430 MANUAL OF BOTANY 



function are the hygrometric condition of the air, the tempera- 

 ture of the soil in which the roots of the plant are embedded, 

 and the mechanical disturbances to which the plant is subjected. 

 If the air be dry, transpiration is vigorous ; if moist, then it is 

 checked. If a plant be exposed to a warm, dry atmosphere till 

 the leaves droop from too great a loss of water and then be trans- 

 ferred to one saturated with moisture, after a short time they 

 again become turgid. This is not due to an absorption of water 

 in the form of vapour by the leaves, but to a diminished loss by 

 the checking of transpiration. The return of turgidity is caused 

 by the accumulation of the store drawn from the earth by the 

 roots. Warming the soil increases the amount of vapour given 

 off by the leaves. The mechanical disturbance of shaking has 

 already been alluded to, and it has been seen that when thus 

 stimulated, the protoplasm allows transpiration to increase. 

 This helps to explain the flaccid condition observable in some 

 trees after the prevalence of a high wind. The continued 

 renewal of the air around the transpiring organs may also have 

 the effect of increasing the removal of vapour. 



When transpiration is excessive, the leaves and branches 

 lose their turgescence, become flaccid and droop. A branch 

 which has reached this condition may be revived by forcing 

 water into it, which can be done by fastening it in one arm of a 

 U-tube containing water and pouring mercury into the other. 

 The restoration of the water restores the turgidity of the tissues. 



Transpiration is of the greatest importance to the plant in 

 many ways. By maintaining a copious flow of liquid to the cells 

 of the leaves, it brings to the metabolic cells of their mesophyll a 

 supply of the materials needed by the plant for the construction 

 of its nutritive substances, and it assists in the transportation of 

 the stored material laid up in various parts of the stem and root to 

 the growing parts and other seats of consumption. Its influence 

 in maintaining the turgidity of the tissues is also very consider- 

 able, and this turgescence is of the highest importance to the 

 growing cells, growth itself and many adaptations to changes in 

 the plant's environment being dependent upon it. The negative 

 gaseous pressure in the vessels, already alluded to, is dependent 

 upon transpiration. The importance of the presence of the 

 water to the life of the protoplasm has already been shown. 



