Page images

effect: it is, however, doubtful whether it ever happens in nature, that the act of germination takes place under conditions so simple as those; it is usually a more complicated phenomenon.

Water is the agent to which we are most in the habit of assigning the power of causing the growth of seeds; to air and heat they are generally exposed more or less, and it is by the addition of water that the two latter are popularly considered to be brought into active operation. According to De Candolle, it is a general property of seeds to absorb, during this period of germination, more than their own weight of water; but no regular proportions have been remarked, and it is probable that the respective power of different seeds depends upon the nature of the matter deposited in their tissue. The effect of water may be supposed to be that of softening the tissue, of enabling all the parts to distend, and of dissolving the soluble parts so as to render them fit to be taken into the circulation, as the young plant becomes capable of absorbing them.

Germination cannot take place in vacuo ; nor in an atmosphere of nitrogen or hydrogen, and still less in carbonic acid; or at least, if in this latter gas some traces of germination manifest themselves, they rapidly disappear: it can only occur in free oxygen.

Of this but a small proportion is really necessary; from što je, according to different observers. But 1 part of oxygen and 3 of nitrogen are the proportions which seem to be the most favourable, and this is not very different from the proportions in atmospheric air ; viz. 1 of oxygen and 4 of nitrogen. A too large dose of oxygen weakens the young plant, by abstracting its carbon too rapidly.

Experiments show that oxygen is not absorbed by the seed, but combines with its carbon, forming carbonic acid, which is thrown off. When a seed ripens, a considerable quantity of carbon is stored up in its tissue, apparently for the purpose of enabling it to “ maintain the unalterability” to which its preservation is owing. This superfluous carbon renders it scarcely soluble in water. To enable the parts to be sufficiently moistened, it is therefore necessary that the seed should be decarbonised by oxygen. This explains why Peas scarcely ripe will germinate much more rapidly than those which are fully matured; the former contain more pure water and less carbon. In fact, the effect of the abstraction, by oxygen, of the fixed carbon, is, to bring back the seed to the state in which it was, before it was provided with the means of remaining unchanged in a torpid state. The sweet taste of germinating barley is, in reality, what the seeds possessed before they were finally hardened. The destruction of oxygen, by the carbon of the seed, produces a sensible heat in germination, just as a similar cause produces a similar effect in flowers, when the fæcula of their disk is converted into sugar (see p. 331.). Hence the heat of masses of Barley which are made to germinate in darkness in order to become malt: and it can scarcely be doubted, that the change of the starch of that grain into sugar is chemically owing to the abstraction of a proportion of its carbon, and the addition of some other proportion of oxygen.

It has been asked, Whence comes the oxygen which, combining with the carbon of the seed, forms the carbonic acid expelled in germination ? The usual answer is, From the air; and it is necessary that seeds should have access to the atmosphere in order to germinate. But Messrs. Edwards and Colin have shown, by recent experiments, that the oxygen of which germinating seeds make use is obtained by the decomposition of water, and not necessarily from the air. These physiologists placed Beans in water, under such circumstances that they were completely cut off from access to the air. The Beans disengaged bubbles of air from their sides in great abundance for the space of 4 days, a part of such air collecting in a receiver, but the greater part dissolving in the water. This air consisted chiefly of carbonic acid; there was also a trace of oxygen, and a small quantity of what appeared to be nitrogen. The hydrogen left after the decomposition of the water appeared to be absorbed by the seed, either wholly or in great part. This proof of the decomposition of water by the vital energies of the seed is justly stated, by the authors now quoted, to be a fact of the first importance. (Comptes rendus, vii. 922.)

It also appears that the carbon of seeds is lost, not only by the formation of carbonic acid, but by the production of acetic acid, during germination, a phenomenon which Messrs. Becquerel and Boussingault consider constant. (Comptes rendus, vi. 109.)

In the opinion of some persons, oxygen also acts as a stimulant of the vital actions of the embryo. Humboldt remarked that seeds plunged in chlorine, and taken out before the radicle appears externally, germinate more rapidly than ordinary; Cress, for instance, may thus be made to germinate in 6 hours instead of 24 or 30. He even succeeded, by this process, in bringing about germination in old seeds which appeared destitute of the power.

