2 i 2 



SCIENCE OF AGRICULTURE. Part II. 



enquiry, was, tli.it the pith was analogous to the li-.irt and i»r:»in of animals, aa related by Malpighi; who 

 did not hiraiclf adopt it, but belli ved the pith to be, like the cellular tissue, the visa ra In which the sap 

 waa elaboi itod for the nourishment ol the plant, and for the protrusion of future buds. Magnol thought 

 thai it produced the flower and fruit, bul not the wood. Do Hamel regarded it at being merely an exten- 

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



Ution Bul Linnaeus was of opinion thai II even the wood: regarding it not only as the 



ratable nourishment, but i I the vegetable what the brain and spinal marrow 



!tv to animal — the sour e il life In these opinions there ma] be someth as ol truth, but 



they have all the common fault ol : pith either too little or too much. Mr Land 



■ i ,i new opinion on I rdingU as being the^seatol tin- irritability of the 



lea 

 tin 



Mill 



been thought to be increased from the circumstance ol it- seeming to b ilj of a temporary use in the 



tation, bj it- disappearing in the aged trunk. Hut although it is thus only temporary 



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



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



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



annually made to perennials. The pltn, 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 he an organ 

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



the tip, „ , , , ■ ., 



1578 The generation of the layer of wood in woody plants, or of the parts analogous to wood in the rase 

 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 developemenl by 

 means of the introsusception and assimilation of sap and proper juice: but il we suppose them to tie 



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



can on ij i ,,,. the resull ol operations that have been so long continued as to present an effect cognisable 

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



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

 virtu encj of the vital principle operating on the proper juice, always make their appearance at 



la-t in a uniform ami 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 plints; a 

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

 and woody flbre 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 Uie 

 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 hark, wood, 

 and pith, as in the shoot of the former year; and in diameter b) 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 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. 



157 1. 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 l)u Ilainci 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 lias 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 del iching a ring of bark above and below it, the wood of the insulated portion that is above the 

 leaf is not augmented : tins 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 augment ition 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 

 lir-t 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 building, 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. 



\o~C. But how is the formation of the wood that note occupies the place of the pith to be accounted for? 

 It appears thai 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 tilling 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, 1 "the phenomena which took place at the period of 

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

 ot the fibres which were seiit 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 tibres. Hence arises another 

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



