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evident stem or vessels; such plants are called cellular, in contradistinction to the stem-producing plants, which are vascular.

An exogenous plant produces, during its first year of growth, a ring or circle of vascular bundles between the pith and bark. During the second year a second ring is formed inside the previous one; and so on year after year. It is an outward-grower. The oldest, most mature and hardest wood is in the interior of such a stem, and its age can be determined by counting these annual rings or layers (Fig. 13, p. 216). An endogenous stem (Fig. 14, p. 217) exhibits no rings. Its vascular bundles are developed towards the interior, so that the oldest, most mature, and hardest, are outside, and the newest inside. It is an inward-grower. The ages of palms cannot be determined in the same way as our ordinary trees. Their growth in height, however, is uniform, and by this means their ages can be estimated. An acrogenous stem (Fig. 15, p. 217) increases by its summit; its growing point is carried upwards by the union of the bases of the leaves. It is a summit-grower. tree-fern stem exhibits the acrogenous structure.

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Exogenous stems in general have provision for lateral buds, and these form branches; while the endogenous and acrogenous stems have usually no such provision, and hence do not branch. The mode in which the branches are given off by exogens gives rise to the different habits of the cedar, the spruce, and the poplar. The nature of a landscape depends much on the mode in which the stems and branches of trees are developed.

The Leaves of Plants.

Leaf-buds are produced on stems and branches at regular intervals. There is a tendency to a spiral arrangement of buds. Leaves are placed alternately on the stem in a certain number of rows, according to the following series of numbers: 1, 2, 3, 5, 8, 13, 21, 34, &c., the sum of two succeeding numbers making up the following one. Thus there is a two-rowed leaf arrangement where the third leaf is situated directly above the first, the fourth above the second, and so on; a three-rowed arrangement, in which the fourth leaf is above the first, and the fifth above the second, and so on; a five-rowed arrangement, in which the sixth is above the first, the seventh above the second, and so on. Sometimes leaves are placed opposite to each other, and apparently

at the same level, and in that case the leaves in each set alternate with those in the sets next them. By this law of alternation the leaves are fully exposed to light and air, and a wise provision is made for the production of the rounded stem, and for the elaboration of the fluids which reach the leaves from the roots. The leaves consist of cells and vessels in the same way as the stem. The vessels are seen in the ribs and veins. In some leaves the veins form an angular network, and are said to be reticulated (Fig. 16). Such leaves are usually associated with a dicotyledonous embryo, and exogenous stem. In other instances the veins run parallel, and do not form angular meshes (Fig. 17).

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Such leaves are associated with monocotyledonous and endogenous plants. A third set of leaves have forked veins, and these are usually found in acotyledonous and acrogenous plants.

When a leaf consists of one piece, either undivided or divided, but not jointed to the stalk, it is called simple (Figs. 16 and 17). When it is composed of separate leaflets, which are articulated to

1 (Fig. 16.) Reticulated simple leaf, showing angular meshes produced by the veins. These veins consist of different kinds of tubes or vessels combined, while the spaces between them consist of cells. The midrib (1) is a continuation of the leaf-stalk. It gives off primary veins (2), and these divide into secondary veins (3).

2 (Fig. 17.) Simple parallel-veined leaf of a palm. There are no angular meshes in the leaf.

the stalk, it is compound (Fig. 18). Occasionally leaves are folded, and their edges become united so as to form pitcher-like appendages. There are openings in leaves called stomata (Fig. 19). These are opened and closed according to the moist or dry nature of the atmosphere. They exist chiefly on the lower surface of

FIG. 18.1

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leaves. Leaves have the power of absorbing and of exhaling fluids and gases. The watery fluid exhaled by leaves has a marked effect on the climate of a country. The presence of forests thus causes the climate to be more or less moist. The felling of forests occasionally makes a climate dry. Leaves in their green state absorb carbonic acid, and under the influence of light they give out oxygen gas. Thus while the breathing of man and animals, and the processes of combustion, are constantly sending into the atmosphere a large amount of carbonic acid gas, plants are taking up the noxious gas by means of their leaves, applying the carbon to important purposes in their economy, giving out oxygen gas. How wondrous are the adaptations of different parts of creation to each other! Plants are employed to prepare solid food for man and animals, as well as carbon for combustion, and at the same time to purify the atmosphere which living beings inhale. In the case of aquatic plants, we find that the power of giving out oxygen gas under the influence of the sun

and

1 (Fig. 18.) Compound leaf of melilot, consisting of three leaflets which are connected by joints to a common leaf-stalk.

