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are wanting, as in the Prickly Pear, (Cactus,) the green sur. face of the stem appears to perform the office of leaves. 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 slight. ly, 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; this forms what is called the vascular system. (See Fig. 42.) Fig. 42.
In external appearance the organs which compose
the vascular system are analogous to the bones which consti. tute the founda. Stion of the ani. mal system, but
are considered as performing the office of veins or arteries. They are found to be tubular; in some cases, this is ascertained by the naked eye; in others, beautifully illustrated by immersing the fibres of the leaf in some coloured liquid; on taking it 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-work of the leaf is the cuticle and a pułpy 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 glands, having a com. munication with the vascular system.
The covering of the leaf, or the cuticle,* guards the vascular * The cuticle is sometimes called epidermis, from epi, around, and derma,
Skeleton of the leaf-Vascular system-How ascertained to be tubular and transparent-Cellular texture-Cuticle,
system and cellular texture 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 capable 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.
Some of the uses of Leaves. Leaves perform a very important office, in sheltering and protecting the flowers and fruit; the fact of their inhaling or absorbing air, is thought to have been proved, by placing a plant under an exhausted receiver, permitting the leaves only to receive the influence of air; the plant remained thrifty in this situation for a length of time; but as soon as the whole plant was placed under the receiver, it withered and died.*
The upper surface of leaves is usually of a deeper green, and supposed to perform a more important part in respiration, than the under surface. The upper surface repels moisture; you may perceive upon a cabbage leaf after a shower, or heavy dew, that the moisture is collected in drops, but has no appearance of being absorbed by the leaf. It has been found that the leaves of plants, laid with their upper surface upon the water, wither almost as soon as if exposed to the air; although the leaves of the same plants, placed with their under surfaces upon the water, retain their freshness for some days. But few among the vegetable tribes are destitute either of leaves, or green stems, which answer as a substitute. The Monotropa, or Indian pipe, is of pure white, as if made of wax; the mushrooms are also destitute of any green herbage. It is not known in what man. ner the deficiency of leaves is made up to these vegetables.
The period in which any species of plant unfolds its leaves, is termed its Frondescence. Linnæus paid much attention to this skin; the true skin being not the outer covering, but a cellular substance beneath; thus the thin skin upon the hand which so easily becomes rough, is the cuticle, or epidermis (sometimes called the scarf-skin), while the real skin is below.
* I give this experiment on the authority of Barton; but although the respiration of leaves seems not to be doubted, this experiment may not be thought a. fair one; for it would seem very difficult, to place a plant under a receiver, with the leaves exposed to the air, without at the same time admitting any air into the receiver.
Important office of the leaf--Few plánts destitute of leaves--Frondescence
subject; he stated as the result of his investigations, that the opening of the leaf buds of the Birch tree, (Betula,) was the most proper time for the sowing of barley. The Indians of our country had an opinion, that the best time for planting Indian corn, was when the leaves of the White oak, (QUERCUS alba,) first made their appearance; or according to their expression, are of the size of a squirrel's ears.
One of the most remarkable phenomena of leaves, is their irritability, or power of contraction, upon coming in contact with other substances. Compound leaves possess this property in the greatest degree; as the sensitive plant, (MIMOSA sensitiva:) and the American sensitive plant, (CASSIA nictitans :) these plants, if the hand is brought near them, seem agitated as if with fear, but as plants are destitute of intelligence, we must attribute this phenomenon to some physical cause, perhaps the warmth of the hand, which produces the contractions, and dilatations of the leaves.
The effect of light upon leaves is very apparent, plants being almost uniformly found to present their upper surfaces to the side on which the greatest quantity of light is to be found. It has already been observed that plants throw off oxygen gas; but for this purpose they require the agency of light.
Carbonic acid gas is the food of plants; this consists of car. bon and oxygen, and is decomposed by the agency of light; the carbon becomes incorporated with the vegetable, forming the basis of its substance, while the oxygen is exhaled or thrown off into the atmosphere.
