59 



transpiration.* When the air is very damp, and near to its 

 point of saturation, or when the light is dull, or it is night, the 

 stomata are closed, and the guard cells remain side by side. 

 Some other explanation, then, of the function of these stomata, 

 than the old one that they are regulators of the amount of 

 transpiration, must be sought. Have the stomata, then, much 

 importance as regulators of the absorption of gases, i. e., do they 

 materially assist the leaf in taking up Carbon Dioxide from the 

 air ? for the conditions under which the stomata are most 

 widely opened, viz., dry air, high temperature, and intensely 

 bright sunlight, are also those under which the greatest supply 

 of Carbon Dioxide is taken up by the cells of the leaf. It was 

 thought until recently that they were of great importance, and 

 it was believed that the wide open condition of the stomata had 

 a distinct relation to the ready passage of Carbon Dioxide from 

 the exterior air into the intercellular passages present on the 

 lower surface of the leaf, whence the parenchyma cells of the 

 mesophyll absorbed it with great rvctivity ; since the cells 

 bounding these passages, which had absorbed as much of the gas 

 as they could, when they were robbed of their store continually by 



* 'I he opinion stated in a previous part of this essay — viz., that the independent 

 action of the guard cells, under conditions favouring transpiration, was to close the 

 stomata, but that they were restrained from doing so through the greater force produced 

 by the flaccid condition of the other epidermal cells exetting a stronger influence upon 

 the guard cells than their own flaccid condition does, and that, consequently, the open 

 state of the stomata, under the conditions of bright sunlight and high temperature, 

 was due to the total or resultant action of two opposite and non-concurrent forces — 

 was held until very recently. But it has lately been shown that the action of bright 

 sunlight, which increases the amount of transpiration, also causes separation, not 

 closure — turgidity, not flaccidity, of the guard cells themselves ; and hence opening of 

 the stomata. Von Mohl attributes this to the mechanical stimulus of light inducing 

 the formation in the cell-sap of the guard cells of osmotic substances capable of attract- 

 ing great amounts of water j other observers think that the action of light is to increase 

 the resistance of the living protoplasm in the cell to the loss of water from it. At 

 present the balance of evidence is about equal on both sides j perhaps it slightly 

 inclines more to favour the last. Whatever be the view taken, however, the fact is 

 plain that the action of the guard cells under sunlight is concurrent with that of the 

 ordinary epidermal cells, though the two actions are due to exactly opposite conditions 

 — the former to turgidity, the latter to a flaccid condition — and it is owing to their com- 

 bined (not mean) action tint the stomata remain widely open. 



