Fig. 54, a, is seven-lobed, denticulate, peltate.

b. is a leaf of the Passiflora serrata; it is seven wobed; the divisions are lanceolate, denticulate, veined, glandular.

c, is a leaf of the Alchemilla hybrida, it is nine-lobed, denticulate, plicate.

Fig. 54.

parted, decompound.

Fig. 55, a, is a

leaf of the Jatropha multifida; it is manyparted; the di visions are pinnatifid.

b, is a leaf of the Helleborus niger; the leafets are sub-peti

(Peony ;) it is three

b, is a leaf of the Geranium pratense; it is seven-parted, laciniate. c, is a leaf of the Leontodon taraxacum, (dandelion ;) it is runcinate

Fig. 56.

米米

Fig. 57, a, is a trifoliate leaf; the leafets are ob-cordate, entire. b, is digitate, five-leaved; the leafets are lanceolate, denticulate. c, has the petioles stipuled and articulated; the leafets are oval and acuminate.

NOTE. It is recommended to the pupil to practise drawing the various leaves which are given for examples; and to collect as many specimens of leaves as possible.

Explain Fig. 58-Fig. 59-Fig. 30.

Leaves with respect to Magnitude.

Leaves vary in size, from the small leaves of some of the forest trees of our climate, to the spreading Palms and Bananas of the tor. rid zone. As we approach the torrid zone, the leaves increase in magnitude; we can, however, scarcely credit the reports of travellers, who say, that the Talipot-tree, in the Island of Ceylon, produces leaves of such size, that twenty persons may be sheltered by one sin-. gle leaf. Although this account may be exaggerated, there is no doubt of the fact, that the leaves of the torrid zone are of a wonderful size; and that whole families, in those regions, can make their habitations under the branches of trees. Here we see the care of a kind Providence, which, in countries parched the greater part of the year by a vertical sun, has formed such refreshing shelters. Mungo Park, in his travels in Africa, remarks upon the many important uses of palm-leaves; serving as covering to cottages, baskets for holding fruit, and umbrellas for defence against rain or sun. These leaves answer as a substitute for paper, and were so used by the eastern nations. Many suppose that the scriptures of the Old Testament were originally committed to palm-leaves.

The magnitude of leaves often bears no proportion to the size of the plants to which they belong. The oak, and other forest-trees, bear leaves, which appear very diminutive, when compared with those of the cabbage, or burdock.

Leaves, with respect to Duration, are,

Caducous, such as fall before the end of summer;

Deciduous, falling at the commencement of winter; this is the case with the leaves of most plants, as far as 30° or 40° from the equator Persistent, or permanent, remaining on the stem and branches amidst the changes of temperature; as the leaves of the pine and box;

Evergreen, preserving their greenness through the year; as the firtree and pine, and generally all cone-bearing and resinous trees; these change their leaves annually, but the young leaves appearing before the old ones decay, the plant is always green.

In our climate, the leaves are mostly deciduous, returning in autumn to their original dust, and enriching the soil from which they had derived their nourishment. In the regions of the torrid zone, the leaves are mostly persistent and evergreen; they seldom fade or decay in less than six years; but the same trees, removed to our climate, sometimes become annual plants, losing their foliage every year. The passion-flower is an evergreen in a more southern cli

mate.

Leaves with respect to Colour.

Leaves have not that brilliancy of colour which is seen in the corolla or blossom; but the beauty of the corolla, like most other external beauty, has only a transient existence; while the less showy leat remains fresh and verdant after the flower has withered away.

The substance of leaves is so constituted as to absorb all the rays of light except green; this colour is of all others best adapted to the extreme sensibility of our organs of sight. Thus, in evident accommodation to our sense of vision, the ordinary dress of nature is of the only colour upon which our eyes, for any length of time, can rest without pain.

But although green is almost the only colour which leaves reflect, the variety of its shades is almost innumerable.

Palm-leaves-Leaves not corresponding in magnitude to the size of the plant-Du ration-Colour of leaves-Different shades in the colour of leaves.

66 No tree in all the grove but has its charms, Though each its hue peculiar; paler some, And of a wannish gray; the willow such, And poplar, that with silver lines his leaf; And ash far stretching his umbrageous arm; Of deeper green the elm; and deeper still, Lord of the woods, the long surviving oak." The contrast between their shades, in forests, where different families of trees are grouped together, has a fine effect, when observed at such a distance as to give a view of the whole as forming one

mass.

