312 



HOW CEOPS GROW. 



to a minimum, as in a Wardian case or fernery, without 

 evident influence on growtii ; but plants of parentage 

 naturally accustomed to copious exhalation of water 

 flourish best where the conditions are favorable to this 

 process. Exhalation is not injurious, unless the loss 

 be greater than the supply. If water escapes from the 

 leaves faster than it enters the roots, the succulent organs 

 soon wilt, and if this disturbance 

 goes on too far the plant dies. 



Exhalation ordinarily proceeds to 

 a large extent from the surface of 

 the epidermal cells. Although the 

 cavities of these cells are chiefly oc- 

 cupied with air, their thickened walls 

 transmit outward the water which is 

 supplied to the interior of the leaf. 

 Otherwise the escape of vapor occurs 

 through the stomata. These pores 

 appear to have the function of facil- 

 itating exhalation, by their property 

 of opening when exposed to sunlight. 

 Thus evaporation from the leaves is 

 favored at the time when root-action 

 is most vigorous, and the plant is to 

 the greatest degree surcharged with 

 water. 



Access of Air to the Interior 

 of the Plant — Not only does the Fig. 59. 



leaf allow the escape of vapor of water, but it admits of- 

 the entrance and exit of gaseous bodies. 



The particles of atmospheric air have easy access to 

 the interior of all leaves, however dense and close their 

 epidermis may be, however few or small their stomata. 

 All leaves are actively engaged in absorbing or exhaling 

 certain gaseous ingredients of the atmosphere during 

 the whole of their healthy existence. 



