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interesting experiment. Some ascribe this phenomenon to the laws of gravitation, by which the root is attracted towards the centre of the earth ; others say that the radicle, stimulated by moisture, extends itself in the natural direction from which the moisture proceeds; and some imagine that the plant is endow. ed with a kind of instinct, similar to that which often appears in animals from their first moments of existence, leading the little duck to seek the water and birds to attempt to fly; but let us call this power by what name we will, or refer it to whatever secondary laws, we must after all attribute it to the will and design of Him, who gave the plant a principle of vi. tality. After the
root has made some progress, the cotyledons swell, and rising out of the ground form two green leaves, called seed leaves. You have no doubt noticed their appearance in the garden bean, when it first appears above the ground. When the plume developes leaves, these seed leaves, being no longer needed, they wither and decay.
You will recollect that the embryo or germ is composed of two parts, the radicle and the plume. The radicle, we have just seen, extends itself downwards. Soon after this part of the
germ has begun its downward course, the plume, (so called from its resembling a little feather,) rises upwards, and soon becomes a tuft of young leaves, with which the stem, if there is one, ascends.
“Some rye,” says a botanist,“ was planted in a good soil, and at the end of the second day its radicle was discernible. At the end of twenty-four hours the embryo had escaped from its integument. On the second day the fibres of the root had augmented, but the leaves had not appeared. On the fourth day the first leaf began to appear above the ground, at which time the colour was red. On the fifth day, it had grown to the length of an inch, and its colour was now green, and on the sixth day the second leaf had appeared.”
Rye, however, belongs to thạt class of plants whose seeds have but one cotyledon; this never rises above the ground to form a seed leaf. Seeds with but one cotyledon are chiefly composed of albumen, which performs the same office of nourishing the embryo during its germination, as do the cotyledons of dicotyledonous plants. In some monocotyledons is perceived under the albumen or white, a part called Vitellus, or the yolk ; this, like the albumen, is entirely converted into nourishment for the young plant; it may be seen in the seeds of grasses, and is conspicuous in the Indian corn.
Causes assigned for the downward course of the radicle-Seed leaves-I lume- Experiment with rye-Seeds with one cotyledon--Vitellus.
Fig. 90 represents a young monocotyledonous plant; at a is the cotyle. don; at b is the second leaf, which, in the example just given of the rye, appeared on the sixth day; at c is the primordial leaf, which at first envelopes and conceals the other leaves ;* at d are the several branches of the root, bearing their radicles, and at their base enveloped by a peculiar covering through which the extremities have
forced their way.t It has been obb
served, that moisture is essential to the germination of the seed. Earth, though not absolutely essential, is use. ful, as affording to the vegetable egg a favourable situation, where it may receive the influence of the various agents, which are to perform their offi. ces in the developement of its parts. It seems, too, not improbable that some of the constituent elements of earth may be absorbed by the germinating plant, and converted into nourishment. It is however sufficiently apparent that plants may vegetate without earth. The parasite grows upon the bark of other plants; many seeds vegetate in water, and some, if moistened and placed on cotton, or other supporting substance.
Air is essential to vegetation; der an exhausted receiver a seed will
not germinate, although possessing d
every other requisite. Seeds that become imbedded deeply in the ground,
do not vegetate, unless accidentally d
d ploughed up, or exposed to the contact d
of the atmosphere. Acorns which are supposed to have lain for centuries, have germinated as soon as they were raised sufficiently near the surface to
receive the influence of air. * This, the French botanists call the pileole. + This covering, Mirbel terms the coleorhize.
Explanation of Fig. 90--Earth useful to vegetation--Air essential to vegetation.
You will recollect that in the process of germination, oxygen gas unites with the carbon of the seed, and carries it off in the form of carbonic acid. Air furnishes that important agent, oxygen, which is the first moving principle of vitality.
Carbon constitutes the greater part of the substance of seeds; and this principle, being in its nature opposed to putrefaction, prevents seeds from rotting, previous to their being sown. Some seeds having an abundance of carbon, are capable of be. ing preserved for ages; while others, in which this element exists but in a small proportion, require to be sown almost as soon as ripe; and such as are still more deficient in carbon, lose their vital principle, before separating from the pericarp.
You can now understand that oxygen is important to ger. mination, on account of its agency in removing the carbon which held the living principle of the sced in bondage.
The absence of light is favourable to the germination of seeds; for light acts upon plants in such a manner as to take away oxygen by the decomposition of carbonic acid gas, and to de. posite carbon ; now this is just the reverse of the process re. quired in germination, where the carbon must be evolved and the oxygen in excess.
