The Vegetable World: Being a History of Plants, with Their Botanical Descriptions and Peculiar Properties

-other by the length of surface (as shown in the vertical section) and the fleshy consistence of the subterranean leaves. We now have to consider the structure of stems in different

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kinds of vegetables. In order to arrive at a correct idea of their structure let us consider, first, the stem of forest trees; secondly, that of palm trees; thirdly, that of aborescent or tree ferns.

An acquaintance with the ligneous stems of forest trees is interesting in more than one respect. Nature has collected all her powers to give to trees the strength necessary to resist the dangers and the causes of destruction which threaten them. Their wide-

Fig. 31.—Subterr tem of Carex, with shoots for four years.

spread and bushy summits, the immense mass of foliage which they support, and the great height to which they attain at the end of their growth, expose them to the fury of tempests. Their trunks must be immovably solid, in order to brave all the violence

THE VEGETABLE WORLD.

32

Fig. 33.—Bulb of the Hyacinth.

Fig. 32.—Maize plant.

Fig. 36.—Vertical section of the Lily.

Fig. 35.— Bulb of the Lily.

of the winds. Nature has constructed them with the particular aim in view of resistance. Year after year, she accumulates in their interior successive layers of increasingly solid substance. In proportion as the vegetable increases in size and needs a more powerful support, the interior concentric rings, which by their combinations form the strong and compact tissues of our forest trees, are compressed, and more and more consolidated. In its origin —that is to say, at the moment when the young stem, just sprung

Fig. 37.—Section of a Crocus Bulb. Fig. 38.—Bulb of the Crocus.

up out of the ground, begins to rear itself in the air—nothing is observable in its interior except an abundance of pith, surrounded by its breathing vessels (trachee). But as the plant increases, new elements interpose between the pith and the bark; and when the trunk has lengthened and strengthened, it presents an internal structure complicated enough, and well calculated for resistance to all outward forces. A mere glance at the section of a log of tire-wood, informs us that the stems of forest trees present three D.

essential parts, namely, pith at the centre, surrounded by woody fibre, and exterior bark. Let us examine more closely each of these parts in an indigenous tree.

The pith forms a sort of column in the centre of the woody axis, as in Fig. 39, which represents an horizontal section of the trunk of the maple. In very " thick and old stems, the diameter of the pith appears very small, and for a long time it “® was even supposed that in the trunk of very

But itis not so. It is j asserted that accord- ¥_.. ing to exact measurement and observation the diameter retains \ perceptibly and inva- é = e riably the same pro-

Fig. 39.—Horizontal section of a Maple-tree. portion from the time when the young ligneous axis has begun to solidify, up to the period of its maturity. The pith is formed by a combination of cellules, or cells, to use the scientific term.

Cells are simple, primitive organs, which are present in every vegetable structure. It is a sort of sac or cavity, surrounded by walls of transparent membrane; a vegetable cell, in short, is well represented by a soap bubble. A cellular mass without intercellular spaces may be compared to an aggregation of soap bubbles, pressed against each other. The cavity is completely closed; sometimes it is empty, sometimes it is filled with vegetable matter.

Fig. 40 represents the transverse section of a cluster of young vegetable cells; they are, as we see, nearly circular in form. When they have become larger, they mutually compress each other, so that their form, at first nearly circular, becomes polyhedral, as represented in Fig 41. The pith of young trees, as represented in Fig. 40, is, as we have

said, an aggregation of cells, at first nearly circular, which become polyhedral from compression as the stem increases in size, and this is the medulla or medullary tissue of which the central column of

Fig. 40.—Polyhedral tissues of the Pith, Fig. 41.—Prismatic cells of the Pith.

most forest trees is formed. This medullary tissue is totally deficient in vital energy in its central parts.

