460 VENTILATING SYSTEM 



advantage of this high degree of sensitiveness of the stomata towards 

 any decrease in the relative humidity of the atmosphere is sufficiently 

 obvious. Doubtless the strong influence which dry air exerts upon 

 the guard-cells, accounts for the fact that many stomata (e.g. those of 

 Berberis vulgaris, Syringa vulgaris, Ribes aureum, Saponaria officinalis, 

 etc., according to Leitgeb) remain closed in a high wind even after 

 prolonged insolation. 



There is a natural inclination to regard the fall in the turgor of 

 the guard-cells which leads to the closure of the stomata in dry air, as 

 a direct result of an increase in the transpiratory activity of these cells, 

 especially as they occupy a relatively exposed situation, their outer, 

 inner and ventral walls all bordering either upon intercellular 

 spaces or upon the outer air. But active transpiration on the part of 

 the guard-cells tends to increase the concentration and hence also 

 the osmotic strength of their cell-sap ; these cells ought, therefore, to 

 be able to withdraw water from the less actively transpiring ordinary 

 cells of the epidermis, and thus to maintain their turgor-pressure at a 

 more or less constant value. As a matter of fact, the relation between 

 transpiratory activity and the condition of the stomata is not of a 

 simple or direct nature at all ; the action of variations of humidity 

 upon the turgidity of these guard-cells must rather be regarded like 

 the effect of light as a process of stimulation. 



We may conclude the present section with a brief reference to the 

 chloroplasts and starch-grains which so frequently occur in the guard- 

 cells of stomata. The turgor- variations that take place in these cells 

 in response to external stimuli, are in their turn dependent upon 

 alterations in the quantity and quality of the osmotically effective 

 contents of the cells. A very plausible view is that which regards the 

 chlorophyll-apparatus of the guard-cells as the source of these osmoti- 

 cally active materials ; according to this theory, the starch-grains 

 represent a store of solid material which can easily be transformed into 

 soluble and osmotically effective substances (most probably sugars), 

 when such are required. This suggestion finds some support in 

 Schellenberg's discovery of the fact, that plants which have been kept 

 for some days in an atmosphere devoid of carbon- dioxide, and have 

 thus been prevented from forming starch during that period, always 

 keep their stomata tightly closed, even when they are exposed to bright 

 light in a moist atmosphere. 



4. The structure of stomata in relation to climatic find edaphic conditions. 



In the preceding section, the anatomical structure of stomata has 

 been considered solely with reference to the mechanism of opening and 



