Book I. 



INTERNAL STRUCTURE OF PLANTS. 



145 



620. The pith, as has been already shown, 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 longitudinal 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 containing a watery and colorless juice, or of cellular 

 tissue and a parenchyma. 



GEL The cortical 'layers, or interior and concentric layers, constituting the mass 

 of the bark, are situated immediately under the cellular integument, where such 

 integument 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 {fig. 30.), 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. 51. a), and varied by bundles of 

 longitudinal fibre (6). But the liber of daphne lagetto is remarkable 

 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 destroved 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 rege- 

 nerated. 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 oT the liber of the 

 graft and stock. 



622. 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. 50.) 



62i The concentric layers, which constitute by far the greater part of the mass of the wood, are suffi- 

 ciently conspicuous for the purpose of exemplification 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 con- 

 centric, 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 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 circum- 

 ference, 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, though in the 

 case of some plants, as in that of the poplar and lime-tree, this peculiarity of character is not very ap- 

 parent From the peculiarity of external character, however, which it possesses in general, it was at one 

 time thought to be a substance essentially different from that of the layers which it invests. The ancients, 

 whose phytological opinions were often very whimsical, supposed it to be something analogous to the fat 

 of animals, and intended perhaps to serve as a sort of nutriment to the plant in winter. But it is now 

 known to be merely wood in a less condensed state, being yet lighter and softer than the interior layers, 

 but acquiring strength and solidity with age. It does not, however, acquire its utmost degree of solidity 

 till after a number of years, as is plain from the regular gradation observable in the solidity of the different 

 layers. 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. 



624. 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, 



625. 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 ; the two sets of fibres being interwoven and interlaced 

 together, so as to form a firm and compact body in the matured layers ; and thus corresponding exactly 

 to the description given of them by Grew and Malpighi, in which the longitudinal fibres are compared to 

 the warp, and the transverse fibres to the woof of a web. 



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

 woody plants, is distinguished by a humber of notable and often insulated fibres passing longitudinally 

 throughout its whole extent, as* in the stipe of aspidium filix-mas, 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 spht 

 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. 



