126 PROCESS OF NUTRITION. CHAP. III. 



Parenchyma, swelled also with sap, which they 

 have taken up by suction or filtration, is conse- 

 quently so compressed, that the sap therein is 

 forced upwards a second stage, and so on till it 

 reaches the summit of the plant.* But if the 

 vesiculae of the Parenchyma receive their moisture 

 only by suction or filtration, it is plain that there 

 is a stage of ascent beyond which they cannot be 

 thus moistened, and cannot consequently act any 

 longer upon the longitudinal tubes. The supposed 

 cause, therefore, is inadequate to the production of 

 the effect. 



Of Mai- Malpighi was of opinion that the sap ascends by 

 means of the contraction and dilatation of the air 

 contained in the air-vessels. This supposition is 

 perhaps somewhat more plausible than either of 

 Grew's ; but in order to render the cause efficient, 

 it was necessary that the tubes should be furnished 

 with valves, which were accordingly supposed ;-}- 

 but of which the existence has been totally dis- 

 proved by succeeding phytologists. If the stem or 

 branch of a plant is cut transversely in the bleed- 

 ing season, it will bleed a little from above, as well 

 as from below : and if the stem of any species of 



* Veg. of Trunks, chap. i. 



t Subintrans itaque humor, sursum asccndit et quasi suspen- 

 ditur; singula namque portio, quae invicem fibrarum frustula 

 unit, cum parum interius emineat, valvulae vices supplet, et ita 

 minima quaelibet guttula, veluti per funem, seu per gradus, ad 

 ingens deducitur fastigiuin. Anat. Plant, vol. v. 



