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membranes or cell-walls being absorbed; and at the same time the membrane which encloses the united cells thickens, by deposits taking place in its interior. Thus vessels are formed, which are closed tubes tapering to both extremities, and a substance called vascular tissue,1 which is less soft and succulent than cellular tissue, and often becomes

very tough and hard. Vascular tissue is generally found wherever strength is required, as in stems, the ribs of leaves, and the shells of nuts. A most important kind of vascular tissue is woody tissue, which in shrubs and trees becomes compacted into wood, the vessels being filled up by the deposits which take place in them, so that great solidity is attained. Not only is the wood of many trees of great value to man, but woody tissue, which never becomes compacted into wood, is also of much importance, as affording the fibrous materials of which cordage and clothing are made. Another kind of vascular tissue consists of spiral vessels, which are tubes of extreme fineness, sometimes not more than one three-thousandth part of an inch in diameter, coiled up within the young shoots and leaf-stalks of plants. These spiral vessels may be pulled out to a great length. If a young shoot or leafstalk of a geranium be broken, and the parts gently pulled asunder, the spiral vessels will be seen, delicate as the threads of a cobweb. Their great number, however, and their closely compacted coils, add to the strength of the shoot or leaf-stalk, whilst they contribute also to the circulation of its sap.

Organs. The organs of which plants consist are divided into two classes -Nutritive Organs and Reproductive Organs; the former being those which are essential to the life of the individual plant, the latter those which have for their purpose the perpetuation of the race. The nutritive organs are the root, stem, and leaves; the reproductive organs are the flowers and all organs concerned in the production of seeds.

Nutritive Organs of Plants.

The Root. The root not only serves the purpose of drawing nourishment from the soil, but also that of fixing the plant in its place, and it has often been remarked with admiration that trees growing in exposed situations send out more numerous roots than those which grow in sheltered places, as if to anchor themselves more firmly, and to secure themselves against storms.

Roots are often branched, and the branching is repeated again and again, particularly in hard and woody roots, which are divided at last into fine fibres. Softer roots are often quite unbranched, as in the hyacinth. The extremities of roots consist of loose cells, forming a minute spongy

1 From Latin vasculum, a little vessel.

mass called a spongiole or little sponge, and by these cells the fluid nourishment of the plant is imbibed. Animal and vegetable membranes have the property of allowing fluids to pass through them, not by any special pores, but at all parts, through minute interstices in their whole structure; and when the membrane is very thin, as that is which forms the cells of a spongiole, and the fluid on one side of it differs considerably in density from that on the other, an exchange pretty rapidly takes place by what is called osmotic action. This may be exemplified by filling a bladder with brine from salted meat, and placing it in a vessel full of pure water, when much of the salt of the brine will soon be found to pass through the bladder into the water by which it is surrounded. The animal substances contained in the brine will, however, remain in the bladder, which will receive a quantity of water greater than that of the substances which have passed out. Osmotic action is carried on to a great extent in nature, the cells both of animals and plants thus imparting their contents to those which are in contact with them, so that a motion of fluids is kept up even where there are no proper veins or tubes, and changes are at the same time effected in the fluids themselves. And thus it is that the roots of plants take in nutriment, whilst they are also continually giving forth substances which the plant may be said to desire to be quit of, and the continual growth of the root pushes forward the spongiole into a new place, where it finds fresh soil. The soil in which a plant grows therefore undergoes a twofold change, being deprived of the substances most beneficial for the nourishment of the plant, and more or less filled with others which are specially unsuitable to it; but these are often very suitable for other plants; and thus the farmer finds a rotation of crops more advantageous than a continual repetition of the same kind of crop on the same land.

The roots of many plants extend
Many plants also send forth roots

Roots are covered with a bark like that of stems; and the roots of some plants, if by accident or design exposed to the air, produce buds from which shoots or branches proceed. underground and send up shoots. from the stem or branches, when these are brought into contact with the soil, or partially buried in it; and advantage is taken of this to propagate some kinds of trees and shrubs by cuttings, which soon produce roots when planted in moist earth, or by layers, which are branches bent down and partially covered with earth, but not separated from the parent plant till the new roots are formed. Some plants, in their ordinary growth, send forth roots from the stem or branches, which, as they elongate, seek the ground, and form new stems or props to the stem, adding also to the supply of nourishment from the soil. A remarkable

1 From Greek osmos, impulsion.

example of this is seen in the banyan-tree of India, from the widespreading branches of which roots descend straight to the ground, so that a vast canopy of branches and leaves is formed, supported by numerous

[graphic][merged small]

tall columns. A banyan-tree has been described having no fewer than 350 stems equal to large oaks, and more than 3000 smaller ones, covering sufficient to contain 7000 persons.

a space

Tubers and Bulbs.-Some soft roots swell very much in favourable circumstances, storing up in their cells nourishment for the future wants of the plant. The swelling takes place in various forms. The turnip and the carrot afford instances of two of the most common forms. The tubers 1 of the potato are not properly parts of the root, but short and thick subterranean stems, the eyes being buds ready to send up shoots into the air in the spring of the following year, and the substance of the tuber being a provision for the nourishment of these shoots, until they become independent of the parent plant. Bulbs also, as those of the onion, the lily, and the hyacinth, are not really parts of the root, although we familiarly speak of such plants as bulbous-rooted, and of the potato and the Jerusalem artichoke as tuberous-rooted. A bulb is, in fact, a subterranean bud, and the true stem is the very short hard part at its base from which the roots spring.

