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CHAPTER II.

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GERMINATION.

THE NATURE OF A SEED.-ITS DURATION.-POWER OF GROWTH.-CAUSES OF GERMINATION.-TEMPERATURE.— LIGHT. HUMIDITY.-CHEMICAL

CHANGES.

A SEED is a living body, separating from its parent, and capable of growing into a new individual of the same species. It is a reproductive fragment, or vital point, containing within itself all the elements of life, which, however, can only be called into action by special circumstances.

But while it will with certainty become the same species as that in which it originated, it does not possess the power of reproducing any peculiarities which may have existed in its parent. For instance, the seed of a Green Gage plum will grow into a new individual of the plum species, but it will not produce the peculiar variety called the Green Gage. This latter property is confined to leaf-buds, and seems to be owing to the seed not being specially organised after the exact plan of the branch on which it grew, but merely possessing the first elements of such an organisation, together with an invariable tendency towards a particular kind of development.

Under fitting circumstances a seed grows; that is to say, the embryo which it contains swells, and bursts through its integuments; it then lengthens, first in a direction downwards, next in an upward direction, thus forming a centre or axis round which other parts are ultimately formed. No known power can overcome this tendency, on the part of the embryo, to elevate one portion in the air, and to bury the other in the earth. It is an inherent property with which nature has endowed seeds, in order to insure the young parts, when first

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PHENOMENA OF GERMINATION.

called into life, each finding itself in the situation most suitable to its existence; that is to say, the root in the earth, the stem in the air.

The conditions required to produce germination are, exposure to moisture, and a certain quantity of heat; in addition, it is necessary that a communication with the atmosphere should be provided, if germination is to be maintained in a healthy state. A seed, when fully ripe, contains a larger proportion of carbon than any other living part, and so long as it is thus charged with carbon, it is unable to grow. The only means it possesses of ridding itself of this principle, essential to its preservation, but forming an impediment to its development as a new plant, is by converting the carbon into carbonic acid, for which purpose a supply of oxygen is necessary. It cannot obtain oxygen in sufficient quantity from the air, for it is cut off from free communication with the air by various means, either natural, as being inclosed in a thick layer of pulp, or in a hard shell or stone; or artificial, as being buried to a considerable depth below the surface of the soil. It is from the water absorbed in germination that the seed procures the requisite supply of oxygen; fixing hydrogen, the other element of water, in its tissue and thus it is enabled to form carbonic acid, which it parts with by its respiratory organs, until the proportion of fixed carbon is lowered to the amount suited to its growth into a plant.

It has been objected that the evidence adduced in support of this explanation is not conclusive; and that there is nothing to show that the hydrogen of decomposed water enters into new combinations or is fixed in tissue. But since no hydrogen is evolved during germination, it must necessarily be fixed or recombined after water has been decomposed. That this last phenomenon occurs is proved by the experiments of Edwards and Colin, as given in the Comptes rendus (vii. 922), and quoted in Lindley's Introduction to Botany (4th edit., II. 261 and 272).

But the formation and respiration of carbonic acid takes place most freely, though not exclusively, in darkness; if exposed to light, the seed again parts with some of its oxygen, and again fixes its carbon by the decomposition of its carbonic acid.

INFLUENCE OF WATER AND HEAT UPON GERMINATION.

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In addition to this, the absorption of water causes all the parts to soften and expand; many of the dry, but soluble, parts to become fluid; sap, or vegetable blood, to be formed; and a motion of fluids to be established, by means of which a communication is maintained between the more remote parts of the embryo.

Heat seems to set the vital principle in action, to expand the air contained in the numerous microscopic cavities of the seed, and to produce a distension of all the organic parts, which thus have their irritability excited, never again to be destroyed except with death. What degree of heat seeds find most conducive to their germination, probably varies in different species. Chickweed (Stellaria media) and Groundsel (Senecio vulgaris) will germinate at a temperature but little above 32° Fahr.

It has been imagined that ELECTRICAL ACTION also promotes the germination of seeds. Sir H. Davy found that seeds placed in the vicinity of the positive pole of a voltaic pile, germinated sooner than those near the negative pole; and judging from the known powers of electricity it was not unreasonable to expect, that, like light and heat, it would exert influence on the growth of vegetables. Professor Edward Solly, however, has shown, experimentally, by an extensive series of trials in the Garden of the Horticultural Society, that this is not so. Seeds of Barley, Wheat, Rye, Turnip, and Radish, were, in several different experiments, found to germinate with increased rapidity, when exposed to the influence of a feeble current of electricity of very low tension, and the plants not only came up sooner, but were more healthy than others; but, on the other hand, a number of experiments on other seeds had given quite opposite results, proving either that the germination of some seeds was retarded, whilst that of others was facilitated, by electricity; or, that the effects, observed in both cases, were merely accidental. Out of a series of fifty-five experiments on different seeds, twenty appeared in favour of electricity, ten against it, and twenty-five showed no effect whatever; and, on carefully counting the whole number of seeds up in the entire series, it was found that twelve hundred and fifty of the electrified, and twelve hundred and fifty-three of the non-electrified seeds had grown.

Germination being established, by the absorption and decomposition of water, and by the requisite elevation of temperature, all the parts enlarge, and new parts are created, at the expense

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INFLUENCE OF WATER AND HEAT UPON GERMINATION.

of a mucilaginous saccharine secretion which the germinating seed possesses the power of forming. With the assistance of this substance, the root, technically called the radicle, at first a mere point, or rather rounded cone, extends and pierces the earth in search of food; the young stem rises and unfolds its cotyledons, or rudimentary leaves, which, if they are exposed to light, decompose carbonic acid, fix the carbon, become green, and, by processes hereafter to be explained, when speaking of leaves, form the matter by which all the pre-existing parts are solidified. And thus a plant is born into the world; its first act having been to deprive itself of a principle (carbon) which, in superabundance, prevents its growth; but, in some other proportion, is essential to its existence.

CHAPTER III.

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GROWTH BY THE ROOT.

ROOTS LENGTHEN AT THEIR POINTS ONLY.-ABSORB AT THAT PART CHIEFLY. -INCREASE IN DIAMETER LIKE STEMS.-THEIR ORIGIN.-ARE FEEDING ORGANS.-WITHOUT MUCH POWER OF SELECTING THEIR FOOD.-NATURE OF THE LATTER.-MAY BE POISONED.-ARE CONSTANTLY IN ACTION. SOMETIMES POISON THE SOIL IN WHICH THEY GROW.-HAVE NO BUDS. -BUT MAY GENERATE THEM.

THE root, being the organ through which food is conveyed from the earth into the plant, is the part which is the soonest developed. Even in the embryo, at the earliest commencement of germination, it is the part immediately connected with the root, that first begins to move, by lengthening all its parts, and protruding itself beyond the seed-coats into the earth.

But as soon as this primitive lengthening of the root has taken place, and the upper part of the embryo, namely, the young stem, has begun to exist as a separate organ, the root changes its property, ceases to grow by a general distension of its tissue, and simply increases in length by the addition of new matter to its point. A root is therefore extended much in the same way as an icicle by the constant superposition of layer over layer to its youngest extremity, with this difference however, that an icicle is augmented by the addition of matter from without, while the root lengthens by the perpetual creation of new matter from within.

For this reason, the extreme points of the roots are exceedingly delicate, and are injured by very trifling causes; moreover, since all newly formed vegetable matter is extremely hygrometrical, they have the power of absorbing, with rapidity,

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