Fig. 53, a, is a leaf of the Menispermum canadense; it is sub-orbicular, three-lobed, peltate.

b, is a leaf of the Passiflora biflora; it is two-lobed; the lobes are divergent.

C, is a leaf of the Passiflora incarnata; it is three-parted; the divisions are lanceolate, denticulate; the petiole glandular.

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

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

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

Fig. 54.

Fig. 55.

oled, mostly acuminate, denticulate, veined.

Fig. 55, a, is a

leaf of the Jatropha multifida; it is many

parted; the di visions are pinnatifid.

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

Fig. 56, a, is a leaf of the Paonia officinalis, (Peony ;) it is threeparted, decompound.

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.

a

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.

Fig. 57.

Explain Fig. 54-Fig. 55-Fig. 56-Fig. 57.

Fig. 60.

Fig. 60, at a, is cylindrical, and fistulous, as in the onion.

b, is a fleshy leaf, deltoid and dentate. c, a leaf which is sub-ovate, and bearded at the summit.

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. 60.

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 torrid 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 single 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 leaf 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-Duration-Colour of leaves-Different shades in the colour of leaves.

"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 as performing the office of veins and arteries. They are found to be

+ This idea coincides with the supposition, that the green colour of leaves is changed to brown by the loss of an acid principle; that the petals of flowers change from purple 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 vegetable economy-Skeleton of the leaf-Vascular system.