Book I. PROCESS OF VEGETABLE NUTRITION. 235 



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. (Vun Morceau de Boiu 

 Hort. Tour. 481.) 



1516. That the sap does not ascend exclusively hy the hark is thus rendered sufficiently 

 evident. 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 ali 

 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 otherwise 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. 



1,517. The sap ascends neither by the bark nor pith, but by the ivood 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 ascertain the 

 cliannel 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 matured 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 remaining channel 

 through vvhich 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 alsa 

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

 jirogress of the sap, and the vessels through which it passes, might be traced or ascer- 

 tained 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 honey- 

 suckle 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 ia 

 the bark. 



1518. Thus it is proved that the sap ascends through the vessels of the longitudinal Jibre 

 coinposing the alburnum of woody j^lanls, and through the vessels of the several bundles of 

 longitudinal Jibre 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 

 infusions obtained by macerating the skins of very black grapes in water ; and, on 

 examining the transverse section at the end of the experiment, it was found that the 

 infusion 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 ob 

 ject 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 plants 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 such bundles. Now the colored fluid was found in each case to have passed 

 through the centre of the several bundles, and through the centre only, tinging the tubes 

 throughout almost the whole length of the leaf-stalk. In tracing their direction from the 

 leaf-stalk upwards, they were found to extend to the extremity of the leaves; and in 

 tracing their direction from the leaf-stalk downwards, they were found to penetrate 

 the bark and alburnum, the tubes of which they join, descending obliquely till they. 



