242 SCIENCE OF AGRICULTURE. Part II. 



layer of wood and of bark, generated between the wood and bark of the former year, and 

 covering the orio^inal 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 increased in diameter. Malpighi was of opinion that the new layer of wood is 

 formed from the liber of the former year. 



1645 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 thougiit 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 re- 

 garded 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. 



1547. Knight has thrown the highest degree of elucidation on this, one of the most obscure and intri- 

 cate processes of the vegetable economy, in having shown that the sap is elaborated, so as 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 augmented 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 man- 

 ner 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 sup- 

 posed 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, observed that the wood formed under the bark of the in- 

 serted 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 evi- 

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



1548. But how is the formation of 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. 



1549. 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 

 necessarily 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 wootl 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 indura- 

 tion 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 solidity 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 of!" again by the 

 gap as it ascends in the spring. 



1550. 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. But the doctrine, as it appears, has 

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

 modern phytologists. Hedwig is said to have declared himself to be of opinion, that plants 

 have a circulation of fluids similar to that of animals. Corti is said to have discovered a 

 species of circulation in the stem of the chara, but confined, it is believed, within 

 the limits of the internodia. Willdenow has also introduced the subject, and de- 

 fended the doctrine [Princijdes of Botany, p. 85.) ; but only by saying he believes a cir- 

 culation to exist, and that it is impossible for the leafless tree to resist the cold if there be 

 not a circulation of fluids. Knight has given his reasons somewhat in detail ; and 



