PLANT PHYSIOLOGY AS RELATED TO PRUNING 15 



27 trees to the acre would transpire 540 tons in a season ! 

 Such facts, estimations and deductions as these indicate 

 the importance of water conservation in trees and shrubs 

 by pruning and in the soil by cultural practices. 



17. The transpiration apparatus. Water escapes from 

 plant tissue into the air mainly through minute openings 

 (stomata) in the epidermis of 



leaves and growing shoots. 

 Lenticels in the bark of older 

 shoots and branches perform a 

 similar function. Some water 

 may also escape more or less di- 

 rectly through the epidermis of 

 delicate and very young leaves 



and shoots before the cutiniza- FjQ 12 _ EPIDERMAI CELLS 

 tion of this epidermis. AND STOMATA FROM A LEAF 



18. Stomata (singular, stoma). "Openings in the epi- 

 dermis of plants for the admission of air and the libera- 

 tion of water; breathing pore" (Crozier). 



19. How stomata operate. Stomata are doors through 

 the epidermis to the chambers and contorted corridors 

 which constitute the aerating, intercellular spaces of leaf 

 and other green tissue. They open and close according 

 to the intricate action of growth factors, which operate 

 mostly in the tissues, though external factors light, tem- 

 perature, motion and humidity of the air, water content 

 of the soil, etc. may also operate to a greater or lesser ex- 

 tent. Since the protoplasm in every cell of the transpir- 

 ing tissue is rich in water, and since every cell borders an 

 intercellular space, the air in the intercellular parts be- 

 comes heavy with water vapor. When the stomata are 

 open and the external air conditions favor it, the outward 

 passage of this water-saturated air is rapid. Plants in 

 nature check transpiration in many ways, such as de- 

 velopment of hairs or wax on their cuticles, by thickened 

 cuticles, placing their stomata in cavities, reducing the 



