Vegetable Physiology. 619 



they ascend till they gain the air, when they suddenly turn outwards, to allow 

 of nearly horizontal expansion. 



Having said so much of the downward motions of the sap and of woody 

 bundles to form both wood and roots, he adds what he considers decisive 

 proof, by instancing the effects of strangulation of a stem or branch of an exo- 

 genous tree. A band, he says, " will offer little impediment to the ascent of the 

 sap from the roots, but it will obstruct any descent. In consequence, there 

 will be a deficiency of the leaf-elaborated nourishment to the parts beneath, 

 and a superfluity above the band ; so that a protuberance will arise from the 

 stem just at the point where the downward flow of the sap is checked. This 

 protuberance will increase in progress of years, if the tree survives, so as 

 almost to bury the band beneath it; but most commonly the tree is destroyed, 

 ere long, by the insufficient supply of nourishment to the roots." The author 

 also alludes to the well-known effect produced by the constriction of a twining 

 shoot of the honeysuckle around a stem of any exogenous plant, which ob- 

 structs, rather than prevents, the descent of the sap, by causing a spiral ridge to 

 be formed above the whole length of the embracing shoot. 



Different effects are pi-oduced by different descriptions of ligatures : if metal 

 wire be used singly, the swellings on each side are nearly equal, and very soon 

 cover the wire ; if a common woollen shred be used, the protuberance is 

 largest on the upper side ; and if a plurahty of bands of common twine be 

 tightly tied on a stem, at short distances from each other, swollen rings will 

 be produced on the spaces between the ties. From all such experiments, it is 

 perfectly evident that the cambium, or living membrane of the tree, is only 

 struggling to get free from the compressive action of the bands, and, of course, 

 forms those protuberances so visible on strangulated or ringed stems. The 

 author avows that " the cambium is gradually organised into cells, and from 

 these are formed the ducts and cellular portion of the woody layer ;" and he 

 might have added, with great truth, all the tubes and vessels ever found as 

 belonging to perfect alburnum and liber. The gradual growth of all these 

 components exhibiting exogenous expansion, requiring space laterally, must be 

 deranged by any resisting band or unyielding body applied by art or accident 

 to the exterior. 



It is perfectly true that the protuberances above a band are not easily 

 explained, unless we admit that there is a descent of sovie constituent of the 

 system ; that is, either organisable sap or fibrous processes from the superior 

 parts, either leaves or buds. In seeking information on this point, by cutting 

 into those protuberances, we find neither accumulations of proper or elabo- 

 rated juice, nor any unusual assemblage of contorted fibres, as we might be led 

 to expect. The interior of these swellings is of similar character to the other 

 parts of the living cambium, and remains of the same alburnous texture after 

 the growth ceases in the autumn ; only with this difference, the cellular parts 

 are more extended laterally, which, indeed, is the cause of the protuberance. 



The descent of the sap is a very old idea ; not only the elaborated sap, as 

 stated by our author and others, but the whole body of the juices of the tree, 

 which, it was supposed, retreated to the roots in winter. This doctrine, I 

 believe, was held by Buffon, and it was confirmed by the experiments of the 

 late Mr. T. A. Knight, who could not otherwise account for the swelling 

 above a ligature. The notion, that woody fibres or actual roots descended 

 from the buds or foliage, was first suggested by Darwin, and afterwards by Du 

 Petit Thouars, and is now adopted by most modern professional botanists 

 and a great majority of gardeners, though positively denied by the late Mr. 

 Knight, who, in speaking of a grafted tree, declared (what is well known to be 

 a fact) that not a particle of the graft ever descended below the place of junc- 

 tion with the stock. Practical gardeners, however, are rarely guided by the 

 physiologist's notions relative to the functions of the leaves, the buds, or 

 superior parts of the tree, as sources of elaborated sap, or as productive of 

 alburnous matter, trusting more to the agency of the roots than to that of 

 the foliage. This is particularly evident in their management of the grape- 



