VEGETABLE PHYSIOLOGY. 29 



particular fluids in the bark. Lymphatic vessels have also been 

 described ; but we have met with no satisfactory account of them. 

 The functions of the vessels of plants have been as variously 

 described as the organs themselves. Malpighi supposed them 

 to be air vessels ; Grew declares, that they sometimes contained 

 moisture ; and Du Hamel suspected that they contained " highly 

 rarified sap." The experiments of Darwin and Knight, have, 

 to a certain degree, determined their uses. The former placed 

 twigs of the common fig tree into a decoction of madder, and 

 on taking them out after some hours' emersion and cutting 

 them across, the colored fluid was found to have ascended into 

 each branch, and the cut ends of the vessels formed a circle of 

 red dots around the pith, and these vessels again were surrounded 

 by other vessels containing the milky juice, so very remarkable 

 in the fig tree. The latter (Mr. Knight) made similar experi- 

 ments with cuttings of the horse chesnut and of the apple tree, 

 with an infusion in water of very black grapes. The result 

 corresponded with those of Darwin. He, however, pursued the 

 investigation still further, and traced the fluid into the leaves ; 

 and during the whole course it did not give the slightest tinge to 

 the bark, nor to the sap between it and the \vood. The pith 

 was very slightly, if at all affected. The radicles are probably 

 elongations of these vessels which absorb the proper fluids from 

 the earth, and convey it into the body of the root, where it 

 becomes sap by some process which we cannot develop ; it is 

 then conveyed to the stem and leaves, where certain other 

 changes take place, that are to be hereafter noticed. The 

 functions of the alburnous vessels appear to be two fold, according 

 to the views of Mr. Knight. At one period, they convey sap to 

 the leaves in common with the central vessels : and during the 

 winter, they serve as reservoirs of the juices of the plant, which, 

 after having undergone certain changes in the leaves, are there 

 deposited until the approach of spring, when they contribute to 

 the formation of those new parts which are necessary for the 

 vital action of the vegetable. 



The cortical vessels seem to carry the sap back to the roots 

 through the bark, and, in its course, it possibly forms alburnum, 



