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with pores for exhaling and inhaling gases; and as they present to the air a more extended surface than all other parts of a plant, they are of great utility to the vegetable, by imbibing suitable nourishment, and throwing off such gases as would be useless or injurious.
We have seen how the bud is formed, and by what curious means the principle of vegetable life which it contains is preserved and protected through the cold and dampness of winter. In the spring, when the sun has turned his course towards the north, recrossed the equator, and is advancing towards the tropic of Cancer; the vegetable world, quickened by its influence, begins to awaken from its dormant state; the leaf buds expand, and soon bursting their envelopes, the green leaves come forth. The manner in which the leaf, before it expands, lies wrapped up in the scales of the bud, is called Foliation; this presents an interesting study, and is said to be sufficiently various, in different families of plants, to afford a mark of distinction between them.
In the buds of grasses the leaves are said to be rolled together; in the rose, one side of the leaf is doubled upon the stem; in the maple, the leaves are plaited or folded up like a fan; in the blue bell, imbricated, or laid over one another like tiles on the roof of a house; in the monk's hood they are bent downwards towards the petiole.
Some plants are destitute of leaves; they are then called Aphyllous, which term signifies wanting leaves.
In determining the species of plants, the leaves are much regarded; specific names are often given from some circumstance of the leaf, for example; the HEPATICA triloba is that species of the Hepatica, which has leaves with three divisions called lobes; the VIOLA rotundifolia is a species of violet with round leaves.
A knowledge of the various appearances presented by leaves, is of great importance; in order to become acquainted with all their varieties, considerable practice in the analysis of plants is necessary. Engravings will assist you in understand. ing the definitions, but you must chiefly consult nature. are many terms to express the varieties observable in leaves; we shall here explain some of the most important.
* Technically called stomas.
Utility of leaves to the whole plant-The period at which leaves appearFoliation-Aphyllous leaves-Leaves furnish specific characters.
Leaves considered with regard to the manner in which they succeed each other in different stages of the plant.
1. Seminal, leaves which come up with the plant when it first appears above the surface of the earth: as in the garden bean; these leaves are only the cotyledons or lobes of the seed, which, after nourishing the young plant, decay.
2. Primordial, leaves growing immediately after the seminal leaves, and resembling them in position, form, and size. The primordial leaf, according to the fanciful idea of a French botanist, is a sketch which nature makes before the perfection of her work.
3. Characteristic, leaves which are found in the mature state of the plant; or according to the idea above advanced, nature in them, perfects her design.
It is not always, however, that this process with regard to change of leaves takes place; as in many cases the proper, or characteristic leaf, is the only one which appears.
There are many terms to express the mode of insertion of the leaf; such as radical growing from the root (radix), cauline growing from the stem (caulis), &c.
To express the position of leaves we find the terms, opposite, alternate, &c.
The form of the leaf is expressed by various terms, borrowed from the names of different objects; as digetate (from digitus, the finger), &c. For the explanation of these different technical terms we must advise you to consult the vocabulary as often as you find those you do not understand. We will, however, illustrate some of the most common forms of simple leaves.
Orbicular, or the round leaf; the Nasturtion (Tropaeolum), affords an example of this kind (See Fig. 30, a); this is also
Leaves with respect to succession-Insertion of leaves-Position-FormOrbicular.
peltate, having its petiole inserted into the centre of the leaf, and thus resembling a shield.
Reniform (from the Latin ren, the kidney), or as it is sometimes called kidney-form; the Ground-ivy (Glechonia), has a leaf of this kind. (See Fig. 30, b;) it is crenate, or has a margin with scalloped divisions; ciliate, being fringed with hairs like eyelashes.
Cordate (from the Latin cor, the heart), or heart-shaped. (See Fig. 30, c;) this form resembles more the figure of a heart as seen in a pack of cards, than of a real heart; this figure represents a cordate leaf with an accuminated point, that is acute and turned to one side; the margin is serrated, or notched like the teeth of a saw; an example of this kind may be seen in the aster cordifolium. Fig. 31.
Ovate, obovate, oval; these are terms derived from the Latin ovum, an egg; suppose the figure at 31, a, to represent an egg; you observe that one end is broader than the other; now if to this broad end you add a petiole prolonging it into a mid-rib with some lateral divisions, you have, as at b, the representation of an ovate leaf. If the petiole were placed at the narrowest end, it would be an obovate leaf. An oval leaf (c), is when both the ends are of equal breadth. When the length is much greater than the breadth, the leaf is said to be elliptical, as at d. Fig. 32.
Lanceolate, this kind of leaf may be
seen in the peach tree; it
is represented in Fig. 32, a; this is an accuminated, or slightly accuminate, with a serrulated or slightly notched margin, at (b,)
may be seen the cleft stipules or appendages of the leaf. Linear, as the grasses and Indian corn, (Fig, 32, c,) represents a leaf of this kind; it is sheathing, or encloses the stem by its base, as may be seen at d.
Deltoid, from the Greek letter delta; this kind of leaf is represented at e, Fig. 32; the Lombardy poplar affords an example of the same.
Sagittate (from sagittus an arrow), or arrow-shaped leaf; this is represented at a, Fig. 33; the Sagittaria, an aquatic plant, affords an example of this leaf.
Acerose, or needle-shaped; this is represented at b, Fig. 33. Leaves of this kind are mostly clustered together, as in the pine; they are subulate or pointed like a shoemaker's awl; they are rigid and evergreen.
Trees with acerose leaves, are usually natives of mountainous or northern regions; any other kind of leaves would in these situations be overpowered by the weight of snow or the violence of tempests; but these admit the snow and wind through their interstices; their many points or edges, presented even to a gentle breeze, produce a deep solemn murmur in the forest; and when the storm is abroad and the tempest high,
"The loud wind through the forest wakes,
With sound like ocean's roaring, wild and deep,
Burns in describing such a scene, says; "this is my best season for devotion: my mind is wrapt up in a kind of enthusiasm to Him who, in the pompous language of the Hebrew bard, 'walks on the wings of the wind.""
Pinnatifid, may be seen at Fig. 33, d; leaves of this form are sometimes finely divided, like the teeth of a comb; they are then said to be pectinate.
Lyrate, differs from pinnatifid in having its terminating segment broader and more circular. (See Fig. 33, c.)
Palmate, or hand shaped (Fig. 34, a); one species of the pas sion flower (Passiflora cœrulia) affords a good example of this kind of leaf. The oblong segments like fingers, arise from a space near the petiole, which may be considered as resembling the palm of the hand.
Digitate, or fingered leaf (Fig. 34, 6), differs from the palmate leaf in having no space resembling the palm of a hand; but several distinct leafets arise immediately from the petiole, as may be seen in the Horse Chesnut.
Connate (Fig. 34, c); the bases of opposite leaves are united so as to appear one entire leaf.