EXHALATION OP MOISTURE. 75 



after being cut from the tree, and again after an exposure after three 

 minutes, and computing the superficial measure of all the leaves of 

 the tree, concludes that an oak tree evaporates, during the season of 

 growth, eight and a-half times the mean amount of rainfall on an 

 area equal to that shaded by the tree." Corresponding observations 

 made by others are not awanting. 



Marshal Vaillant writes : — " Even the most humble plants, such as 

 chickweeds and meadow grasses, evaporate considerable quantities 

 of water. Independently of all the other actions, which ought not 

 to be overlooked, this is the chief cause why the abundant summer 

 rains make so little impression on our rivers. This is why the 

 gardener uses the hoe so carefully, and why he removes all weeds 

 which would exhaust the moisture of the soil. 



" The intelligent gardener is not deceived, he knows that if any 

 weed has long numerous roots, like the triticum repens (couch grass), 

 they do more harm than others, because by spreading to a distance 

 they deprive a larger extent of ground of its share of moisture. This 

 is why a paillis, although on account of its dark colour it becomes 

 sooner warm than turf or other grass plants, does so much good to 

 cultivated vegetables, by not depriving their roots of moisture. 



" The quantity of water exuded by rapidly-growing plants is far 

 greater than could be supposed possible by people who have not made 

 direct experiments ; it is especially plants of soft texture with hollow 

 stems that excite our surprise. A branch of helianthns annuus (a 

 sunflower), placed in a caraffe full of water, and exposed to the sun, 

 exhausts the water very rapidly; thus this plant, with its far- 

 spreading roots, is a very dangerous neighbour to others, and we may 

 be thankful that the passage of water into the roots does not go on so 

 quickly as the evaporation by its leaves, otherwise the exhaustion 

 caused would be much greater than it really is. It is this difference 

 between the quantity of water transmitted from the soil to the roots, 

 and from the leaves to the air, that causes the latter to wither in warm 

 weather. Rain does not penetrate the leaves, but only diminishes 

 their evaporation, so that an equilibrium is re-established. 



" If from herbaceous plants or modest shrubs we turn to our large 

 forest trees, we may expect that, compared with the weeds of which 

 we have just spoken, they will transpire a great quantity of water, 

 which is probably in proportion to the number of leaves and their 

 extent of surface ; and it is our belief that this summer function of 

 the leaves is carried on by the trunk and branches during the whole 



