Book I. PROCESS OF VEGETABLE NUTRITION. 235 



and reached, at its maximum, to a height of thii-ty-eight inches. But this was equivalent 

 to a column of water to the height of forty-three feet three and one third inches; demon- 

 strating a force in the motion of the sap that, without the evidence of experiment, would 

 have seemed altogether incredible. 



1541. Thus the sap, in ascending from the loiver to the upper extremitij of the plant, is 

 propelled with a very considerable force, at least in the bleeding season. But is the as- 

 cending sap propelled indiscriminately throughout the whole of the tubular apparatus, or 

 is it confined in its course to any particular channel ? Before the anatomy of plants had 

 been studied with much accuracy, there was a considerable diversity of opinion on the 

 subject. Some thought it ascended by the bark ; others thought it ascended by the 

 baric, wood, and pith, indiscriminately ; and others thought it ascended between the bark 

 and wood. The first opinion was maintained and supported by Malpighi ; and Grew 

 considered that the sap ascends by the bark, wood, and pith, indiscriminately. Du Hamel 

 stripped several trees of their bark entirely, which continued, notwithstanding, to live for 

 many years, protruding new leaves and new branches as before. Knight stripped the trunks 

 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 perforated. 

 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 each case tlie tree lived, so that he concludes 

 that both the bark and wood are competent to act as conductors to the-^ap. (Hist, d'un 

 Morccau de Bois, Hort. Tour, 481.) 



1.542. That the sap does not ascend exclusively by 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 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 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. 



1543. 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 which has acquired its last degree of 

 solidity, does not in general aflPord 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 

 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 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 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 ascer- 

 tained by means of making plants vegetate in coloured 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 coloured 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 coloured circle surrounding the pith, but none in the pith itself, nor 

 in the bark. 



1544. 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 



