Book I. 



INTERNAL STRUCTURE OF PLANTS. 



215 



bah 



tough and leathery membrane, or it is a crust of considerable thickness, forming a notable portion of the 

 bark, and assuming some peculiar shade of colour ; while in the leaves, flowers, and tender shoots, it is a 

 fine, colourless, and transparent film, when detached ; and when adherent, it is always tinged with some 

 peculiar shade, which it borrows from the parts immediately beneath it. 



1370. The pulp is a soft and juicy substance, constituting the principal mass of succulent plants, and a 

 notable proportion of many parts even of woody plants. It constitutes the principal mass of many 

 of the Fungi and f fici, and of herbaceous plants in general. Mirbel compares it to clusters of small 

 hexagonal cells or bladders, containing for the most part a coloured juice, and formed apparently of the 

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

 distinguished. 185 



1371. 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 colourless juice, or of cellular tissue and a parenchyma. 



1372. 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 distinguishable 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 {jig. 185.), 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. 186. a) and varied by bundles of longitudinal fibre (6). But the liber of Daphne Lag6tto is remarkable 



186 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 

 diflferent 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. 



1373. 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 sorts, concentric layers and divergent layers. 

 {fig. 185.) 



1374. 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 fourteen exceeded the twenty in thickness by one fourth 

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

 consistence 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 ; by which character, as well as by 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. 



1375. 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. 



1376. 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 fining up the interstices of the network. 



1377. 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 Aspidium i^llix-mas or in the 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. 



1378. Fibre, cellular tissue with or without parenchyma, and reti ulated membrane are 

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



P 4 



