232 BOTANY PART i 



differences in the area of the evaporating surfaces and to structural peculiarities 

 (the number and size of the stomata, presence of a cuticle, cork, or hairy covering, 

 etc.). But even in the same shoot transpiration is not always, uniform. This is 



attributable to the fact that, both from internal 

 and external causes, not only the size of the 

 openings of the stomata varies, but also that 

 transpiration, just as evaporation from a surface 

 of water, is dependent upon external con- 

 ditions. Heat, as well as the dryness and 

 motion of the air, increases transpiration for 

 purely physical reasons; while light, for 

 physiological reasons, also promotes it. From 

 both physical and physiological causes, tran- 

 spiration is more vigorous during the day than 

 night. Plants like Impatiens parviflora, which 

 droop on warm days, become fresh again at the 

 first approach of night. Information as to the 

 condition of opening ( 15 ) of the stomata can be 

 obtained by the use of cobalt paper (cf. p. 230) 

 or by the method of infiltration. If the stomata 

 are open, fluids such as petroleum, alcohol, etc., 

 easily penetrate and inject the whole system 

 of intercellular spaces ; the leaf thus becomes 

 translucent. If a strip of black paper is laid 

 across a leaf the underlying stomata close. On 

 treatment with alcohol the appearance repre- 

 FIG. 241 .-A leaf of Lilac darkened in the sented in Fig. 241 is then obtained. The open 

 middle while the ends were exposed to condition of the stomata may also be demon- 

 light. Only the illuminated stomata . 

 remain open and allow the absolute strated by the method of gaseous diffusion. 

 alcohol to enter. (After MOLISCH.) If a red leaf containing anthocyan with its 



stomata open is placed in air containing 



ammonia, a blue colour develops in a few seconds ; this does not take place if 

 the stomata are closed. 



Plants of dry habitats which require to economise the absorbed 

 water show numerous arrangements which protect them against 

 excessive transpiration (cf. p. 168). In plants living in very damp 

 situations, on the other hand, arrangements to further transpiration 

 are found. When the leaf is able, either by absorption of heat from 

 without or by the production of heat within itself (p. 276), to raise 

 its temperature above that of its surroundings, transpiration is still 

 possible even in an atmosphere saturated with aqueous vapour. In 

 the process of exudation the plant has a further means of giving off 

 water even after transpiration has completely stopped. 



Exudation ( 16 ) 



The discharge of water in a liquid state by direct exudation is 

 not of so frequent occurrence as its loss by transpiration, but is found 



