48 HISTORY OF BOTANY 



noticed that leaves gave off water " perspired " or 

 transpired as we say, and forthwith he proceeded to 

 measure the amount transpired and to compare it with the 

 amount absorbed by the root. He studied the variations 

 in the quantity of water evaporated during the twenty- 

 four hours and demonstrated the reduction in transpira- 

 tion at night, and from a comparison of the amounts 

 absorbed and transpired in a given time, he estimated the 

 rate of ascent of the water-current. The forces concerned 

 in the ascent were root pressure in the bleeding season 

 and the " capillary sap vessels out of the bleeding season." 

 " Though these have little power to protrude sap in any 

 plenty beyond their orifices " (a comparison with the 

 spurts of blood from a cut artery) " they can by their 

 strong attraction (assisted by the genial warmth of the 

 sun) supply the great quantities of sap drawn off by 

 perspiration." 



Having obtained successful results from one set of 

 experiments, Hales at once uses these results as a basis 

 for further enquiry. In this respect he exemplifies Grew's 

 remark that " the generation of experiments is like that 

 of discourse, where one thing introduceth an hundred 

 more which otherwise would never have been thought 

 of." One of these suggested investigations is into the 

 connection between the plant and the atmosphere. 

 You will remember that Malpighi was doubtful 

 whether the air in plants enters by the root or by 

 the leaf. Malpighi figures stomata, but is hazy as to 

 their function ; Grew, though he also knew of the 

 existence of stomata, is equally vague as to their use to 

 the plant. Hales, on the other hand, has some quite 

 definite notions as to the part leaves play in plant 

 nutrition. Transpiration is of course their chief function, 

 but they have other duties as well. " For the air is full 

 of acid and sulphureous particles which, constantly 

 forming in the air, are doubtless very serviceable in 

 promoting the work of vegetation ; when being imbibed 



