WATER 15 



is caused by the increase in the concentration of the cell sap : 

 a sufficiency of water favours the hydrolysis of starch to sugar, 

 which is, presumably, the active osmotic substance of the 

 guard cells ; on the other hand, a deficiency of water, up to a 

 certain point, favours the formation of starch, but if this point 

 be passed, there is an excessive concentration of the cell sap 

 which inactivates the enzyme responsible for the change, 

 which inactivation is not reversible, thus a recovery to the 

 normal conditions is prevented and the guard cells no longer 

 function. It was further observed that in starch-free guard 

 cells, severe desiccation resulted in the disappearance of the 

 sugar, presumably owing to the increased rate of respiration 

 which obtains in these circumstances. Dastur * also finds 

 that the decline in the photosynthetic activity of leaves is 

 due to the shortage of the water supply, the inadequacy of 

 which eventually terminates this particular activity of the 

 leaf. The cessation of photosynthesis is, however, not abrupt ; 

 the marginal and intervascular regions are the first to feel the 

 drought and from these regions the ending of photosynthesis 

 spreads towards the centre and the main veins of the leaf. 

 This shortage of water is due to two causes : the increase of 

 the leaf area creates an increased demand for water owing to 

 the increase in photosynthesis and transpiration, but the 

 specific conductivity of the wood f remains unchanged after 

 the tracheae have attained their permanent form, hence the 

 increased demand is not satisfied ; secondly, as more leaves 

 unfold, the pull exerted by the living cells of the upper leaves 

 on the water of the conducting columns increases whilst the 

 pull exerted by the older leaves remain the same, hence there 

 is a deficient supply to the older leaves. 



There is, in fact, a distinct correlation between the decrease 

 in the rate of assimilation and the fall in water content per 

 unit of leaf area, and if the two values for a set of leaves are 

 plotted, the points lie very nearly on a straight line. This 

 is shown by the following observations on Abutilon Darwini 

 made in carefully controlled experiments : — 



* Dastur : " Ann. Bot.," 1924, 38, 779 ; 1925. 39. 7 6 9- 

 f See Farmer : " Proc. Roy. Soc," B, 1919, 90, 218, 232. 