Many plants close their leaves at a certain period of the day, and
open them at another; almost every garden contains some plants, in which this phenomenon may be observed; it is par. ticularly remarkable in the sensitive plant, and the tamarind tree. This folding up of the leaves at particular periods, has been termed the sleep of plants; it may seem a singular term to apply to plants; but a celebrated botanist* remarks, “ this fold. ing up of the leaves may be as useful to the vegetable constitution, as real sleep is to the animal.” Linnæus was led to observe the appearance of plants in the night, from a circumstance which occurred in raising the Lotus plant; he found one morning some very thrifty flowers, but at night they had disappeared ; this excited his attention, and he began to watch the plants through the night, in order to observe the period of their unfolding. He was thus led to investigate the appearance of other plants in the night, and to observe their different manner of sleep. He found that some folded their leaves together,
* Sir J. E. Smith.
Irritability-Effect of light-Sleep of plants.
others threw them back upon their stems, and exhibited a va. riety of appearances. This phenomenon has been attributed to the absence of light. A curious experiment was made by a botanist, who placed the sensitive plant in a dark cave, at midnight, and then lighted up the cave with lamps ; the leaves which were before folded up, suddenly expanded, and when on the following day the lights were extinguished, the leaves again closed.
The period at which the leaves fall off is termed the Defoli. ation* of the plant ; about the middle of autumn, the leaves of all annual, and of many perennial plants, gradually lose their vi. gour, change their colour, and at length fall from their stems.
The « fall of the leaf” may be referred to two causes; the death of the leaf, and the vital action of the parts to which it is attached. If a whole tree is killed by lightning, or any sudden cause, the leaves will adhere to the dead branches, because the latter have not the energy to cast them off. The richness and variety of colouring exhibited about the end of autumn, by our groves and forests, is splendid beyond the power of the painter to imitate. Yellow, red, and brown, are the most common colours of the dying leaf; but these colours vary from the brightest scarlet, and the deepest crimson, intermixed with every shade of yellow, from the deep orange, to the pale straw colour.
Although we have said considerable upon leaves, yet we have merely touched upon the most important circumstances with re. spect to them. You will, perhaps, be induced to pay more at. tention than formerly to them, in their different stages ; from their situation in the bud, to their full growth and perfection; and will feel a new interest in their change of colour, when you understand something of the philosophy of this change; even the dry skeletons of leaves, which the blasts of autumn strew around us, may not only afford a direct moral lesson, but, indu. cing you to examine their structure, lead you to admire and adore the power which formed them.
Appendages to Plants. Plants have a set of organs, the uses of which are less apparent than those we have been considering; but we should not infer, because the design for which they have been formed is in some measure concealed from us, that they were made for no purpose, or exist by mere accident ; let us rather with humility,
* From de, signifying to deprive of, and folium, leaf.
Defoliation-Fall of the leaf-Concluding remark-Reflections upon the use of certain organs.
acknowledge that this blindness must be owing to the limited nature of our own faculties. It would be impious for us to ima. gine, that all the works of God which we cannot comprehend, are useless.
The organs to which we now refer are called by the gene. ral name of appendages; they consist of seven different kinds, Stipules, Prickles, Thorns, Glands, Stings, Scales, Tendrils, Pubescence, and Bracts. Fig. 43.
1st. Stipules, are membranous or leafy
scales, usually in pairs, b
at or near the base of the leaf or petiole. The stipules furnish charac. ters used in botanical distinctions. They are
various in their forms and situations, found in most plants, but sometimes wanting. In the garden violet, VIOLA TRICOLOR (Fig. 43, a, a), the stipúles are of that form called lyrate pinnatifid, while the true leaf (6), is oblong and crenate. The most natural situation of the stipules is in pairs, one on each side of the base of the foot. stalk, as in the sweet pea; some stipules fall off almost as soon as the leaves are expanded, but in general, they remain as long as the leaves.
2d. Prickles, arise from the bark; they are sometimes straight, sometimes hooked, and sometimes forked. They are usually found
upon the stem, as in the rose; but in some cases, they cover the petiole, as in the raspberry; in others, they are found upon the leaf or the calyx, and in some instances, upon the berry; as in the gooseberry.
3d. Thorns, seem to be a kind of short pointed stem, easily distinguished from prickles, as they grow from the woody part of the plant, while the prickle proceeds only from the bark. On stripping the bark from a rose bush, the prickles will come away with it, but let the same experiment be made with a thorn bush, and although the bark may be separated, the thorn will still remain projecting from the wood.
Different kinds of appendages-Stipules-Prickles--Thorns-Difference between the thorn and the prickle.