180 FLOW OF SAP. 



Bult, and it is now generallj held that no water is taken in bj the 

 pores of vegetables. Late observations by Cailletet in France, 

 however, tend to the establishment of a new doctrine on this sub- 

 ject which solves many difficulties and will probably be accepted 

 by botanists as definitive. Cailletet finds that under normal con- 

 ditions, that is, when the soil is humid enough to supply sufficient 

 moisture through the roots, no water is absorbed by the leavos, 

 buds or bark of plants, but when the roots are unable to draw 

 from the earth the requisite quantity of this fluid, the vegetable 

 pores in contact with the atmosphere absorb it from that source. 



Popular opinion, indeed, supposes that all the vegetable fluids, 

 during the entire period of growth, are drawn from the bosom of 

 the earth, and that the wood and other products of the tree are 

 wholly formed from matter held in solution in the water abstract- 

 ed by the roots from the ground. This is an error, for the solid 

 matter of the tree, in a certain proportion not important to our 

 present inquiry, is received from the atmosphere in a gaseous 

 form, through the pores of the leaves and of the young shoots^ 

 and, as we have just seen, moisture is sometimes supplied to trees 

 by the atmosphere. The amount of water taken up by the roots, 

 however, is vastly greater than that imbibed through the leaves 

 and bark, especially at the season when the sap is most abundant, 

 and when the leaves are yet in embryo. The quantity of water 

 thus received from the air and the earth, in a single year, even 

 by a wood of only a hundred acres, is very great, though experi- 

 ments are wanting to furnish the data for even an approximate 

 estimate of its measure ; for only the vaguest conclusions can be 

 drawn from the observations which have been made on the imbi- 

 bition and exhalation of water by trees and other plants reared in 

 artificial conditions diverse from those of the natural forest.* 



Flow of Sarp. 



The amount of sap which can be withdrawn from hving trees 

 furnishes, not indeed a measure of the quantity of water sucked 



* The experiments of Hales and others on the absorption and exhalation of 

 vegetables are of high physiological interest ; but observations on sunflowers, 

 cabbages, hops, and single branches of isolated trees, growing in artificially 

 prepared soils and under artificial conditions, furnish no trustworthy data foi 

 computing the quantity of water received and given off by the natural wood. 



