mumj important Truths in Phytology. 105 



Sir J. E. Smith is perfectly right in saying that the wood can 

 be divided into such diminutive pieces, it is impossible that they 

 should form sap-vessels. They are not sap vessels, but sap va- 

 cancies; which convey the liquid up the tree with more freedom 

 than vessels could, as the escape of the buds horizontally would 

 be apt bv moving the vessels to twist them, and thus impede the 

 sap; whereas the vacancies formed in the wood, move to and 

 fro with it, onlv tied together everv inch by a spiral wire which 

 surrounds the top of the aperture (see Plate I. fig. 8), and by 

 contracting and dilating brings them again to their proper places, 

 after thev have been disturbed by the passing out of the bud ; 

 and as the aperture or passage for the sap is really very large, 

 it will bear a little reducing, without injury to the flow of the 

 liquid. The spiral also prevents their deviating too far from 

 their proper situation. Let a piece of wood be cut quite straight 

 and horizontal, and it will (if much magnified) easily show the 

 spiral vessels pa'^sing from one aperture to another (see fig. 8), 

 preventing the wood from warping too much, or obliging it to 

 return to its proper situation. 



I shall now show Mr. Knight's opinions (as far as I can un- 

 derstand them), and contrast them with my own. We certainly 

 both agree as to the existing ves'jels, though differing in the pur- 

 pose for which they are intended. It is astonishing that, not 

 dissecting proijressively, he should so well have ascertained the 

 ves<iels\ this I think wonderful: — but it is only by tracing them 

 daily, from their Jirst existence^ that their purpose can he really 

 proved. 



Mr. Knight believes that the sap flows up the spiral vessels iu 

 the wood ; that they are the sap-veswls, wb.ich convey all this liquid 

 up the tree ; and that, arrived at the branches, they run up to the 

 bottom of the loaves, and convey the returning sap to the bark 

 downwards. I (on the contrary) am convinced " that the spirals 

 are the mu^cla of the tree, can convey no sop, that the-, uiean- 

 <U'r in every part of the leaf to accelerate its inotioi) ; and that 

 the large vessels meeting the spirah, at the liottom of th'' leaf, 

 are the inner bark -vessels running downwards, filling the bark 

 anev.' which was lost by the lale winter, and v/hich is all formed 

 i;i tlie leaves. 



My first reason against Mr. Knight's opinion is taken from 

 llic size of the spirals. That so small a vessel should be chosen 

 for the purpose of nourishing a great tree, wiien sixty spirals 

 would scarcely make a small thread, seems preposterous. The 

 next reason against it is its extreme twisting and e erval agita- 

 tio7i, which iu so diminutive a thread must continually nnpede 

 the progress of the 'sap. Place a stem of a plant or tree in a co- 

 loured licjuorj the spirals run only iu the three or four laotrowsof 



the 



