572 SCIENCE PROGRESS 



quantity. In most analytical work they would be returned as 

 hexoses. 



Factors influencing Osmotic Pressures in Plants 



The effect of light upon leaves and other chlorophyll-contain- 

 ing plant organs is by far the most powerful of the many causes 

 which alter the pressure. In light, sugars accumulate very 

 rapidly, accordingly rapid increases take place in the pressure. 

 Sucrose is the sugar chiefly responsible for this. Its formation 

 causes about half the increase in pressure, weight for weight, 

 that a hexose would, since its molecular weight is almost twice 

 as great. The same remark applies to maltose, which has the 

 same molecular weight as sucrose. But starch also appears 

 in leaves exposed to light, and this substance exerts no pressure. 

 Thus far larger supplies of carbohydrates can be stored in the 

 leaf without unduly high pressures than could be if sugars only 

 were formed, as in the leaf-blade of the iris. There is, however, 

 another limit to the accumulation of sugar in a leaf, with its 

 consequent rise in pressure . This is the translocation of the sugar 

 downwards through the bast to be stored as starch in the white 

 leaf bases and rhizomes of the iris, for example, as sucrose in the 

 beetroot, as inulin in the dahlia tuber. In trees storage takes 

 place in the medullary rays and wood parenchyma of the woody 

 tissues. The mechanism of this translocation may perhaps be 

 that of simple diffusion from a region of high sugar content, the 

 photosynthetic cells to one of low content, the storage cells, in 

 which as it were an upper limit is set to the pressure by the 

 formation of some colloidal substance. In many woody tissues 

 hemi-cellulose layers are deposited on the walls as reserve pro- 

 ducts. The normal permeability of the protoplasm appears to 

 be increased in such cases to allow of the passage of the sugars. 

 By these means a pressure gradient from leaf to root is main- 

 tained. 



Within the leaf itself many other factors may cause a rise 

 in pressure. For example, in conditions favouring rapid 

 evaporation a certain amount of contraction in volume may 

 take place if the water supply is insufficient ; this will lead to 

 loss of turgidity, as the cellulose walls contract, and if it proceeds 

 far enough will result in the wilting of the leaf. 



When photosynthesis is in abeyance the losses in sugars 

 due to respiration and continued translocation are made good 



