CHAPTER IX.

OF THE SEED.

THE action of the seed is confined to that phenomenon which occurs when the embryo that it contains is first called into life, and which is named germination.

Excepting at this time the seed is an inert mass, often containing nutritious matter, or some of the secretions of the species, and covered with a skin, which is in many cases so efficient a protection to the young embryo, that the seeds are enabled to resist, not only, as is well known, the powerful action of the gastric juice, after having been taken into the stomach of animals, but even that of exceedingly high temperatures. Spallanzani has stated, that he caused seeds to germinate after immersion in boiling water; and a case is mentioned by Duhamel, in which seeds retained their vitality after an exposure to 235° of Fahrenheit. These are, however,

rare cases.

The earliest indication of germination consists in the parts of the seeds swelling, in consequence of the absorption of water by their cuticle, and a chemical change taking place in the nature of its juices. This is said to depend upon a loss of carbon, and an addition of oxygen, without which latter in abundance, it is believed that seeds cannot germinate at all. It is this which causes the starch of barley to be converted into sugar in malt. For this reason it has been recommended, that muriatic acid, chlorine, and various metallic oxides, should be used in inducing the rapid germination of seeds. Humboldt, Willdenow, and others, have declared their use to be advantageous. Link says the same; others deny the utility; and I believe that their action is very doubtful.

It is well known, that seeds will not germinate in the light. This is caused by light decomposing the carbonic acid gas, expelling the oxygen, and fixing the carbon, whence all the

parts become hardened; a condition under which vegetation cannot proceed.

If seeds are sown as soon as they are gathered, they generally vegetate at the latest in the ensuing spring; but if they are dried first, it often happens that they will lie a whole year or more in the ground without altering. This character varies extremely in different species: the power of preserving their vitality is also extremely variable; some will retain their germinating powers many years, in any latitude, and under almost any circumstances. Clover will come up from soil newly brought to the surface of the earth, in places in which no clover had been previously known to grow in the memory of man. Many of the rarest plants in our gardens have been raised from old seeds, taken off specimens in herbaria: others perish so soon, that a few days' exposure is sufficient to destroy them. This is particularly the case with such as contain much oil.

As those conditions which are necessary to the germination of seeds are, heat, moisture, and darkness, it follows, that, in order to prevent this occurring, these three conditions are, if possible, to be obviated. Thus, in packing seeds for travelling to a long distance, it is found, by experience, that no mode is so suitable for their conveyance as being packed loosely in coarse canvass bags, hung to the ceiling of the cabin of a ship; where they are exposed to light and air, and where they are protected from damp: this is much better than enveloping them in wax, or mixing them with sugar, as has been sometimes done; both pernicious practices. It has been thought, that if seeds were mixed with charcoal, as their carbon would, under such circumstances, always be in excess to their oxygen, their vegetation might be safely suspended. But it has not been found, from experience, that any practical advantage arises from this method; seeds perishing in charcoal as quickly as in most other media. The best material is the English coarse brown paper, made from old tarred rope, in which a large quantity of tar is incorporated. No material will preserve seeds so long a time as this; while cartridge paper offers them no protection whatever.

The germination of seeds may be retarded by excluding them from light, and surrounding them with moist earth, rammed very hard: mango-seed, packed in hard clay, will thus travel safely from the West Indies; it was thus that the Araucaria Dombeyi was first brought to England from Chili; and many other seeds, which cannot otherwise be transported will live a long time under such circumstances.

As soon as the chemical changes now spoken of have taken effect, the embryo swells and bursts its envelopes, protruding its radicle, which pierces the earth, deriving its support at first from the cotyledons or albumen, but subsequently absorbing nutriment from the earth, and communicating it upwards to the young plant. The manner in which the embryo clears itself from its integuments differs in various species; sometimes it dilates equally in all directions, and bursts through its coat, which thus becomes ruptured in every direction; more frequently the radicle passes out at the hilum, or near it, or at a point apparently provided by nature for that purpose, as in Canna, Commelina, &c. If the radicle has a coleorhiza or rootsheath, this is soon perforated by the radicle contained within it, which passes through the extremity; as in grasses, and most monocotyledonous plants. The cotyledons either remain under ground, sending up their plumula from their centre, as the oak; or from the side of their elongated cauliculus, as monocotyledons; or they rise above the ground, acquire a green colour, and perform the ordinary functions of leaves, as in the radish and most plants. In the mangrove germination takes place in the pericarpium before the seed falls from the tree; a long thread-like radicle is emitted, which elongates till it reaches the soft mud in which such trees usually grow, where it speedily strikes root, and separates from its parent. Trapa natans has two very unequal cotyledons; of these, the larger sends out a very long petiole, to the extremity of which are attached the radicle, the plumula, and the smaller cotyledon (Mirbel). Cyclamen germinates like a monocotyledon: its single cotyledon does not quit the seed till the end of germination; and its radicle thickens into a fleshy knob, which roots from its base (Mirbel). The Cuscuta,

which has no cotyledons, strikes root downwards, and lengthens upwards, clinging to any thing near it, and performing all the functions of a plant without either leaves or green colour.

In monocotyledons the cotyledon always remains within the seminal integuments; while its base lengthens and emits a plumula. In Cycas, which has two cotyledons, the seminal integuments open, and the radicle escapes. The cotyledons remain within the integuments, in the oak; but their petioles lengthen and liberate the plumula.

T

CHAPTER X.

OF COLOUR, SMELL, AND TASTE.

UPON this obscure subject Link has some good observations, the substance of which is as follows:

The ordinary membranes and juices of plants are destitute of colour they acquire colour by losing oxygen and acquiring carbon. Mucilage, sugar, starch, gluten, and other principles of that kind, are entirely devoid of colour or white. So also are vegetable membranes freed from foreign matter. The addition of oxygen makes no difference; for all the pure acids of plants are white: but azote seems to possess a great power of discolouration.

Young roots are white; but their epidermis becomes burned, as it were, by the surrounding atmosphere parting with its hydrogen in forming water, and thus acquires a brown or blackish cast. Some remarks upon this mode of staining bodies are to be found in the Annales de Chimie, vol. v. p. 80., by Fourcroy; and in volume vi. p. 238. of the same work, by Berthollet. In woody roots the colouring principle generally becomes red.

Young stems are green like the leaves: as they grow older they are scorched like the roots. Within the stem a colouring principle is formed, which seems to own its colour to extractive matter in itself colourless; leaves are green from loss of oxygen. If light is absent, and oxygen accumulates in the green parts, those lose their colour; but become green again upon exposure to light. Humboldt has some excellent remarks upon this in the Journal de Physique, vol. xl. p. 151. He has there demonstrated, that not only light, but every other agent which attracts oxygen from plants, will equally produce green colour.

When leaves become sickly, they admit an unusual quantity of oxygen, in consequence of which the green principle