1910.] 



Vegetable Assimilation and Resjnration. 



429 



Thus the fully turgid leaves appear to have had their stomata open to 

 approximately the same extent, Leaf 21 showing no difference from the 

 others ; whereas Leaf 22 had its stomata markedly less open.* The 

 anomalous result with Leaf 21 may be due to an exceptional experimental 

 error, or an unusually high degree of asymmetry; or it may be that the 

 pressure of the excessively turgid epidermal cells upon the guard cells 

 partially closed the stomata, and that increased evaporation under the head 

 of water, which had freer play in this leaf than in the others, neutralised the 

 effect of this closure upon the hygroscope. 



N'ote on the Use of the Horn Hygroscope. — As the observations in connection with this 

 and other experimentst had often to be made out of doors, it was necessary to protect 

 the hygroscope from air currents, which greatly disturb its readings. 



The protective mount was made of two parallel slips of glass, such as are used for 

 microscopic preparations, attached to one another by lateral strips of card. The hygro- 

 scope just fitted into the space between the two glass slips and was affixed to them with 

 shellac. This arrangement is shown in fig. 2. Readings were 

 taken after about 10 seconds. After that interval a fall was 

 observed, due presumably to the diffusion of water vapour into 

 the partially enclosed air above the leaf, and the consequent 

 decrease in the gradient of humidity. J 



The conditions under which the hygroscope is used are variable 

 in the open air, but it is probable that the readings obtained at 

 the same time on different leaves are a true relative indication 

 of the diS'ering conditions of their stomata. Thus, in leaves 

 which are losing their turgidity, the closing of the stomata is 

 unmistakably revealed. 



On the other hand, changes in the readings obtained with the 

 same leaf from hour to hour are sometimes of doubtful signifi- 

 cance, for the increasing internal temperature of the leaf, by 

 directly increasing the rate of evaporation, must affect the 

 hygroscope apart from any change in the stomata themselves. 

 An effect of this nature is observed when readings are taken on 

 a piece of wet blotting paper surmounted by fine wire gauze. If 



a series of observations be made first in the shade and then in the sun, the reaaings are 

 found to increase as the temperature of the evaporating surface rises. The reverse effect 

 is observed when the sunshine is intercepted. 



In the sun, the occurrence of variations of leaf-temperature will make the hygroscope 

 a less delicate indicator of the conditions of the stomata even in comparing different 



* The detailed observations show that in the case of this leaf the stomata, more 

 especially those on the lower surface, began to close just after the leaf had begun to flag, 

 at 11 A.M. 



t I am indebted to Miss M. G. Sjkes, Fellow of Newnham College, Cambridge, for 

 many of the hygroscope observations taken in connection with the experiments. Without 

 her kind assistance the series of observations would necessarily have been far from 

 complete. 



+ The readings on the scale of the instrument were roughly degrees of arc with the 

 point of attachment of the horn as centre. 



