Popular Science Monthly/Volume 51/May 1897/The Latent Vitality of Seeds
| THE LATENT VITALITY OF SEEDS. |
By M. C. de CANDOLLE.
SEEDS that remain in keeping without losing the faculty of germination are said to be in a state of latent life. The term is not exact, for it leaves us still to ask whether the life of the seeds is completely stopped, or is simply slackened in its activity—questions to which the same answer can not be given under all circumstances. It may be that a seed will continue to respire without producing any formation of new histological elements, when a loss of substance results to the plantule it contains which is compensated for by the assimilation of reserve materials from the energides, or living protoplasmic masses of the cells. A plantule may be supposed to live in this way for a considerable time if the temperature is favorable and the seed and the surrounding air are not too dry. Under these conditions the latent life may be considered one of slackened activity.
An experiment by MM. Van Tieghem and Bonnier proves that seeds may retain their vitality for a considerable period in this condition. Three lots of peas and beans were left—one in the open air, a second in a sealed glass tube containing common air, and a third in a sealed tube containing pure carbonic-acid gas. At the end of two years the seeds of the first lot had perceptibly increased in weight, and nearly all germinated; those kept in confined air had increased less in weight, and fewer of them germinated; the air inclosed with them in the tube had changed in composition, having lost oxygen and gained carbonic acid. Of those sealed up in carbonic acid, the weight had not changed, and none germinated.
While these results show that the seeds continued to lead a retarded life in open and in confined air, it is possible that the retarded life was only of short duration, and that it had ceased, before the end of the experiment, to give place to a complete stoppage of respiration, assimilation, and life. But to admit this we have to suppose that the protoplasm in seeds in latent life finally becomes wholly inert, while it preserves its composition and its internal chemical structure. This view seems to be confirmed by a number of experiments and observations which I am about to describe.
I have already several times related experiments that prove that seeds may be subjected to a very intense cold for many hours in succession without losing their germinating faculty. A recent experiment of this sort, made with M. Raoul Pictet's apparatus and under his direction, proves that some peas and beans and fennel seeds germinate quite well after having endured for four days a temperature of −200° C. (−328° F.). The seeds had not undergone any previous desiccation, and no precautions were taken to adjust the depression of temperature. Others of M. Pictet's researches have demonstrated that the chemical reactions which take place at ordinary temperatures cease to be produced at very low temperatures, like those reached in the experiments just mentioned. If this is so, we may suppose that the protoplasm of seeds exists during these experiments in a condition of complete inertia, without either respiring or assimilating. In other words, life is then really stopped; yet this does not prevent their vegetating anew when the conditions of temperature and moisture permit it. The seeds in these experiments were cooled so very rapidly that it is natural to suppose that their protoplasm was already quite inert before the test began. It would be hard otherwise to explain its complete indifference to abrupt variations of temperature, which would certainly have been more harmful if they affected protoplasm still active.
Another experiment I have recently tried casts more light on this point. Wrapping seeds of wheat, oats, fennel, and the sensitive plant in packages of tinned paper and inclosing the whole in a sheet-iron box, hermetically sealed, I placed them under the cover of a wooden box in a compressed-air refrigerator for meats, where they were exposed for a hundred and eighteen days to repeated but not continuous refrigerations, most of which lasted twenty hours each. The lowest temperature reached was −53·89° C. (−65° F.); the highest, −37·78° C. (−36° F.); and the mean, −41·93°C. (−43·4° F.). After each refrigeration the temperature rose to that of the interior of the receiver, but slowly, while the refrigerations were rapid.
After the conclusion of the experiment, when taken out of the refrigerator and planted, the wheat, oats, and fennel came up promptly; only thirteen out of sixty seeds of sensitive plants germinated, and of lobelia seeds, which were too small to be counted, only ten. The failures of the sensitive-plant seeds could not all be attributed to the cold, for others of the same species which were not refrigerated did but little better. The lobelia seeds were, however, certainly killed by the cold, for the control seeds germinated abundantly. It is safe, too, to infer that seeds can remain inert and unharmed in a medium unsuitable for respiration, provided nothing is present to injure their protoplasm through chemical action. Such a medium, for example, would be an atmosphere of carbonic acid.
I desired to ascertain the effects on germination of keeping seeds in vacuum. The most obvious way of trying this experiment, by the formation of a barometrical vacuum, was liable to the objection that the abrupt removal of the air and moisture might disturb the tissues and modify the structure and composition of the protoplasm of the seeds, and thereby produce a complication of results. I therefore tried another way, by immersing them in mercury under such precautions that no air could reach them other than what they contained within themselves. The results agreed substantially with those obtained by refrigeration, and go to confirm the view that seeds can continue to subsist in a condition of complete vital inertia, from which they recover whenever the conditions of the surrounding medium permit their energides, or the living masses of their cells, to respire and assimilate.
At first sight, this return to life resembles the resumption of motion by a machine that has been resting when it is put into communication with its motor—a comparison which has been often made. But the phenomena are not of the same nature in the two cases, and the energides, of which the total constitutes the living individual, are not machines in the usual sense of the word. For a machine works without changing its structure, while the energides segmentate after they have grown, and their segmentations operate in their turn as energides. This is because the matter assimilated by living protoplasm augments its mass without diminishing its energy. For it to be so, this mass must evidently continually receive new portions of energy, and this can come only either from the surrounding medium or from the reactions that go on in the protoplasm itself. In the former case the agency consists of radiations of different sorts, and is of a purely physico-chemical order; while this can not be in the second case. In fact, the life of protoplasm is manifested by movements which are combined in such a way as to produce an orientation of its parts according to certain structural dispositions succeeding one another in a determined order; phenomena to which ordinary physico-chemical actions never give rise. We are therefore necessarily led to suppose the existence of a special class of reactions of which assimilated matters become capable only after their absorption into this special medium, living and pre-existing protoplasm, into which they penetrate.
