428 MANUAL OF BOTANY 



may be seen if a vigorous plant be covered over by a bell -jar, 

 the water condensing copiously upon the sides of the latter. 



The evaporation takes place to a certain extent through all 

 the epidermal cells of the transpiring organ, but not to a very 

 great one, the degree of development of the cuticle having 

 considerable influence upon the amount. It is carried out much 

 more freely through the thin walls of the cells abutting upon the 

 intercellular spaces, which, as we have seen, communicate with 

 the external air b}' ixieans of the stomata and the lenticels. 

 The transpiration is most copious in the leaf, the structure of 

 the lower side of which, in dorsiventral leaves, is especially 

 favourable to it. If a leaf be taken which has stomata upon 

 its under side only, and the rate of transpiration from its two 

 surfaces be compared, it will be found that the stomatal gives 

 off considerably more vapour than the other surface. 



The amount of moistm-e given off in transpiration varies in 

 different plants. In the sunflower {Helianthus) the amount has 

 been stated to be ^l^ cubic inch of water per square inch of 

 sm^face in twelve hours. Doubtless individual plants show a 

 considerable variety, however, in the amount. This copious 

 evaporation readily explains why bleeding of plants from wounds 

 cannot be observed when the leaves are expanded and active. 



The removal of the water accumulated by root-pressure in 

 the closed system of vessels leads to a diminution of the pressure 

 of the air they contain, so that while transpiration is active there 

 is a negative gaseous pressure existing in them. This is of con- 

 siderable importance in assisting in the movements of gases in 

 the plant, and it further helps the pumping action of the root 

 in pressing forward the water by exerting a suction upon the 

 parenchymatous cortex. 



Transpiration is not a process of simple evaporation. As in the 

 other phenomena we have examined, the protoplasm exercises a 

 regulating mfluence upon the escape of watery vapour from the 

 cell. If the amount given off from a measured area of leaf 

 surface be compared with the amount evaporated from an equal 

 area of free water, the latter is found to be nnich the greater. 

 That this is due to the life of the leaf, and therefore to the 

 protoplasm, is seen from the fact that a dead leaf gives off its 

 water and dries up more rapidly than a surface of freely exposed 

 water. The cuticle of the living leaf and its cell-walls are con- 

 sequently not the causes of the differences observed. 



If the protoplasm of the cells of the turgid leaves of a branch 

 be stimulated by violently shaking it, the leaves become flaccid. 



