veins, more especially the midrib, of leaves. 

 It might be expected that sugar would be found to be present 

 in the mesophyll-cells themselves so long as they contain any 

 starch ; but this is not the case. De Vries, who has minutely 

 studied the distribution of sugar in various plants, has failed 

 to find it in the mesophyll in the cases which he has investi- 

 gated. But, according to the principle laid down at the out- 

 set, we cannot conclude from this that sugar is not formed : the 

 more probable explanation of the facts is that the sugar is 

 formed and is rapidly conveyed away from the mesophyll to 

 the parenchyma around the veins, where, as we have seen, it 

 can be readily detected. 



I have found that if a leaf be removed from the plant and be kept for 

 some hours in a moist atmosphere, in the light or in the dark, sugar can 

 be readily detected in its mesophyll-cells, although no trace could be 

 found at the beginning of the experiment. The accumulation of sugar in 

 the leaf is evidently due to the isolation of the leaf, the sugar formed in it 

 being no longer conveyed away to other parts of the plant. 



With regard to the nitrogenous plastic substances formed 

 in the leaves, we have seen that these are amides and pro- 

 teids. If the proteids are to be conveyed away from the 

 leaves, they must, like the starch, be converted into sub- 

 stances which are diffusible. They may possibly be con- 

 verted into peptone : as a matter of fact traces of peptone 

 have been detected in leaves and shoots (Kern), and from 

 the researches of Schulze and Barbieri it appears probable 

 that these organs contain a ferment which can convert proteid 

 into peptone. The information on this point is, however, 

 very incomplete. But it is probable in any case that the 

 changes do not stop here. The peptones are substances 

 which, according to the most recent investigations, do not 

 very readily diffuse through membranes. If, then, it is in 

 the form of peptone that nitrogenous organic substance 