Under this relation we might, in a certain way, compare assimilation to what occurs when combustible matter takes fire on being heated in a furnace in which a combustion is already going on, and is kept up by the new matter. So, one might say, it is only after having been previously put into a special condition by their mixture with protoplasm that assimilable substances react among themselves in such a way as to produce a new quantity of living matter. So one may suppose that protoplasm in the condition of latent life, having become inert but retaining the faculty of reviving, resembles those mixtures formed of substances that do not react except under certain conditions of temperature or other influences, and which, so long as those conditions are not fulfilled, continue indefinitely in contact without combining. Such, for example, are explosive mixtures.
The presence of assimilable matter in protoplasm or within its range is not sufficient for the production of the phenomena of assimilation and orientation. Certain conditions of temperature, moisture, and aeration have to be realized. As long as they are not realized, and if nothing occurs to change the composition or structure of the energides, they will remain inert, while they retain the faculty of evolving anew when the circumstances become favorable again.
Such condition of chemical and vital inertia may probably endure for a long time, possibly indefinitely. This, as it seems to me, is at least the only way of accounting for the preservation of seeds during very many years. Cases are in fact known where seeds have germinated after so prolonged a rest that it is impossible to assume that they have lived during the interval even a retarded life. We cite a few examples. M. A. P. de Candolle[1] speaks of seeds of the sensitive plant that germinated after more than sixty years of rest. Girardin[2] saw beans germinate that were taken from Tournefort's herbarium, where they had been kept more than a hundred years.
In 1850 Robert Brown, out of curiosity, sowed some seeds from the collection of Sir Hans Sloane, of which they had formed a part for more than a hundred and fifty years. He succeeded in making several of them germinate, particularly a seed of Nelumbium speciosum. The plant has been preserved in the galleries of the British Museum,[3] where I saw it a few years ago.
The pretended germination of wheat from mummies is said to be a fable. It seems, besides, that wheat was always sterilized before being introduced into the sarcophagi, so that the possibility of its being brought to life again was excluded in advance. On the other hand, various well-verified facts have demonstrated that seeds may preserve their faculty of germinating after an extremely prolonged abode underground—that is, when sheltered from atmospheric influences. The most extraordinary case of this kind was observed a few years ago by Prof. de Heldereich,[4] director of the Botanical Garden at Athens. While herborizing around the mines of Laurium, this naturalist discovered, in 1875, a glaucium, which he unhesitatingly considered a new species, and described under the name of Glaucium serpieri. The plant had just made its appearance on a tract from which had recently been removed a thick bed of scoria produced in the workings of the mines by the ancients, or at least fifteen hundred years ago. Unless we assume a spontaneous generation, this glaucium must be regarded as a species which existed formerly in the place, the seeds of which had been preserved intact under the protection of the ground and the scoria that covered them.
Many instances are mentioned in which the opening of deep trenches or the clearing of forests has been followed by the appearance of species formerly unknown in the place. Prof. Peter, of Göttingen,[5] has very recently made a long series of methodical researches, the results of which are of great interest. His method consists in collecting specimens of forest earth, the age and all the anterior conditions of which are fully known. He cultivates them, taking all precautions against introducing foreign seeds. These specimens of earth are always taken from thickly shaded spots, destitute of all other vegetation except the moss that carpets the surface of the soil. Holes are dug under this moss, from which the earth is taken at depths successively of eight, sixteen, and twenty-four centimetres. The specimens taken from these several depths are cultivated separately. The cultivations, prolonged for more than three months, have all ultimately given rise to plants the seeds of which must of necessity have remained under the earth for a greater or less length of time.
M. Peter has carefully indicated in detail the plants that corresponded to each of the specimens of earth on which he operated. It resulted from the experiments that the specimens of earth from very old forests gave plants of the woods, while those from forests of more recent date yielded species the nature of which was manifestly related to the previous disposition of the soil—that is, plants of the fields or the meadows, according as forestation had replaced one or the other of these methods of cultivation. While he is extremely reserved as to the probable duration of the abode of the seeds in the soil, M. Peter concludes in these words: "Although the experiments in cultivation just described do not furnish a solution to the question of the length of time during which seeds at rest preserve their faculty of germinating, the conclusion results from this demonstration that for many field and meadow plants this duration may considerably exceed a half century."
These researches of M. Peter's deserve careful attention, and it is to be hoped that they will, without delay, be imitated in other countries and different kinds of land, for they may reveal very important facts in biology and prehistoric botanical geography. Alphonse de Candolle[6] insisted a few years ago on the desirability of making soundings beneath the snows of the Alps with a view of recovering vestiges of the vegetation anterior to the Glacial period. It is to be regretted that no one has carried out this idea, for the facts I have just summarized almost permit us to hope that research of this kind may lead to the recovery of still vital seeds dating from very remote epochs.—Translated for the Popular Science Monthly from the Revue Scientifique.
- ↑ Physiologie, p. 621.
- ↑ Sur la propriété qu’ont certaines espèces de graines de conserver longtemps leurs vertues germinatives.
- ↑ See Gartenflora, 1873, p. 323
- ↑ These facts have been recently confirmed by Mr. W. Carruthers, director of the botanical galleries in the British Museum.
- ↑ Nachrichten v. d. königl, Gesellschaft der Wissenschaften u.d. George Augustus Universität zu Göttingen, November, 1893, and December, 1894.
- ↑ Extract from the Archives des Sciences physiques et naturelles.