13. 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; but it is an inherent property with which nature has endowed seeds, in order to insure the young parts, when first 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.

14. 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 developement 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 enclosed 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.

15. 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.

16. 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 sort of circulation to be established, by means of which a communication is maintained between the more remote parts of the embryo.

17. 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 (Alsine media) and Groundsel (Senecio vulgaris) will germinate at a temperature but little above 32° Fahr.

18. 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 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.

CHAP. II.

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.

19. 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 seedcoats into the earth.

20. 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.

21. For this reason, the extreme points of the roots are exceedingly delicate, and are injured by very trifling causes; they, moreover, as all newly formed vegetable matter is extremely hygrometrical, have the power of absorbing, with rapidity, any fluid or gaseous matter that may be presented to them. On this account they are usually called spongelets.

22. In the roots of ordinary exogens, when the tissue is very young, the spongelet (fig. 1. a) con