HALES' EXPERIMENTS. 339 



could only be proportionable to the excess of the quantity of water 

 drawn off, above the quantity of air which issued out of the wood. 



' And if the quantity of air, which issued from the wood into 

 the tube, had been equal to the quantity of water imbibed, then 

 the mercury would not have risen at all, because there would 

 have been no room for it in the tube. 



' 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 equalized 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 con- 

 cludes 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 can- 

 not 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 constantly re- 

 curring 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 transplant 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 proportion between it and the imbibing surface 

 under ground.' In the following observations on hop vines, Hales 

 shows the effect of suppressed perspiration : 



' The soil of an acre of ground on which 9,000 hop-vines are 

 growing, must supply to the plants, through the roots, in July, 

 36,000 ozs. of water in twelve hours. This is the quantity of 

 water which during this time is exhaled by them, and which they 

 must have to be in a thriving condition. 



' In a kindly state of the air, this moisture is daily carried off 

 in sufficient quantity to keep the hops in a healthy state ; but in a 



