An introduction to physiological and systematical botany/Chapter 11
THE PROCESS OF VEGETATION. USE OF THE COTYLEDONS.
When a seed is committed to the ground it swells by the moisture which its vessels soon absorb, and which, in conjunction with some degree of heat, stimulates its vital principle. Atmospherical air is also necessary to incipient vegetation, for seeds in general will not grow under water, except those of aquatic plants, nor under an exhausted receiver: and modern chemists have determined oxygen gas, which is always an ingredient in our atmosphere, to be absorbed by seeds in vegetation. An experiment is recorded in the Philosophical Transactions, No. 23, of sowing Lettuce-seed in two separate pots, one of which was placed iu the common air, the other in the vacuum of an air-pump. In the former the young plants rose to the height of two inches, or more, in a week's time; in the other none appeared, till after the pot had been removed for a similar period into the air again. Seeds buried in the ground to a greater depth than is natural to them do not vegetate, but they often retain their power of vegetation for an unlimited period. Earth taken from a considerable depth will, when exposed to the air, be soon covered with young plants, especially of Thistles, or of the Cress or Mustard kind, though no seeds have been allowed to have access to it. If the ground in old-established botanic gardens be dug much deeper than ordinary, it frequently happens that species which have been long lost are recovered, from their seeds being latent in the soil, as I have been assured by Mr. Fairbairn of Chelsea garden, and others.
The integuments of the seed, having fulfilled their destined office of protection, burst and decay. The young root is the first part of the infant plant that comes forth, and by an unerring law of Nature it is sent downwards, to seek out nourishment as well as to fix the plant to the ground. In sea-weeds, Fuci, Ulvæ and Confervæ, it seems merely to answer the latter purpose. In the Dodder, Cuscuta, a parasitical plant, the original root lasts only till the stems have established themselves on some vegetable, on whose juices they feed by means of other roots or fibres, and then withers away.
The descent of the root, and the ascent of the leaf-bud in a contrary direction, are ingeniously explained by Dr. Darwin, Phytologia Sect. 9. 3, on the principle of the former being stimulated by moisture, and the latter by air, whence each elongates itself where it is most excited. This is perhaps more satisfactory than any mechanical hypothesis. In whatever position seeds happen to lie in the earth, the root makes more or less of a curve in order to shoot downwards. Mr. Hunter sowed a number of seeds in a basket of earth placed on an axis, by which their position was a little altered every day. After the basket had thus made two or three circumvolutions, the young roots were found to have formed as many turns in attempting to attain their natural perpendicular direction. Mr. Knight has ascertained, Phil. Trans. for 1806, that a strong centrifugal force applied to vegetating seeds will considerably divert the root from this direction outwards, while the stem seems to have a centripetal inclination.
The young root, if it grew in a soil which afforded no inequality of resistance, would probably in every case be perfectly straight, like the radical fibres of bulbous roots in water; but as scarcely any soil is so perfectly homogeneous, the root acquires an uneven or zigzag figure. It is elongated chiefly at its extremity, and has always, at that part especially, more or less of a conical or tapering figure.
When the young root has made some progress, the two lobes, commonly of a hemispherical figure, which compose the chief bulk of the seed, swell and expand, and are raised out of the ground by the ascending stem. These are called the Cotyledons. Between them is seated the Embryo or germ of the plant, called by Linnæus Corculum or little heart, in allusion to the heart of the walnut. Mr. Knight denominates it the germen, but that term is appropriated to a very different part, the rudiment of the fruit. The expanding Embryo, resembling a little feather, has been for that reason named by Linnæus Plumula; it soon becomes a tuft of young leaves, with which the young stem, if there be any, ascends. Till the leaves unfold, and sometimes after, the cotyledons, assuming their green colour, perform their functions; then the latter generally wither. This may be seen in the Radish, Lupine, Garden Bean, and various umbelliferous plants, in all which the expanded cotyledons are remarkably different from the true leaves. Such is the general course of vegetation in plants furnished with two cotyledons, or dicotyledones; but I have already mentioned a very distinct tribe called monocotyledones, having but one. These are the Grass and Corn tribe, Palms, the beautiful Orchis family, and many others. In these the cotyledon, or body of the seed, does not ascend out of the ground, and some have considered them as having no cotyledon at all. See Mr. Salisbury's paper in the Transactions of the Linnean Society, v. 7, on the germination of the Orchis tribe. We reserve more particular remarks on this subject till we examine the structure of seeds.
