Conversations on Chemistry Part 97

CAROLINE.

True; I wonder that I did not recollect that. The temperature and moisture required for the germination of the seed is then employed in producing the saccharine fermentation within it?

MRS. B.

Certainly. But, in order to understand the nature of germination, you should be acquainted with the different parts of which the seed is composed. The external covering or envelope contains, besides the germ of the future plant, the substance which is to const.i.tute its first nourishment; this substance, which is called the _parenchyma_, consists of fecula, mucilage, and oil, as we formerly observed.

The seed is generally divided into two compartments, called _lobes_, or _cotyledons_, as is exemplified by this bean (PLATE XV. Fig. 1.)--the dark-coloured kind of string which divides the lobes is called the _radicle_, as it forms the root of the plant, and it is from a contiguous substance, called _plumula_, which is enclosed within the lobes, that the stem arises. The figure and size of the seed depend very much upon the cotyledons; these vary in number in different seeds; some have only one, as wheat, oats, barley, and all the gra.s.ses; some have three, others six. But most seeds, as, for instance, all the varieties of beans, have two cotyledons. When the seed is buried in the earth, at any temperature above 40 degrees, it imbibes water, which softens and swells the lobes; it then absorbs oxygen, which combines with some of its carbon, and is returned in the form of carbonic acid. This loss of carbon increases the comparative proportion of hydrogen and oxygen in the seed, and excites the saccharine fermentation, by which the parenchymatous matter is converted into a kind of sweet emulsion. In this form it is carried into the radicle by vessels appropriated to that purpose; and in the mean time, the fermentation having caused the seed to burst, the cotyledons are rent asunder, the radicle strikes into the ground and becomes the root of the plant, and hence the fermented liquid is conveyed to the plumula, whose vessels have been previously distended by the heat of the fermentation. The plumula being thus swelled, as it were, by the emulsive fluid, raises itself and springs up to the surface of the earth, bearing with it the cotyledons, which, as soon as they come in contact with the air, spread themselves, and are transformed into leaves. --If we go into the garden, we shall probably find some seeds in the state which I have described--

[Ill.u.s.tration: Plate XV. Vol. II. p. 250

Germination.

Fig. 1 & 2.

A.B Cotyledons.

C Envelope.

D Radicle.

Fig. 3.

A.B Cotyledons.

C Plumula.

D Radicle.

Fig. 4.

A.B. Cotyledons.

C Plumula.

D Radicle.

Fig. 5. Apparatus to ill.u.s.trate the mechanism of breathing.

A.A Gla.s.s Bell.

B Bladder representing the lungs.

C Bladder representing the Diaphragm.]

EMILY.

Here are some lupines that are just making their appearance above ground.

MRS. B.

We shall take up several of them to observe their different degrees of progress in vegetation. Here is one that has but recently burst its envelope--do you see the little radicle striking downwards? (PLATE XV.

Fig. 2.) In this the plumula is not yet visible. But here is another in a greater state of forwardness--the plumula, or stem, has risen out of the ground, and the cotyledons are converted into seed leaves. (PLATE XV. Fig. 3.)

CAROLINE.

These leaves are very thick and clumsy, and unlike the other leaves, which I perceive are just beginning to appear.

MRS. B.

It is because they retain the remains of the parenchyma, with which they still continue to nourish the young plant, as it has not yet sufficient roots and strength to provide for its sustenance from the soil. --But, in this third lupine (PLATE XV. Fig. 4.), the radicle had sunk deep into the earth, and sent out several shoots, each of which is furnished with a mouth to suck up nourishment from the soil; the function of the original leaves, therefore, being no longer required, they are gradually decaying, and the plumula is become a regular stem, shooting out small branches, and spreading its foliage.

EMILY.

There seems to be a very striking a.n.a.logy between a seed and an egg; both require an elevation of temperature to be brought to life; both at first supply with aliment the organised being which they produce; and as soon as this has attained sufficient strength to procure its own nourishment, the egg-sh.e.l.l breaks, whilst in the plant the seed-leaves fall off.

MRS. B.

There is certainly some resemblance between these processes; and when you become acquainted with animal chemistry, you will frequently be struck with its a.n.a.logy to that of the vegetable kingdom.

As soon as the young plant feeds from the soil, it requires the a.s.sistance of leaves, which are the organs by which it throws off its super-abundant fluid; this secretion is much more plentiful in the vegetable than in the animal creation, and the great extent of surface of the foliage of plants is admirably calculated for carrying it on in sufficient quant.i.ties. This transpired fluid consists of little more than water. The sap, by this process, is converted into a liquid of greater consistence, which is fit to be a.s.similated to its several parts.

EMILY.

Vegetation, then, must be essentially injured by destroying the leaves of the plant?

MRS. B.

Undoubtedly; it not only diminishes the transpiration, but also the absorption by the roots; for the quant.i.ty of sap absorbed is always in proportion to the quant.i.ty of fluid thrown off by transpiration. You see, therefore, the necessity that a young plant should unfold its leaves as soon as it begins to derive its nourishment from the soil; and, accordingly, you will find that those lupines which have dropped their seed-leaves, and are no longer fed by the parenchyma, have spread their foliage, in order to perform the office just described.

But I should inform you that this function of transpiration seems to be confined to the upper surface of the leaves, whilst, on the contrary, the lower surface, which is more rough and uneven, and furnished with a kind of hair or down, is destined to absorb moisture, or such other ingredients as the plant derives from the atmosphere.

As soon as a young plant makes its appearance above ground, light, as well as air, becomes necessary to its preservation. Light is essential to the development of the colours, and to the thriving of the plant. You may have often observed what a predilection vegetables have for the light. If you make any plants grow in a room, they all spread their leaves, and extend their branches towards the windows.

CAROLINE.

And many plants close up their flowers as soon as it is dark.

EMILY.

But may not this be owing to the cold and dampness of the evening air?

MRS. B.

That does not appear to be the case; for in a course of curious experiments, made by Mr. Senebier, of Geneva, on plants which he reared by lamp-light, he found that the flowers closed their petals whenever the lamps were extinguished.

EMILY.

But pray, why is air essential to vegetation, plants do not breathe it like animals?

MRS. B.

At least not in the same manner; but they certainly derive some principles from the atmosphere, and yield others to it. Indeed, it is chiefly owing to the action of the atmosphere and the vegetable kingdom on each other, that the air continues always fit for respiration. But you will understand this better when I have explained the effect of water on plants.

I have said that water forms the chief nourishment of plants; it is the basis not only of the sap, but of all the vegetable juices. Water is the vehicle which carries into the plant the various salts and other ingredients required for the formation and support of the vegetable system. Nor is this all; part of the water itself is decomposed by the organs of the plant; the hydrogen becomes a const.i.tuent part of oil, of extract, of colouring matter, &c. whilst a portion of the oxygen enters into the formation of mucilage, of fecula, of sugar, and of vegetable acids. But the greater part of the oxygen, proceeding from the decomposition of the water, is converted into a gaseous state by the caloric disengaged from the hydrogen during its condensation in the formation of the vegetable materials. In this state the oxygen is transpired by the leaves of plants when exposed to the sun's rays. Thus you find that the decomposition of water, by the organs of the plant, is not only a means of supplying it with its chief ingredient, hydrogen, but at the same time of replenis.h.i.+ng the atmosphere with oxygen, a principle which requires continual renovation, to make up for the great consumption of it occasioned by the numerous oxygenations, combustions, and respirations, that are constantly taking place on the surface of the globe.

EMILY.

What a striking instance of the harmony of nature.

MRS. B.