Book I. ' PROCESS OF VEGETABLE NUTRITION. 167 



sap ascends by the bark, wood, and pith, indiscriminately. Du Hamel stript several trees 

 of their bark entirely, which continued, notwithstanding, to live for many years, protrud- 

 ing new leaves and new branches as before. Knight stript the trunk of a number of 

 young crab -trees of a ring of bark half an inch in breadth, but the leaves were protruded, 

 and the branches elongated, as if the operation had not been performed. Du Petit 

 Thouars removed the central wood and pith from the stems of several young sycamore 

 trees, leaving the upper part to be supported only by four pillars of bark : in others he 

 removed the bark, liber, and alburnum, leaving the upper part of the tree to be supported 

 solely by the central wood. In both cases the trees lived, so that he concludes the bark 

 and wood can alternately act as the sap's conductor. (Hist, d'un Morceau de Bois. 

 Hort. Tour. 481.) 



744. It is evident, therefore, that the sap does not ascend exclusively by tJie bark. But it 

 is equally evident that it does not ascend by the pith, at least after the first year ; for then, 

 even upon Grew's own supposition, it becomes either juiceless or wholly extinct: and 

 even during the first year it is not absolutely necessary, if at all subservient to the ascent 

 of the sap, as is proved by an experiment of Knight's. Having contrived to abstract from 

 some annual shoots a portion of their pith, so as to interrupt its continuity, but not other- 

 wise materially to injure the fabric of the shoot, Knight found that the growth of the 

 shoots which had been made the subject of experiment was not at all affected by it. 



745. Thus the sap ascends neither by the bark nor pith, but by the wood only. But the 

 whole mass of the wood throughout is not equally well adapted for the purpose of con- 

 veying it. The interior and central part, or that part that has acquired its last degree of 

 solidity, does not in general afford it a passage. This is proved by what is called the 

 girdling of trees, which consists in making a circular gap or incision quite round the 

 stem, and to the depth of two or three inches, so as to cut through both the bark and 

 alburnum. An oak-tree on which Knight had performed this operation, with a view to 

 ascertaining the channel of the sap's ascent, exhibited not the slightest mark of vegetation 

 in the spring following. The sap then does not ascend through the channel of the ma- 

 tured wood. But if the sap ascends neither through the channel of the bark, nor pith, 

 nor matured wood, through what other channel does it actually ascend ? The only re- 

 maining channel through which it can possibly ascend is that of the alburnum. In 

 passing through the channel of the alburnum, does the sap ascend promiscuously by the 

 whole of the tubes composing it, or is it confined in its passage to any peculiar set ? 

 The earliest conjectures recorded on this subject are those of Grew and Malpighi, who, 

 though they maintained that the sap ascends chiefly by the bark, did not yet deny that it 

 ascends also partly by the alburnum or wood. It occurred to succeeding phytologists 

 that the progress of the sap, and the vessels through which it passes, might be traced or 

 ascertained by means of making plants vegetate in colored infusions. Du Hamel steeped 

 the extremities of branches of the fig, elder, honeysuckle, and filbert in common ink. 

 In examining the two former, after being steeped for several days, the part immersed 

 was found to be black throughout, but the upper part was tinged only in the wood, 

 which was colored for the length of a foot, but more faintly and partially in proportion 

 to the height. The pith, indeed, exhibited some traces of ink, but the bark and buds 

 none. In some other examples the external layers of the wood only were tinged. 

 In the honeysuckle the deepest shade was about the middle of the woody layers ; and in 

 the filbert there was also observed a colored circle surrounding the pith, but none in the 

 pith itself, nor in the bark. 



746. Thus it is proved that the sap ascends through the vessels of the longitudinal fibre 

 composing the alburnum of woody plants, and through the vessels of the several bundles of 

 longitudinal fibre constituting the woody part of herbaceous plants. But it has been already 

 shown that the vessels composing the woody fibre are not all of the same species. There 

 are simple tubes, porous tubes, spiral tubes, mixed tubes, and interrupted tubes. Through 

 which of these, therefore, does the sap pass in its ascent ? The best reply to this enquiry 

 has been furnished by Knight and Mirbel. Knight prepared some annual shoots of the 

 apple and horse-chestnut, by means of circular incisions, so as to leave detached rings of 

 bark with insulated leaves remaining on the stem. He then placed them in colored in- 

 fusions obtained by macerating the skins of very black grapes,, in water ; and, on exa- 

 mining the transverse section at the end of the experiment, it was found that the infu- 

 sion had ascended by the wood beyond his incisions, and also into the insulated leaves, 

 but had not colored the pith nor bark, nor the sap between the bark and wood. From 

 the above experiment, Knight concludes that the sap ascends through what are called 

 the common tubes of the wood and alburnum, at least till it reaches the leaves. Thus 

 the sap is conveyed to the summit of the alburnum. But Knight's next object was to 

 trace the vessels by which it is conveyed into the leaf. The apple-tree and horse- 

 chestnut were still his subjects of experiment. In the former the leaves are attached 

 to the plant by three strong fibres, or rather bundles of tubes, one in the middle of the 

 leaf-stalk, and one on each side. In the latter they are attached by means of several 



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