38 THE WORK OF THE LEAF [chap. 



assimilation and in its turn exhales oxygen ; through 

 them again it exhales carbon dioxide when it is respiring, 

 and gives off its transpiration water in the form of 

 vapour. As a rule, the water escapes as vapour and 

 continues to be invisible, but when the surrounding 

 atmosphere is saturated it may condense as drops upon 

 the tips or edge of the leaf. Thus arises the greater 

 part of the dew which covers the grass in the early 

 morning ; were dew only water that had been condensed 

 from the atmosphere by the ground cooled by radiation, 

 dew would be as plentiful on a garden path or on a 

 stone as on the grass. 



Many experiments have been made to ascertain how 

 much water a plant transpires during its growth, and 

 this information is most easily applied to practical 

 problems if we establish a connection between the 

 amount of water transpired by a plant and the increase 

 of weight (reckoned, of course, as dry matter) which takes 

 place within the same period. At bottom there is no 

 real connection between assimilation and transpiration ; 

 the two processes go on simultaneously, and to some 

 extent are similarly affected by the same external 

 conditions, but they are in so many respects inde- 

 pendent that any ratio we may trace between them can 

 only be a sort of average, true for the general conditions 

 prevailing in the place of experiment. For example, in 

 England, Lawes and Gilbert concluded that for every 

 pound of dry matter elaborated by such plants as wheat, 

 barley, clover, and peas, about 250 lb. of water was 

 evaporated from the surface of the leaves. Hellriegel in 

 Germany with a drier atmosphere obtained results 

 about 50 per cent, higher, while WoUny in Vienna and 

 King in Wisconsin, with still hotter and drier climates, 

 obtained even higher ratios. In England, however, we 

 may assume that every pound of dry matter produced 



