HALES EXPERIMENTS. 3G1 



* But if nine parts in twelve of the water be imbibed by the 

 branch, and in the meantime but three such parts of air issue 

 into the tube, then the mercury must needs rise near six inches, 

 and so proportionately in different cases.' 



When, in Hales' experiments, the root, the stem, or a branch 

 had been wounded in any part by cutting off root-fibres, or 

 buds, or smaller twigs, the imbibing power was found to be 

 diminished in the other parts (because at those wounded spots 

 the difference in the pressure was more readily equalised by 

 air finding its way in). The imbibing power was greatest about 

 fresh cuts, but it gradually diminished until, after a few days, 

 it remained no stronger about the cut than about the uninjured 

 parts. Hales further concludes the exhalation from the surface 

 to be the powerful cause that conveys nutriment to the plant 

 from the parts surrounding it. If the proper proportion 

 between the exhalation and the supply of food is in any way 

 disturbed, the plant sickens and dies. If, in hot summers, the 

 soil is unable to supply to the roots the moisture carried off in 

 the course of the day by exhalation from the leaves, &c., and 

 the tree or a branch of it is dried up, the motion of the sap 

 ceases in such parts. Once dried up, the original action cannot 

 be restored by capillary attraction alone. Exhalation is the 

 chief condition of the life of the plant, serving, as it does, to 

 effect and maintain a continual motion of the sap, and a con- 

 stantly recurring change in its condition. 



* By comparing,' says Hales, ' the surface of the roots of a 

 plant with the surface of the same plant above ground, we see 

 the necessity of cutting off many branches from a transplanted 

 tree. Suppose upon digging the plant up, in order to trans- 

 plant it, half the roots be cut off (which is the case of most 

 young transplanted trees), then it is plain that but half the 

 usual nourishment can be carried up through the roots, and 

 that accordingly the perspiring surface above ground must be 

 correspondingly reduced in order to restore the proper propor- 

 tion between it and the imbibing surface under ground.' In 

 the following observations on hop-vines. Hales shows the effict 

 of suppressed perspiration : — 



* The soil of an acre of ground on which 9000 hop-vines 

 are gi-owing, must supply to tlie plants, through the roots, in 



