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Leaf Water-content , and Transpiration Rate . 
Stomatal changes appear to be chiefly influenced by conditions of 
illumination rather than by small changes in the leaf water-content. Since 
the water-content may vary within fairly wide limits without causing any 
stomatal change, the stomata cannot be said to function in maintaining the 
requisite quantity of water in the leaf by response to changes of water- 
content, although there is no doubt that stomatal changes can influence the 
rate of transpiration. 
The present work has shown the importance of leaf water-content in 
influencing transpiration, thus confirming the work of Livingston and Brown 
(18) in demonstrating that a small decrease in leaf water, producing incipient 
drying, can reduce the transpiration rate. 
The relative importance of incipient drying and stomatal size as the 
factor controlling transpiration probably depends upon other conditions. 
From the results obtained in these experiments it appears that in a bright 
light the stomata are wide open, and that therefore changes in intrinsic 
transpiring power of the plant are brought about chiefly by changes of leaf 
water-content. On the other hand, in a dull light it is probable that the 
lower transpiration rate is the result of the failure of the stomata to open 
widely, and incipient drying will not occur so long as the supply of water 
through the stem is maintained. 
The direct effect of light on the transpiration of a plant, i.e. the effect 
of light on the rate of water loss from the evaporating tissues, has not yet 
been investigated in the present work. Darwin (10) by a new method 
experimented on this factor and concluded that the direct effect of light 
might be considerable, the transpiration rate, irrespective of the influence of 
stomatal change, being as much as 36 per cent, greater in light than in dark- 
ness, but the nature of this direct effect is as yet obscure. 
Summary. 
Experiments have been carried out with various plants under con- 
trolled atmospheric conditions to determine the relationship between stomatal 
aperture and the rate of transpiration. 
1. The results have shown that in many cases there is no agreement 
between the two, a decreasing transpiration rate being often accompanied 
by stomatal opening, and vice versa. 
2. The water-content of the leaf was found to be a factor playing 
a large part in the control of transpiration, a high water-content tending 
to produce a high transpiration rate, and a lower transpiration rate resulting 
from decreased water-content. 
3. The stomatal aperture is not reduced by slight water deficiency in 
the leaf ; hence the ordinary view that stomata, by their response to incipient 
drying, are the chief factors in maintaining the water-content of the leaf is 
not tenable. 