Between the pith and the bark we find concentric zones, which bear the name of igneous layers, the aggregation of which form what is commonly called mood. If we examine the trunk of the oak, the apple, or the cherry-tree, a very sensible difference is observable between the innermost woody layers, which are of a darker colour and denser tissue than the exterior ones, which are, on the contrary, of a paler hue and softer texture. In Fig. 42, which represents a vertical section of an oak of eighteen years’ growth, the sap-wood (alburnum) is represented by the letter a, the wood by the letter B the bark by the letter E. The pith is in the centre, with the starry appearance, which, in the oak, it often presents. The medullary rays, to which we shall now return, are very ap-

Fig. 42.—Vertical section of the Oak of eighteen parentin this section. The years’ growth, name of sap-wood (alburnum) is given to the outside sie of wood, and that of heart- D2

‘wood (duramen) to the innermost ones. In some trees, and notably in those which are not hard grained, as the poplar, willow, and chestnut, the line of demarcation between the wood and the sap-wood is slightly marked. In hard woods, on the contrary, it is strongly defined. Thus, in ebony the heart-wood is of an intense black, whilst the sap-wood is white; in the J udas tree the heart is yellow and the sap-wood white; in the PAillyrea (mock privet) the heart is red and the auder, or sap-wood, white. Workmen who work in wood are well aware that the sap-wood is much less solid than the heart, and that the latter only ought to be employed for wood work. Examined in masses, the figneous layers are hardest at the centre; but (Fig. 43, woody fibres _ magnified) studied individually, each layer is more com-ii pact towards the exterior. Neither are all the layers of i; ]} an equal thickness, whether compared with each other,

| or in their several parts.

i!) The substance which prevails in the wood, and that

| which gives it its hardness, is the woody _fibre, represented

; in Fig. 43. This is an elongated cellular substance, terminating in a point at the two extremeties, as there represented. The walls of the cells of which it is composed

| are very thick, generally so thick that their interior

Hii} cavity is much reduced. This thickness, as well as the colouring of the fibres, varies with the different parts of wood, with the age of the stem, and even with the nature of the tree that is under examination. The woody fibres press end to end one against the other, and become 80

| entangled as to constitute what is called a fibrous tissue,

j very difficult to pierce when cut across, but, on the contrary, easy enough to divide when cut longitudinally. This ligneous fibre is not the only element composing

| wood. Cut transversely a branch of the vine (a plant in

Fiz.43, Which the substances we are going to speak of acquire 4 eeviiea considerable volume), and apply the eye to one end; if the branch is straight, you will see the light at the other end. Examine the surface of the section of the branch, either with the naked eye or with a magnifying glass, and it will be observed that it is perforated with a considerable number of small a rm et eee ete on aT gS 0s s ee pine Sect irs ee i ve J D. ee

holes, of unequal size. If you introduce a hair or a very fine thread into one of these openings, you will succeed in passing the thread to the other end of the branch. Continuous canals, therefore, exist in the interior of the vine branch. These canals, formed of a membrane peculiar to them, are the sap vessels.

If a portion of the transverse surface of a log of oak or elm is neatly cut, it will be observed that the inside edge of each ligneous zone presents a certain number of small holes, clearly perceptible to the naked eye, or at least with the help of a common lens: these are the orifices of rather voluminous vessels. In the centre of the ligneous zone the vessels are much smaller, and sometimes almost imperceptible. Examine the woed of the hornbeam, lime, or maple-tree, in the same manner, and it will be observed that the internal edge of the zone is not now occupied by large vessels, but is almost entirely riddled by the orifices of smaller and more evenly-sized vessels, which become indistinct towards ‘the external edge of each zone. These are called the lymphatic vessels.

What is the structure and function of these different vessels? They resemble a cylinder, with obstructions placed at intervals, more or less marked, having also transverse lines or diaphragms intersecting them at brief distances. Sometimes the remains of these diaphragms correspond in the interior of the vessel to these contractions and lines. In one word, these cylinders appear formed of cells, placed end to end, the partitions of which have been destroyed. Their exterior walls show certain points, rays, and net-work, presenting very beautiful forms, which result from the unequal thickness of the walls, inequalities which are the result of certain laws regulating their existence. Fig. 44 represents these vessels in the melon. From the peculiar appearance offered by their external covering, which