A small quantity of iron, united to oxygen in the vegetable substance, and acted upon by rays of light, is said to give rise to the various colours of plants. If this theory is correct, the different shades of colour in plants, must be owing to the different proportion in which the iron and oxygen are combined.

To quote the words of a celebrated chemist "When Nature takes her pencil, iron is the colouring she uses."

LECTURE X.

ANATOMY AND PHYSIOLOGY OF LEAVES-THEIR USE IN THE VEGETABLE SYSTEM-APPENDAGES TO PLANTS.

LEAVES are compared to the lungs of animals; they are organs for respiring, perspiring, and absorbing. When leaves are wanting, as in the Prickly Pear, (Cactus,) the green surface of the stem appears to perform their office. If you will observe a dead leaf which has for some time been exposed to the action of the atmosphere, you may see its skeleton, or frame-work; this consists of various fibres, minutely subdivided, which originate from the petiole. This skeleton of the leaf may be examined to advantage, after boiling the leaves slightly, or rubbing them in water; the cuticle, or skin, easily separates, and the pulp, or cellular texture, may then be washed out from between the meshes of the veined net-work; thus, the most minute cords of the different vessels become perceptible, with their various divisions and subdivisions; these form what is called the vascular system. (See Fig. 61.)

* Cowper

Fig. 61.

Though in external appearance, the organs which compose the vascular system of plants, are analogous to the bones which constitute the foundation of the animal system, yet they are rather considered performing the office of veins and arteries. They are found to be

as

+ This idea coincides with the supposition, that the green colour of leaves is changed o brown by the loss of an acid principle; that the petals of flowers change from pur. ple to red by an increase of acid. The base of this acid is oxygen.

What is the cause of these different shades of colour? -the use of leaves in the veg etable economy-Skeleton of the leaf-Vascular system.

tubular; in some cases, this is ascertained by the naked eye; in oth ers, it may be beautifully illustrated by immersing the fibres of the leaf in some coloured liquid; on taking them out, they are found to contain internally a portion of the liquid; this experiment proves them to be transparent, as well as tubular.

The covering of this frame of the leaf is the cuticle, and a pulpy substance, called the parenchyma, or cellular texture. Some leaves contain much more of this than others, of course they are more pulpy and juicy; it is found, as its name cellular would denote, to consist of a mass of little cells, various in size in different leaves; in some, with the most powerful magnifiers, the cells are scarcely perceptible; in others, they may be seen with the naked eye. These cells are of important use in the secretion and communication of substances through the leaf; and may thus be considered as a kind of gland, having a communication with the vascular system.

The covering of the leaf, or the cuticle,* guards the vascular and cellular system from injury, and is the medium by which the leaf performs the important functions of absorbing nourishment, and throwing off such substances as are useless or hurtful. The cuticle is sometimes covered with downy, or hairy glands, which seem to afford security against changes of weather; such plants are capa ble of enduring a greater degree of heat than others. In some cases, the cuticle is covered with a transparent varnish, which preserves the plant from injury by too much moisture, and adds to the beauty of the leaves. The trees of Abyssinia and some other countries, which are subject to long rains, and continued moisture, are thus shielded from the injurious effects of the weather.

When the surface of the cellular tissue is more ample than the vascular net-work, the leaf is rugose, as seen at Fig. 62, a; where, for every swelling of the upper surface of the leaf, there is a correspondent depression of the under surface; the sage has a leaf of this kind. When the net-work exists, but the meshes are destitute of cellular tissue, the leaf presents the appearance of lattice-work, and is saiċ to be cancellated; the leaves of an aquatic plant of Madagascar (Hydrogeton fenestralis, Fig. 62, b,) are of this kind. Another exainple of this leaf is seen in the Claudea elegans, a species of marine Algæ, found in New Holland, (Fig. 62, c;) the veins are parallel to the sides, and cross the nerves.

The cuticle is sometimes called epidermis, from epi, around, and derma, skin, the true skin being not the outer covering, but a cellular substance beneath: thus, the thin skin upon the back of the hand, which so easily becomes rough, is the cuticle, or epidermis, (sometimes called the scarf-skin,) while the real skin is below.

How ascertained to be tubular and transparent-Cellular texture-Cuticle-Imoc tant office of the leaf-What is a rugose leaf?-What is a cancellated leaf ?-Exdiain Fig. 62.