1839] 



FARMERS' REGISTER. 



206 



a great degree, upon the cellular pystem, and not 

 upon the fibro-vasciilar system ;" and according 

 1o both theories, this grows horizontally, the new 

 portions originating in the cellules of the bark. 

 There is another fact, which Mirbel has urged 

 against the second theory, and in support of the 

 first. "Suppose," says he, that a scion ofrobinia 

 inermis has been grafted ior a long time, say 30 

 years, upon a young stem of rohinia pseudacacia, 

 or common locust, cut off six or eight feet from 

 (he ground. According to the theory of Du Petit 

 Thouars, what has taken place during the thirty 

 years which have elapsed since the grafting? 

 The leaves of robinia inermis have each year 

 sent out roots, which, under the form of a layer 

 of wood, have insinuated themselves between the 

 wood and the bark of the robinia pseudacacia, ex- 

 tending quite to the earih. Thus the trunk has 

 successively acquired thirty layers of the wood of 

 robinia inermis, since they have been produced 

 by the leaves of that species ; and these layers, 

 one after an other, have been divided in the soil, 

 and spread in the I'orm of roots, which are also 

 those of robinia inermis. Now if we cause these 

 roots to develope buds, and thus form now stems, 

 we should, according to this theorj^, obtain stems 

 of robinia inermis. But such is not the fact. 

 These roots give rise to stems of the robinia pseu- 

 dacacia ; and consequently, their specific organi- 

 zation is Ihat which is peculiar to the latter spe- 

 cies." To this objection of Mirbel, an answer of 

 the same kind, as in the last case, may be given, 

 viz. : that the roots are really those of the robinia 

 inermis, so far as the fibro- vascular system is con- 

 cerned ; but that the specific power of originating 

 buds, lies in the cellular system, and both theo"^ 

 riee regard that as produced from the cellular 

 system of the original stump ol^ robinia pseuda- 

 cacia. 



The great objection to the first theor}', i. e. the 

 theory which regards the fibro-vascular svstem, 

 as organized matter deposited by the bark, is, that 

 it does not account for the growth of eniogens, 

 plants which have no regular bark ; or if we con- 

 sider the external covering of the stem as the 

 hark, which form their fibrous bundles, without 

 their having any connexion with that bark. As 

 has been before remarked, we can admit no ex- 

 planation which does not account for the growth 

 of endogens, as well as exogens ; the separate 

 portions of the fibro-vascular system being pre- 

 cisely similar in the two cases, excepting in the 

 one particular of their arrangement ; their origin 

 and mode of developement is presumed to be the 

 same. 



To the theory of Du Petit Thouars no such 

 objection can be urged; it accounts lor the irrowth 

 of both kinds of stem equally well. And here it 

 may be worth while to slate this theory, as modi- 

 fied by Prof Lindley, a littje more at large. "A 

 plant, in the first stages of its growth, is consider- 

 ed a mass of cellular tissue, capable of expansion, 

 and growth in all directions. When there is no 

 leafy system, growth is indefinite ; and not car- 

 ried on in any particular direction, and no woody 

 system is formed ; but when leaves are developed, 

 they send their organic fibres down through the 

 mass of cellular substance, and arrange them 

 sometimes in one way, sometimes in another, ac- 

 cording to the specific power of the plant. Upon 

 this supposition the stem of a woody plant will 



be composed of two essentially distinct systems ; 

 the one cellular, constituting the pith, the principal 

 part of the bark, and the medullary processes (if 

 in an exogen,) and capable of growth in all direc- 

 tions ; the other fibro-vascular, comprising the 

 wood and a part of the bark, and capable of grow- 

 ing longitudinally only." "The stem of a tree 

 has been quaintly, though not inaptly, compared 

 to a piece of cloth ; the cellular system represent- 

 ing the woof, and the fibro-vascular system the 

 warp." The observation which seems first to 

 have suggested this theory to Du Petit Thouars, 

 was his noticing in the case of the dracena, (a 

 plant used for forming hedges, in the Isle of 

 France, where he then residedj the manner in 

 which a bud vegetated upon a stem otherwise 

 dead, a thing which frequently happened, the bud 

 having no longer a series of vital systems, with 

 which it might combine, sent out undoubted roots 

 beneath the bark, instead of the Avoody fibres, 

 which it would have emitted under ordinary cir- 

 cumstances. And also, that in cases, when the 

 wood had died, the living buds formed a sort of en- 

 tangled sheath of roots, by the fibrous matter 

 which they sent down. 



In support of this theory, Profes.«or Lindley 

 urges the following facts. "If you take the branch 

 of a lilac {syringa vulgaris) when it is just 

 clothed with leaves, and while the bark will freely 

 separate from the wood, and peel it, you will find 

 ridges of tubes, passing down from each leafj 

 turning aside at every little obstacle, re-uniting 

 after the obstacle is passed, and together forming 

 a firm fibrous case to the subjacent wood of the 

 previous year. In other plants, the fibres may be 

 traced in like manner from the leaves; but instead 

 of running parallel with each other, they cross each 

 other diagonally, and form a sort of lozenge work. 

 This is wholly inexplicable upon the supposition 

 that the wood is deposited by the bark; but it is 

 clearly intelligable, if the wood is regarded as an 

 organic emanation from the leaves. Again, in 

 endogens, the woody fibres may be distinctly 

 traced into the leaves, Irom which it is plain that 

 ihey originate ; and in this case at least, it is man- 

 liest, that the woody bundles, cannot be organized 

 matter, deposited by the bark ; for they are in the 

 centre, whilst the bark is at the circumference." 

 In addition to this, we may mention the fact, that 

 when plants are propagated by means of slips, 

 the new roots always protrude from between the 

 outer layer of Avood and the bark, which is just 

 the situation from which they should grow, if thia 

 theory be the correct one. Du Petit Thouars at- 

 tributes to leaves and buds, a power similar to that 

 possessed by the seed, of sending a stem in one 

 direction, and a root in another. The root of the 

 leaf^ however, difiers from that of a seed, in being 

 intended to grow in the trunk of the plant produc- 

 ing it, and not in the earth. This theory fur- 

 nishes us with a very simple explanation of the 

 manner in which a bud or graft is enabled to live, 

 and incorporate itself with the stem ol' another 

 plant. In inserting a graft or bud into the stem 

 of a plant, we are placing it in as proper a situa- 

 tion lor it to grow, as we are a seed when we 

 place it in the earth. It will also show us the rea- 

 son on which certain practical rules to be observed 

 in grafting and budding, are tbunded. One of 

 these rules is, that we must make the bark of the 

 graft and that of the stock, coincide exactly witt; 



