Book I. 



INTERNAL STRUCTURE OF PLANTS. 



215 



hexagonal colls or bladders, containing for the most part a colored juice, and formed apparently of the 

 foldings and doublings of a fine and delicate membrane, in which no traces of organization are to 

 be distinguished. _ ^ 



1345. The pith is a soft and spongy, but often succulent substance, occupying the -^-^ ' 

 centre of the root, stem, and branches, and extending in the direction of their longitu- 

 dinal axis, in which it is enclosed as in a tube. The structure of the pith is precisely 

 similar to that of the pulp, being composed of an assemblage of hexagonal cells con- 

 taining a watery and colorless juice, or of cellular tissues and a parenchyma. 



1346. The cortical layers, or interior and concentric layers, constituting the mass of 

 the bark, are situated immediately under the cellular integument, where such integu- 

 ment exists, and where not, immediately under the epidermis ; or they are themselves 

 external. They are distinguishalile chiefly in the bark of woody plants, but particularly 

 in that of the lime-tree. They are composed of two elementary parts bundles of 

 longitudinal fibres constituting a network {fig. 227.), and a mass of pulp more or less 

 indurated, filling up the meshes. The innermost of the layers is denominated the 

 liber, and was used by the ancients to write on before the invention of paper. It is 



the finest and most delicate of them all, and often most beautifully reticulated 



{fig. 228 ), and varied by bundles of longitudinal fibre {b). But the liber of daphne lagetto is remarkable 



228 



a^ a b (t h 



beyond that of all other plants for the beauty and delicacy of its network, 

 which is not inferior to that of the finest lace, and at the same time so very 

 soft and flexible that in countries of which the tree is a native the lace of the 

 liber is often made to supply the place of a neckcloth. If the cortical layers are 

 injured or destroyed by accident, the part destroyed is again regenerated, and 

 the wound healed up without a scar ; but if the wound penetrates beyond the 

 liber, the part destroyed is no longer regenerated. Or if a tree is bent so as to 

 break part of the cortical fibres, and then propped up in its former position, 

 the fractured fibres will again unite. Or if a portion of the stem is entirely 

 decorticated and covered with a piece of bark, even from another tree, the two 

 different barks will unite. Hence the practicability of ascertaining how far the 

 liber extends. And hence also the origin of grafting, which is always effected 

 by a union of the liber of the graft and stock. 



1347. The ligneous layers, or layers constituting the wood, occupy the 

 intermediate portion of the stem between the bark and pith; and are 

 distinguishable into two different sorts, concentric layers and divergent 

 layers. {Fig. 227.) 



1348. The concentric layers, which constitute by far the greater part of the 

 mass of the wood, are sufficiently conspicuous for the purpose of exemplifica- 

 tion on the surface of a horizontal section of most trunks or branches, as on that of the oak and elm. 

 But though they are generally described as being concentric, they are not always strictly so. For they are 

 often found to extend more on the one side of the axis of the stem or branch, than on the other. Some 

 authors say the excess is on the north side, but others say it is on the south side. The former account 

 for it by telling us it is because the north side is sheltered from the sun ; and the latter by telling us it is 

 because the south side is sheltered from the cold; and thus from the operation of contrary causes alleging 

 the same effect, which has been also thought to be sufficiently striking and uniform to serve as a sort of 

 compass, by which the bewildered traveller might safely steer his course, even in the recesses of the most 

 extensive forest. But Du Hamel has exposed the futility of this notion, by showing that the excess is 

 sometimes on the one side of the axis, and sometimes on the other, according to the accidental situation 

 of the great roots and branches ; a thick root or branch producing a proportionably thick layer of wood on 

 the side of the stem from which it issues. The layers are indeed sometimes more in number on the one 

 side than on the other, as well as thicker. But this is the exception, and not the rule. They are thickest, 

 however, on the side on which they are fewest, though not of the same thickness throughout. Du 

 Hamel, after counting twenty layers on the one side of the transverse section of the trunk of an oak, 

 found only fourteen on the other. But the fourteenth exceeded the twenty in thickness by one fourth 

 part. But the layers thus discoverable on the horizontal section of the trunk are not all of an equal 

 consistency throughout, there being an evident diminution in their degree of solidity from the centre, 

 where they are hardest, to the circumference, where they are softest. The outermost layer, which is the 

 softest of all, is denominated the alburnum, perhaps from its being of a brighter white than any of the 

 other layers, either of wood or bark ; from which character, as well as from its softer texture, it is also 

 easily distinguished. It does not acquire its utmost degree of solidity till after a number of years ; but if 

 a tree is barked a year before it is cut down, then the alburnum is converted into wood in the course of 

 that year. 



1349. The divergent layers which intersect the concentric layers in a transverse direction, constitute also 

 a considerable proportion of the wood, as may be seen in a horizontal section of the fir or birch, or of 

 almost any woody plant, on the surface of which they present an appearance like that of the radii of a 

 circle. 



1350. The structure of the concentric layers will be found to consist of several smaller and component 

 layers, which are themselves composed of layers smaller still, till at last they are incapable of farther 

 division. The concentric layers are composed of longitudinal fibres, generally forming a network ; and 

 the divergent layers, of parallel threads or fibres of cellular tissue, extending in a transverse direction, 

 and filling up the interstices of the network. 



1351. The structure of the stem in plants that are purely herbaceous, and in the herbaceous parts of woody 

 plants, is distinguished by a number of notable and often insulated fibres passing longitudinally throughout 

 its whole extent, as in the stipe of apsidium filix mass, or leaf-stalk of the alder. These fibres, when 

 viewed superficially, appear to be merely individuals, but when inspected minutely, and under the 

 microscope, they prove to be groups or bundles of fibres smaller and minuter still, firmly cemented 

 together, and forming in the aggregate a strong and elastic thread, but capable of being split into a 

 number of component fibres, till at last you can divide them no longer. If the fibres of the bark are 

 separated by the destruction of a part, the part is again regenerated, and the fibres are again united, 

 without leaving behind them any traces of a wound. But if the fibres of the wood are separated by the 

 destruction of a part, the part is never regenerated, and the fibres are never united. 



SuBSECT. 3. Elementary or Vascular Organs. 

 1352. Fibre, cellular tissiie with or without parenchyma, and reticulated membrane , are 

 the ultimate and elementary organs of which the whole mass of the plant is composed. 

 If it is asked of what the elementary organs are themselves composed, the reply is, they 

 are composed, as appears from the same analysis, of a line, colorless, and transparent 

 membrane, in which the eye, aided by the assistance even of the best glasses, can discover 

 no traces whatever of organisation ; which membrane we must also regard as constituting 

 the ultimate and fundamental fabric of the elementary organs themselves, and, by conse- 

 quence, of the whole of the vegetable body. It has been asked by some phytologist* 



P 4 



