194 TRANSPIRATION 



ordinary leaves and it may be shown (Expt. 1 14) that in such 

 cases transpiration is most active from the lower sides. 



Unless their surfaces are specially protected by a dense 

 cuticle, plants with leaves of large area usually transpire and 

 need considerable amounts of water for proper growth : they are 

 frequently met with in damp situations unfavourable to trans- 

 piration and therefore where a large transpiring surface is a 

 necessity in order to get rid of surplus water. 



On the other hand the leaves of plants adapted to live in dry 

 situations are frequently small and narrow, the transpiring surfaces 

 being reduced often to a minimum. 



In diastomatic transpiration from a leaf or stem the opening 

 and closing of the aperture between the guard-cells of the stomata 

 {a, Fig. 74) regulates and controls the amount of water-vapour 

 given off, and it is the turgidity of these guard-cells which 

 determines whether the pore is open or shut. When the cells 

 are highly turgid they curve away from each other and the 

 opening is as wide as possible ; when they become flaccid 

 they straighten and the aperture between them decreases until 

 the free edges of the cells touch and completely close the pore. 



The turgidity of the guard-cells, and therefore the possibility 

 of the escape of water-vapour from the leaf, is influenced both by 

 internal and external circumstances. About the nature of the 

 internal vital conditions little is known, but it may be remarked 

 that, when the loss of water is excessive and is not completely 

 compensated by absorption from the soil, the stomata begin to 

 close before actual ' wilting ' is observable. 



The chief external conditions which influence transpiration 

 are : — 



(i) the intensity of the light to which the plant is exposed, 

 (ii) the water-content of the surrounding atmosphere, 

 (iii) the temperature of the air and soil, 

 (iv) the movement of the air, 

 (v) the water-content of the soil and the concentration and 



