30 



complete little contrivances imaginable, with a door and hiages to each, and full power to 

 shut or open them, all as the tree needs). Stop one moment, and notice that when the 

 leaves are cool, and dew is forming on them, as it would on any dead substance 

 which cools by radiation, these doors can be shut. When the tree is warm with life from 

 the sun again, they can open, and transpiration (as the out flow of vapour from the leaves 

 is called), or absorption of moisture if the tree need it, go on in full vigour. We cannot 

 do better, to interest ourselves in the operation, than to read the description written by a 

 distinguished gentleman of France (the Marshal Vaillant), who made it a special study, 

 and gave much time to experiments connected therewith. He says : — 



" Even the most humble plants, such as chickweeds and meadow grasses, evaporate 

 considerable quantities of water. 



" 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 tran- 

 spire a large 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 year. 



" From whence comes the water so rapidly transpired by the foliage? Certainly 

 from the soil. 



" I placed in a large jug of water, tightly closed up to hinder the natural evapora- 

 tion of the water, the end of an oak branch, five feet long and nearly an inch thick at the 

 butt. It was cut from a tree eighty feet high and three feet thick. In three days it had 

 lost thirty ounces of water. 



" If we believe that all the leafy part of a tree will act, as regards the faculty of tran 

 spiration, like the leaves of the above-mentioned branch, we arrive at the astounding result 

 that an oak like the one described will in a summer day cause the evaporation of more 

 than 440 gallons of water. 



" I am not decided as to the value of my experiment, and see that my deductions are 

 not free from objections ; but it must be allowed that supposing even half or one-quarter 

 of the estimated quantity be omitted, the quantity must greatly exceed what might have 

 been expected." 



Let us lise the excellent Marquis's experiment, and draw our own inferences which 



will put us, probably, in a position a step farther advanced. The branch with its leaves, 



no doubt, sent off this vast proportion. But branches on trees probably do not. If 



my readers will remember what is previously stated about the mouths of a tree (the 



spongioles, as they are called, at the ends of the roots), they will notice that these take in 



as much water as the tree needs to carry up its nourishment and act as its vehicle to the 



leaves, where all water not needed is sent off by transpiration int,o the air. Now the 



leaves, it may be supposed, having no such functions, not being the sentinels at the root 



gates, may transpire while they live as much water as is sent up to them, and we may 



suppose the transpiration machine working furiously, when the Marquis had cut off the 



connections with the roots, as a steam engine with the regulating valves left unattended. 



They would, no doubt, take it in at the severed end of the branch, and send it off by 



the leaves. But though the forest draws much up, and transpires it, this experiment does 



not prove that it draws up the enormous quantity spoken of. 



Taken with this understanding and qualification, however, we can well believe the 

 rest of the Marquis's comments, which are in fact generally adopted as correct by meteor- 

 ologists, and of which Mr. Crombie Brown, a high authority, thus speaks :—" With 