Stems. Some plants have their stems entirely underground; not only those which have extremely short stems, and are commonly described as stemless, but some also in which the stem extends to a considerable length. A root-like stem, creeping underground, or along the surface of the

1 Latin, 'a swelling,' from tumeo, to swell.

ground, sending down roots into the soil, and sending up shoots into the air, is called a root-stock. Examples may be seen in the yellow iris and in Solomon's seal. Whilst some plants have strong and erect stems, capable of sustaining all the weight of their branches and leaves, the stems of others are slender and weak, trailing along the ground, or depending upon other plants for their support. Of those which seek the support of other plants, some are provided with tendrils, by which they lay hold of them, and others twine around them. The pea is an instance of a climbing plant, and its tendrils coil in an admirable manner around the branches to which it clings. The scarlet runner is a twining plant: it has no tendrils, but its long stem twines round any pole or branch with which it meets. It is remarkable that twining stems always twine in a particular direction, some from right to left, others from left to right. The scarlet runner twines from right to left, the hop and the honeysuckle twine from left to right. Climbing and twining plants greatly abound in tropical countries, and contribute much to the prodigious density of vegetation in the forests, some of them ascending to the tops of the most lofty trees, and falling down again from their high branches, rich in foliage, and often also in flowers of the greatest beauty, or binding tree to tree, so that the forest becomes impenetrable.

Soft stems are called herbaceous, and the plants to which they belong are called herbaceous plants, in contradistinction to trees and shrubs, which are woody plants. Herbaceous stems are often annual-that is, their existence is limited to a single year. They rise from the ground in spring, and die on the approach of winter, even although the root may survive. Woody stems subsist during the whole life of the plant.

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Buds and Branches.-Stems, as they grow, produce buds. The stem always terminates in a bud; and in some plants, as in the greater number of palms, this is the only bud. In this case, there are no branches, what are often spoken of as the branches of palms being really leaves of extraordinary magnitude. Branches proceed from lateral buds. The buds of some plants arise only from certain well-marked points of the stem, called nodes or knots, which appear as joints, of which a strongly marked example is found in the bamboo; and although this cannot be so clearly observed in other plants, yet in all a certain rule is followed, so that the buds do not appear irregularly scattered, but arranged according to a definite method. Thus the arrangement of leaves is different in different plants; in one they are in pairs on opposite sides of the stem, in another they spring from opposite sides alternately, while in some they are found in whorls. They are very generally arranged in a kind of spiral manner around the stem. A similar diversity of arrangement

1 Greek nodos, a knot.

appears in the branches, which spring from the axils of leaves, every plant having its own peculiar and characteristic mode of branching, which, by giving diversity of aspect to trees, greatly contributes to the beauty of nature. Buds remain dormant during winter, the tender parts being protected from the frost by the scales with which they are covered. In spring, they begin to grow, when the sap ascends in the tree. Many buds, however, remain always dormant, especially on the lower parts of the stem and branches, the strength of the plant being directed to the production of branches and leaves in its upper and outer parts; but if a branch is cut off over above a bud which it has thus seemed to neglect, the bud at once begins to grow, and to send forth a shoot in place of that which has been removed. Some plants send forth branches of peculiar kinds. The runner of a strawberry is a kind of branch, although the plant is one of those in which the stem is scarcely developed. Such a branch, producing a bud at its extremity, serves for the extension of the plant over the surface of the ground, the bud soon sending down roots, and becoming capable of independent existence.

Structure of Stems.-There are great differences in the structure of stems, and, according to these, plants are arranged into different classes, which differ also very much in other important characters. Three modes of structure are distinguished, and stems are accordingly described as exogenous, endogenous, or acrogenous, terms which are also applied to the plants to which they belong.2

Exogenous Stems.-Exogenous stems are those which have a bark distinct and separable from the wood, and in the centre of the stem a pith, which sends out branches, called medullary rays,3

through all parts even of the hardest wood. The medullary rays keep up a connection between the pith and the soft growing part immediately under the bark, and are essential to the nourishment of the wood. Exogenous stems increase in thickness by new layers of vessels formed under the bark, which gradually harden into wood; and the layers produced year after year are very often so distinctly

marked, that the age of a tree may be pretty Fig. 74.-Transverse Section accurately computed by counting the number

of them exhibited in a transverse section of

[graphic]

of Exogenous Stem.

its stem. All the trees and shrubs to be seen in Britain, and very

1 The hollows where the branches unite with the stem, from Latin axilla, the armpit.

2 These terms are derived from the Greek words gennaō, to produce, exu, outwards, endon, inwards, and akros, highest; and they are employed as indicating the manner in which these different kinds of stems grow. 3 From Latin medulla, pith.

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