AIR-VESSELS OP THE LEAVES. 199 



spiral vessels were believed to convey all 

 through the plant, in order that it might act 

 on the sap as it does on the animal blood. 

 The analogy thus understood was not correct, 

 because air is conveyed no further than the 

 lunirs of animals ; but without this hypothesis 

 no use could be found for the supposed longi- 

 tudinal air-vessels. 



The observations of Dr. Hales come next 

 in order to those of Grew and Malpighi. By 

 means of the air-pump, an instrument much 

 in use in his time. Hales obtained abundance 

 of air from every part of the vegetable body, 

 as well as from recently extracted sap. Plants 

 were found to perish very soon in an ex- 

 hausted receiver. Some of this great man's 

 experiments, however, require to be received 

 with caution. He rightly remarked that air 

 was not only taken in by plants very copious- 

 ly along with their food, but also imbibed 

 by their bark ; see Veg. Staficks, chap. 5. 

 But when, from observing that it would 

 freely from the bark pervade the longitudi- 

 nal vessels of a branch, he concluded that 

 Malpighi and Grew were right in their ideas 

 of longitudinal air-vessels, he was misled by 



