6o 



the more internal cells in their immediate proximity, could absorb 

 further supplies rapidly from the store at hand in the intercellular 

 passages. Boussingault has very recently shown, however, from 

 comparative experiments made upon a large number of leaves, 

 between the amount of absorption of the upper surface of the leaf, 

 which has but few stomata, and the lower surface where stomata 

 are present in large numbers, that the upper surface absorbs more 

 actively, and at the same time takes up a much larger quantity of 

 Carbon Dioxide gas, than does the lower surface. Hence the 

 stomata would appear to have no part of importance to play in the 

 absorption of gases by plants, which must take place, then, directly 

 through the cuticularized cell-walls of the epidermal cells. These 

 latter are permeable by dry gases, and so there is no obstacle 

 offered by them to the absorption of gases by the cells of the 

 epidermis ; it is only to the passage of watery vapour that the 

 cuticle offers any resistance. In so far as the stomata are con- 

 cerned in any way with the interchange of gases by the plant, they 

 seem to be channels of exit for these gases, for the stomata open 

 very readily when slight pressure is brought to bear on them from 

 within, while they do not open so readily to pressure applied from 

 without. Pressure from within may be brought about by the gases 

 in the intercellular spaces being expanded, if the temperature in- 

 creases and becomes high ; under the influence of this pressure the 

 stomata readily open, and permit the gases to escape into the 

 surrounding air. 



Since then the only function with which the stomata are con- 

 cerned, apart from this comparatively trivial one of favouring the exit 

 of gases, is that of transpiration, let us see what the real influence of 

 their action is upon it. We have seen that the opening and closing 

 of the stomata are due to variations in the condition of turgidity 

 of the guard-cells, and also that the action of bright sunlight and 

 warmth, which increase the amount of transpiration, cause also 

 separation of the guard-cells, and hence opening of the stomata. 

 When transpiration is most vigorous, the plant is likely to lose the 

 greatest amount of water, and hence the object and use of the 

 stomata is to facilitate and promote the escape of water in the 

 form of vapour from the cell-walls of the leaf -cells of plants. 



