47 ^ Theory of the Nutrition [BOOK in. 



pressure in the case of the bleeding vine, were particularly 

 striking and instructive. His measurements and the figures, on 

 which he founded his calculations, were not so exact as they were 

 often at a later time supposed to be, but he was himself satisfied 

 with obtaining round, approximative numbers ; these under 

 given circumstances supplied a sufficient basis for propositions 

 which were new and afforded a certain amount of insight into the 

 economy of the plant. This mode of proceeding showed his 

 understanding ; for the case of living bodies is different from 

 that of metals and gases ; in these we seek for constants which 

 can then be inserted in general formulae, and to which there- 

 fore the nicest accuracy is applied ; but in plants we have to 

 deal with individual cases, and it is from a right interpretation 

 of the measurements taken from them that we can arrive at 

 general laws of vegetation. 



To show that the forces of suction and pressure which 

 operate in plants are not something sui generis, but prevail also 

 in dead matter, in other words that they are an example of the 

 general attraction of matter, a subject of particular interest at 

 that time, Hales observed the absorption of water by substances 

 with fine pores ; and measured the force employed. These 

 processes he compared with the force which swelling peas 

 exert on the obstacles which they encounter, and thus obtained 

 a more correct idea of the forces concerned in the movement 

 of water in the plant than that given by the capillarity of glass- 

 tubes, which Mariotte and Ray had employed to illustrate 

 them. 



Hales failed to appreciate the value of Malpighi's obser- 

 vations on the function of leaves, and was induced by the 

 copiousness of the evaporation of water from their surfaces 

 to overrate the physiological importance of that process ; hence 

 he saw in leaves chiefly organs of transpiration, which raise the 

 sap by suction from the roots through the stem. In accord- 

 ance with this view he denied the existence of a descending 

 sap in the bark, and only admitted that the ascending sap 



