20 PHENOMENA OF VEGETATION. 



adjacent cells of the epidermis, or skin enveloping the leaf, in swelling 

 when moist, tend to close the stomate, and their contraction when dry 

 to open it ; so that the actual state of the stomate at any one time is 

 the resultant of nicely adjusted opposing forces, and thus is secured 

 a self-regulation of the apparatus far more efficient than could be 

 secured in any laboratory of man, though an attendant were stationed, 

 hydrometer in hand, ready to close partially or entirely, or to reverse 

 such action in the ventilators of the evaporating chambers exactly in 

 regard to time and extent as the ever-varying state of the process of 

 evaporation in the chamber and state of the atmosphere might render 

 desirable. 



But further, it is possible that in some cases, under peculiar 

 conditions or circumstances, it might happen that more moisture 

 might be evaporated than was conducive to the vigorous vitality of 

 the plant — in other words, the process of evaporation might go too 

 far — and we find that provision against this exists in many leaves, 

 in the existence of hairs which by endusmosic action can imbibe 

 additional moisture. 



These hairs are always more numerous on the back of the leaf than 

 on the face, which in some plants is entirely devoid of them, though 

 the back be densely covered with them ; and they may be found in 

 some plants to be always at the junction of veins in the back of the 

 leaf, as if designed only to act when it should become manifest that 

 surplus water which might happen to be retained in one of these 

 would not suffice to counteract a deficiency in the other. 



Such is the beautifully-simple self-regulating ventilating-evaporating 

 apparatus in the leaf of a plant. 



In the absorption of moisture by the spongioles of the rootlets the 

 ascent of the sap, and the evaporation of excess of moisture after the 

 elaboration of the milk sap, we see some of the principal phenomena 

 of vegetation on which meteorological effects of forests depend. 