A certain degree of heat is necessary to germination. Seeds planted in winter, will remain in a torpid state ; but as soon as. the warmth of spring is felt, the embryo emerges into life. By increasing heat, seeds may be hastened in their vegetating pro. cess; 'thus the same seed, which with a moderate degree of heat would germinate in nine hours, may be brought to this state in six hours, by an increase of temperature. Too great heat destroys the vital principle ; thus corn which has been roasted could never be made to vegetate. The process of malting consists in submitting grain of a certain kind, (that of barley is most commonly used,) to a process which causes an incipient stage of germination ; this is done by moistening the grain and exposing it to a suitable degree of warmth ; as soon as germination commences, the process is stopped by increasing the heat. The taste of the grain is then found to have become sweetish. The term malt is given to grain which has been submitted to this process. When mixed with water it forms a sweet liquor ; and the fermentation of this liquor produces beer.
There is a great difference in plants as to their time of ger. minating ; some seeds begin to vegetate before they are sepa. rated from the pericarp.*
* In the month of January, on observing the seeds of a very juicy apple,
Oxygen an important agent--Carbon—The absence of light favourable to the germination of plants—Heat-Effects of too great heat exemplified in the process of malting--Malt--Season of germinating.
In the greater number of vegetables, however, there is no germination until after the opening of the pericarp and the fall of the seed. The time at which different species of seeds, after being committed to the earth, begin to vegetate, varies from one day, to some years. The seeds of grasses, and the grain like plants, as rye, wheat, corn, &c. germinate within two days. The cruciform plants, such as radish, and mustard, the leguminous, as the pea and bean, require a little more time. The peach, walnut, and peony, remain in the earth a year be. fore they vegetate.
All kinds of plants germinate sooner if they are sown imme. diately after being separated from their pericarps.
Most vegetables preserve their vital principle for years : some lose it as soon as they are detached from their pericarps. This is said to be the case in the coffee and tea. The seeds of some of the grasses, as wheat, &c. are said to retain their vital principle even for centuries. It is asserted that mosses, kept for near two hundred years in the herbariums of botanists, have revived by being soaked in water. An American writer* says, that “ seeds, if imbedded in stone or dry earth, and removed from the influence of air or moisture, might be made to retain their vegetative quality or principle of life for a thousand years." But he very rationally adds, “ life is a property which we do not understand; yet life, however feeble and obscure, is always life, and between it and death there is a distance as great as existence and nonexistence.”
The subjects upon which, in this lecture, we have been en. gaged, properly come under the head of vegetable physiology, a department of botany highly interesting, but too complicated in its nature to be, except in a very limited degree, presented to the mind of the youthful investigator. The physician finds in the vegetable organization striking analogies to the internal structure of the animal frame; to him the language of physio. logical botany is familiar, because it is borrowed from his own science. On the other hand, the botanical student, in learning the names and offices of the various internal organs of plants, is making no inconsiderable improvement in the knowledge of the animal economy, and will feel his curiosity excited to search into the mysterious organization of his own material frame.
which had been kept in a warm cellar, I saw that they were swollen, and that the outward coat had burst; examining one seed, by removing the tegument and separating the cotyledons, I saw by the help of a microscope the embryo, as if in a germinating state: the radicle was like a little beak; in the upper part or plume was plainly to be seen the tuft of leaves and the stem.
Time of germinating varies-Vital principle of fruits--Vegetable Physiology -Its language borrowed from animal physiology.
HAVING considered the organs of the plant, from the root to the seed, it might seem as if our inquiries into the vegetable substance were terminated ; but we have yet to investigate more minutely the internal texture of these various organs. Before commencing the study of botany, when you looked at the trunk of a tree, a little herb, or a leaf, you perhaps considered it as very simple in its structure ; you saw it only as one mass : but you now perceive that plants, like animals, are collections of fibres; that they have parts which are analogous to our skin, bones, flesh, and blood ; that they are living, organized beings, composed of solid and fluid parts; and are, like animals, the subjects of life and death.
Plants differ from animals in not possessing any of the organs of sense. . They can neither see, hear, taste, smell, nor touch. Some vegetables, however, seem to have a kind of sensibility like that derived from the organs of touch ; they tremble and shrink back upon coming in contact with other substances ; some turn themselves round to the sun, as if en. joying its rays. There is a mystery in these circumstances which we cannot penetrate, and it is not yet fully known at what point in the scale of existence animal life ends, and vege. table life commences. Some animals, like the sponge and corals, seem almost destitute of any kind of sensation, and yet they are ranked in the animal kingdom. On the subject of the distinctions and analogies between plants and animals, we shall dwell more fully hereafter.
Solid parts of Vegetables.
At present we have to consider the solid parts of the vegetable system; these are all composed of a membranous substance, which exists in every part of the plant, forming by various modifications, the different textures which the vegetable system exhibits. This membranous substance appears chiefly under two elementary forms : viz. Ist, that of cellular texture ; 2d, vascular texture.
Object of the 17th lecture--Plants analogous to animals-Difficult to determine where vegetable life commences--Solid parts of plantsMembranous substance under two forms.