These experiments have not, however, succeeded in all hands : in many cases it is possible that the success that is said to have attended them has been imaginary; and, as the theory upon which the action of chlorine was explained is now abandoned, one cannot avoid entertaining doubts as to the accuracy of the alleged facts.

Heat is that in which the stimulus necessary to call the vitality of seeds into action seems really to reside. No seed can germinate at a temperature so low as that of freezing; and each seems to have some one temperature more proper for it than any other, at the first dawn of its life. If, says De Candolle, the temperature is too high, germination proceeds too rapidly, and the result is weak and languishing plants, in which we cannot avoid recognising beings too much excited and badly nourished. If the temperature is too low, the excitement is not sufficient; and it often happens that the seed cannot resist the decay induced by the water it has absorbed, but not assimilated. It is between these limits that a suitable temperature for every species is to be sought.

Edwards and Colin have instituted some experiments to determine what temperature seeds can bear. They found that Wheat, Barley, and Rye could germinate at 7° centig. (44:6° Fahr.); and that grain of the same description did not


apparently suffer, by being exposed for a quarter of an hour to a temperature equal to freezing mercury: such grains were afterwards placed in a proper situation, and germination took place as usual. Considering that the particles of fæcula of which seeds consist are not liable to bursting below a temperature of 75o centig. (167° Fahr.), these observers were led to ascertain how near an approach to this extreme temperature might be made, without destroying vegetable life.

Seeds of various cereal and leguminous plants were placed for a quarter of an hour in water of this temperature, and they were all killed ; five minutes were afterwards ascertained to suffice for the destruction of three in five. Less elevated temperatures were next experimented

Wheat, Barley, Kidneybeans, and Flax were killed in 27 minutes, by water at 62° centig. (143•6o Fahr.); a few grains of Rye and some Beans required a longer exposure to be destroyed. When the temperature was lowered to 52° centig. (125-6° Fahr.), most of the seeds in experiment retained their vitality ; but even this was fatal to Barley, Kidneybeans, and Flax.

Fluid water has conducting powers very different from those of vapour or of dry air; it was thereupon important, to determine whether the temperature that seeds can bear is regulated by the nature of the medium in which they are exposed to it. In vapour, 75o centig. (167° Fahr.) was sufficient to destroy such seeds as were exposed; but, at 62° centig. (148:6° Fahr.), they retained their vitality, after having been under experiment for a quarter of an hour. But, in dry air, many seeds bore the temperature of 75° centig. (167° Fahr.), for a quarter of an hour, without inconvenience. Hence it appears that seeds in steam can bear 12° centig. more than in water, and in dry air 13° centig. more than in steam.

In these experiments, the action of temperature was extremely rapid. In lowering the temperature and prolonging its action, it was found that, when Wheat, Rye, and Barley were exposed for three days, in water, to a temperature of 350 centig. (95° Fahr.), four fifths of the Wheat and Rye, and all the Barley, were killed. Hence it would appear, that 350 centig. forms the highest limit of temperature which corn can bear under such circumstances. But, in sand or earth, the same grains sustained a prolonged temperature of 40° centig. (104° Fahr.) without inconvenience; at 45° centig. (113° Fahr.) a great part perished; at 50°centig. (122° Fahr.) the whole of them.

These remarkable experiments are calculated to throw great light upon the cause of the impossibility of making certain plants multiply themselves by seeds in hot countries. If Wheat, Barley, &c., cannot endure a prolonged temperature above 40° centig. ; and the temperature of the soil is in some countries and soils as high as 60° centig. (140° Fahr.), as Humboldt asserts, or between 48° and 53° centig. (122° Fahr.), even in some parts of France, as Arago states; it is evident that the seeds of corn placed in such situations will perish.

Exposed to the influence of water, heat, and air, the parts of a seed soften and distend; the embryo swells and bursts its envelopes, extending the neck and the bases of the cotyledons, and finally emitting its radicle, which pierces the earth, deriving its support at first from the cotyledons or albumen, but subsequently absorbing nutriment from the soil, 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 root-sheath, 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 plumule from the centre, as in the Oak; or from the side of their elongated neck, as in 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 pericarp, before the seed falls from the tree; a long thread-like caulicle is emitted, which elongates till it reaches the soft mud in which

« PreviousContinue »