2 (Fig 19.) Skin of the leaf of a lily, showing stomata s t. These consist of two cells surrounding an opening. This opening is closed in dry weather.

is very great. The green herbage floating on a pond has a decided effect on the purity of the water by decomposing noxious gases, and giving out a pure respirable air. If leaves are not exposed to light they lose their green colour, and become blanched. In this condition they give out carbonic acid. The blanching of the green parts of plants by non-exposure to light is resorted to by gardeners for the purpose of procuring tender and delicate vegetables. By this process the plant is prevented from forming woody matter, and delicate cells are alone produced. The blanching of asparagus, celery, sea-kale, and other culinary vegetables are familiar instances of this. By blanching, leaves not only lose their colour, but are also prevented from forming various secretions. A strong-smelling plant may in such circumstances become perfectly inodorous. After performing their functions, leaves fall off and decay. In some cases the leaves remain more or less permanently, and such plants are called evergreen. In our ordinary forest trees the leaves fall off, leaving scars on the stems and branches. Such leaves are called deciduous.

Movements are exhibited by some compound leaves. These movements appear to be connected with changes in the cellular swellings, which occur at the bases of the leaflets and of the leafstalks. Light and darkness have a marked influence on such leaves. During the night the leaflets are folded upon each other, so as to exhibit what Linnæus called the sleep of plants; while during the day the leaflets spread out and expand. In certain plants the leaf-movements are exhibited also under the influence of mechanical or chemical stimuli, as by touch or the contact of fluids or gases of a narcotic or irritant nature. Such plants are called sensitive, and may be illustrated by the Mimosa sensitiva and Mimosa pudica. The moving plant of India (Desmodium gyrans) is provided with small leaflets which are constantly jerking from one side to the other. Venus's fly-trap (Dionaea muscipula) has hairs on its leaf-blades, which, when touched, cause the leaves to fold. The motion of the sap of plants is influenced by a certain power of imbibition called osmose, as well as by the actions going on in the leaves. In ordinary trees, the fluids taken up by the cells of the roots, ascend through the newer woody parts of the stem, reach the leaves, are there exposed to the action of air and light, and finally return by the bark to the lower parts of the stem. This course has been determined by

wounding the stem at separate parts during the spring months, as well as by causing the plant to absorb some chemical substance in a diluted state, the presence of which at different heights in the stem can be easily detected by some other chemical re-agent.

Reproductive Organs of Plants.

The reproductive organs of plants consist of the flower and its parts. These generally arise from the point where a leaf, called a floral leaf or bract, comes off from the axis. Bracts may be the ordinary leaves of a plant, as in some of the common speedwells, or they may be coloured and modified leaves of various kinds. The arrangement of the flowers on the common stalk is called the inflorescence; and it has been divided into two kinds called de

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finite and indefinite. In the former, each floral axis ends in a single flower, and the expansion of the flowers on the different axes takes place from the centre to the circumference, or from above downwards as in gentian and spearwort. In the latter, the floral axis bears a congeries of flowers which expand from the circumference to the centre, or from below upwards, as in the hemlock

1 (Fig. 20.) Flower of common wallflower. The stalk of the flower is called the peduncle, a; the calyx, or first whorl c, consists of four leaves called sepals; the corolla, or second whorl P, is formed of four coloured leaves called petals; the stamens, or third whorl s, are next seen; and in the centre there is the fourth whorl or pistil.

2 (Fig. 21.) The essential organs of reproduction of the wallflower, with the calyx and corolla removed. The stamens s, consist of stalks or filaments bearing anthers which contain pollen; they surround the pistil which is in the centre. The pistil is formed by leaves called carpels, and consists of the ovary, short style, and a stigma at the top b. The place into which the parts of the flower are inserted r, is called the receptacle. Glands at the base of the stamen are marked g.

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