242 SCIENCE OF AGRICULTURE. Part II. 



enquiry, was, that the pith was analogous to the heart and brain of animals, as related by Malpighi ; who 

 did not himself adopt it, but believed the pith to be, like the cellular tissue, the viscera in which the sap 

 was elaborated for the nourishment of the plant, and for the protrusion of future buds. Magnol thought 

 that it produced the flower and fruit, but not the wood. Du Hamel regarded it as being merely an exten- 

 sion of the pulp or cellular tissue, without being destined to perform any important function in the process 

 of vegetation. But Linnjeus was of opinion that it produces even the wood; regarding it not only as the 

 source of vegetable nourishment, but as being also to the vegetable what the brain and spinal marrow 

 are to animals the source and scat of life. In these opinions there may be something of truth, but 

 they have all the common fault of ascribing to the pith either too little or too much. Mr. Lindsay of 

 Jamaica suggested a new opinion on the subject, regarding it as being the seat of the irritability of the 

 leaves of the Mim5sa; and Sir J. E Smith says, he can see nothing to invalidate the arguments oil which 

 this opinion is founded. Plenck and Knight regard it as destined by Nature to be a reservoir of moisture to 

 supply the leaves when exhausted by excess of perspiration. Hence it appears that the peculiar function 

 of the pith has not yet been altogether satisfactorily ascertained ; and the difficulty of ascertaining it has 

 been thought to be increased from the circumstance of its seeming to be only of a temporary use in the 

 process of vegetation, by its disappearing in the aged trunk. But although it is thus only temporary 

 as relative to the body of the trunk, yet it is by no means temporary as relative to the process of 

 vegetation, the central part of the aged trunk being now no longer in a vegetating state, and the 

 pith being always present in one shape or other in the annual plant, or in the new additions that are 

 annually made to perennials. The pith, then, is essential to vegetation in all its stages: and from the 

 analogy of its structure to that of the pulp, or parenchyma, which is known, as in the leaf, to be an organ 

 of elaboration, the function of the pith is most probably that of giving some peculiar elaboration to 

 the sap. 



1572. The generation of the layer of wood in woody plants, or of the parts analogous to wood in the case 

 of herbaceous plants, has been hitherto but little attended to. If we suppose the rudiments of the 

 different parts to exist already in the embryo, then we have only to account for their developement by 

 means of the introsusception and assimilation of sap and proper juice : but if we svippose them to be 

 generated in the course of vegetation, then the difficulty of the case is augmented; and, at the best, we 

 can only state the result of operations that have been so long continued as to present an effect cognisable 

 to the sense of sight, though the detail of the process is often so very minute as to escape even the nicest 

 observation. All, then, that can be said on the subject is merely, that the tubes, however formed, do, by 

 virtue of the agency of the vital principle operating on the proi>er juice, always make their appearance at 

 last in a uniform and determinate manner, according to the tribe or species to which the plant belongs, 

 uniting and coalescing so as to form either a circular layer investing the pith, as in woody plants ; a 

 number of divergent layers intersecting the pith, as in some herbaceous plants ; or bundles of longitudinal 

 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. 



1573. Perennials and their annual layer 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 

 increased 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 

 covering 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 diifered, 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. 



1574. The new layer of wood Linnaus 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. 



1575. 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 layer or more of bark, and the inner forming one 

 layer or more of wood. And hence the origin of the concentric layers of wood and of bark in 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 

 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. 



1576. 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 sheath 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. 



1577. Opinion of Darwin and Du Petit Thouars. According to these philosophers, (and the hypothesis, 

 we believe, was originally proposed by Dr. Darwin,) " the phenomena which took place at the period of 

 germination are renewed by every leaf which successively unfolds itself The cotyledons were the source 

 of the fibres which were sent down into the earth through the root ; in like manner every leaf is enabled 

 to maintain a communication between itself and the soil, by the means of fibres. Hence arises another 

 kind of increase, of which no notice has yet been taken the increase in thickness. A stem, which at the 



