174 SCIENCE OF GARDENING. Part II. 



and woody fibre interspersed throughout the pith, as in others. In the same manner we may account for 

 the formation of the layer of bark. 



768. Perennials and their annual layers. If a perennial is taken at the end of the 

 second year and dissected as in the example of the first year, it will be found to have in- 

 creased in height by the addition of a perpendicular shoot consisting of bark, wood, and 

 pith, as in the shoot of the former year ; and in diameter by the addition of a new layer 

 of wood and of bark, generated between the wood and bark of the former year, and cover- 

 ing the original cone of wood, like the paper that covers a sugar-loaf: this is the fact of 

 the mode of augmentation about which phytologists have not differed, though they have 

 differed widely with regard to the origin of the additional layer by which the trunk is in- 

 creased in diameter. Malpighi was of opinion that the new layer of wood is formed from 

 the liber of the former year. 



769. The new layer of wood Linnaeus considered as formed from the pith, which is absurd, because the 

 opinion goes to the inversion of the very order in which the layer is formed, the new layer being always 

 exterior to the old one. But according to the most general opinion, the layer was thought to be formed 

 from a substance oozing out of the wood or bark first, a limpid fluid, then a viscid pulp, and then a thin 

 layer attaching itself to the former ; the substance thus exuding from the wood or bark was generally 

 regarded as being merely an extravasated mucilage, which was somehow or other converted into wood and 

 bark : but Du Hamel regarded it as being already an organised substance, consisting of both cellular and 

 tubular tissue, which he designated by the appellation of the cambium, or proper juice. 



770. Knight has thrown the highest degree of elucidation on this, one of the most obscure and intricate 

 processes of the vegetable economy, in having shown that the sap is elaborated, so to render it fit for the 

 formation of new parts in the leaf only. If a leaf or branch of the vine is grafted even on the fruit-stalk 

 or tendril, the graft will still succeed ; but if the upper part of a branch is stripped of its leaves the bark 

 will wither as far as it is stripped ; and if a portion of bark furnished with a leaf is insulated by means of 

 detaching a ring of bark above and below it, the wood of the insulated portion that is above the leaf is 

 not augmented : this shows evidently that the leaf gives the elaboration necessary to the formation of new 

 parts, and that without the agency of the leaf no new part is generated: Such then is the mode of 

 the augmentation of the plant in the second year of its growth. It extends in width by a new layer 

 of wood and of bark insinuated between the wood and bark of the former year ; and in height by 

 the addition of a perpendicular shoot, or of branches, generated as in the shoot of the first year. 

 But if the plant is taken and dissected at the end of the third year, it will be found to have aug- 

 mented in the same manner ; and so also at the end of the succeeding year as long as it shall continue 

 to live ; so that the outermost layer of bark, and innermost layer of wood, must have been originally 

 tangent in the first year of the plant's growth ; the second layer of bark, and second layer of wood, in the 

 second year ; and so on in the order of succession till you come to the layer of the present year, which will 

 in like manner divide into two portions, the outer forming one or more layers of bark, and the inner 

 forming one or more layers of wood. And hence the origin of the concentric layers of wood and of bark 

 of the trunk. But how are we to account for the formation of the divergent layers, which Du Hamel 

 erroneously supposed to proceed from the pith ? The true solution of the difficulty has been furnished by 

 Knight, who, in tracing the result of the operation of budding, obs?rved that the wood formed under the 

 bark of the inserted bud unites indeed confusedly with the stock, though still possessing the character and 

 properties of the wood from which it was taken, and exhibiting divergent layers of new formation which 

 originate evidently in the bark, and terminate at the line of union between the graft and stock. 



771. But how is the formationlof the wood that now occupies the place of the pith to be accounted for > 

 It appears that the tubes of which the medullary is composed do, in the process of vegetation, deposit a 

 cambium, which forms an interior layer that is afterwards converted into wood for the purpose of filling 

 up the medullary canal. 



772. Conversion of the alburnum into perfect wood. In consequence of the increase of the trunk by means 

 of the regular and gradual addition of an annual layer, the layers whether of wood or of bark are ne- 

 cessarily of different degrees of solidity in proportion to their age ; the inner layer of bark, and the outer 

 layer of wood, being the softest; and the other layers increasing in their degree of solidity till you reach 

 the centre on the one hand, and the circumference on the other, where they are respectively the hardest, 

 forming perfect wood or highly indurated bark, which sloughs or splits into chinks, and falls off in thick 

 crusts, as in the plane-tree, fir, and birch. What length of time then is requisite to convert the alburnum 

 into perfect wood, or the liber into indurated bark ; and by what means are they so converted ? There is no 

 fixed and definite period of time that can be positively assigned as necessary to the complete induration 

 of the wood or bark, though it seems to require a period of a good many years before any particular 

 layer is converted from the state of alburnum to that of perfect wood ; and perhaps no layer has received 

 its final degree of induration till such time as the tree has arrived at its full growth. The induration 

 of the alburnum, and its consequent durability, are attributed by many to the loss of sap which the 

 layer sustains after the period of its complete developement ; when the supply from the root diminishes, 

 and the waste by evaporation or otherwise is still kept up, inducing a contraction or condensation of 

 its elementary principles that augments the . solhiity of the layer, in the first degree, and begins the 

 process that future years finish. But Knight believes the induration of the alburnum as distinguishable 

 in the winter to be owing rather to some substance deposited in it in the course of the preceding summer, 

 which he regards as being the proper juice in a concrete or inspissated state, but which is carried off again 

 by the sap as it ascends in the spring. 



773. Circulation of vegetable juices. After the discovery of the circulation of the 

 blood of animals, phytologists, who were fond of tracing analogies between the animal 

 and vegetable kingdoms, began to think that there perhaps existed in plants also a circu- 

 lation of fluids. The sap was supposed to be elaborated in the root. The vessels in 

 which it was propelled to the summit of the plant were denominated arteries ; and the 

 vessels in which it is again returned to the root were denominated veins. Du Hamel, 

 while he admits the ascent of the sap, and descent of the proper juice, each in peculiar 

 and appropriate vessels, does not however admit the doctrine of a circulation ; which 

 seems, about the middle of the last century, to have fallen into disrepute. For Hales, who 

 contended for an alternate ascent and descent of fluids in the day and night, and in the 

 same vessels, or for a sort of vibratory motion as he also describes it, gave no countenance 

 whatever to the doctrine of a circulation of juices. I3ut the doctrine, as it appears, has 

 been again revived, and has met with the support of some of the most distinguished of 