Some plants are reckoned by Linnæus to have many cotyledons, as the Fir and Cypress. But the germination of these differs in no respect from that of the generality of dicotyledones. Mr. Lambert, in his splendid history of the genus Pinus, has illustrated this peculiarity of structure in the Swiss P. Cembra; see our tab. 1. fig. 2. In the Dombeya, or Norfolk Island Pine, the cotyledons are very distinctly four: see fig. 3.
The preservation of the vital principle in seeds is one of those wonders of Nature which pass unregarded, from being every day under our notice. Some lose their vegetative power by being kept out of the ground ever so little a while after they are ripe, and in order to succeed must sow themselves in their own way, and at their own time. Others may be sent round the world through every vicissitude of climate, or buried for ages deep in the ground, till favourable circumstances cause them to vegetate. Great degrees of heat, short of boiling, do not impair the vegetative power of seeds, nor do we know any degree of cold that has such an effect. Those who convey seeds from distant countries, should be instructed to keep them dry; for if they receive any damp sufficient to cause an attempt at vegetation, they necessarily die, because the process cannot, as they are situated, go on. If, therefore, they are not exposed to so great an artificial heat as might change the nature of their oily juices, they can scarcely, according to the experience of Mr. Salisbury, be kept in too warm a place. By the preservation of many seeds so long under ground, it seems that long-continued moisture is not in itself fatal to their living powers; neither does it cause their premature germination, unless accompanied by some action of the air.
It is usual with gardeners to keep Melon and Cucumber seeds for a few years, in order that the future plants may grow less luxuriantly, and be more abundant in blossoms and fruit. Dr. Darwin accounts for this from the damage which the cotyledons may receive from keeping, by which their power of nourishing the infant plant, at its first germination, is lessened, and it becomes stunted and dwarfish through its whole duration.
Dr. Thomson of Edinburgh, in his System of Chemistry, vol. 4, 374, has published a very satisfactory explanation of one part of the functions of the cotyledons. Several philosophers have discovered that very soon after the seed begins to imbibe moisture, it gives out a quantity of carbonic acid gas, even though no oxygen gas be present. In this case the process stops here and no germination takes place. But if oxygen gas be present, it is gradually absorbed in the same proportion. At the same time the farina of the cotyledons becomes sweet, being converted into sugar. "Hence, it is evident," says this intelligent writer, "that the farina is changed into sugar, by diminishing its carbon, and of course by augmenting the proportion of its hydrogen and oxygen. This is precisely the process of malting, during which it is well known that there is a considerable heat evolved. We may conclude from this that during the germination of seeds in the earth, there is also an evolution of a considerable portion of heat. This indeed might have been expected, as it usually happens when oxygen gas is absorbed. So far seems to be the work of chemistry alone; at least we have no right to conclude that any other agent interferes; since hay, when it happens to imbibe moisture, exhibits nearly the same processes."
I conceive the evolution of this heat may powerfully further the progress of vegetation by stimulating the vital principle of the embryo, till its leaves unfold and assume their functions. It is necessary to observe, that the above process equally takes place, whether the farinaceous particles be lodged in the bulk of the cotyledons themselves, or compose a separate body called by authors the albumen, as in grasses and corn.
- As may be seen by marking the fibres of Hyacinth roots in water, or the roots of peas made to vegetate in wet cotton wool.
- This is also the opinion of M. de Saussure, Recherches Chimiques sur la Végétation, p. 16.