Darwin writes: “In botanical works, this or that plant is often stated to be ill-adapted for wide dissemination, but the greater or less facilities for transport across the sea may be said to be almost wholly unknown. Until I tried, with Mr. Berkeley’s aid, a few experiments, it was not even known how far seeds could resist the injurious action of sea-water. To my surprise I found that out of 87 kinds, 64 germinated after an immersion of 28 days, and a few survived an immersion of 137 days. It deserves notice that certain orders were far more injured than others; nine Leguminosæ were tried, and, with one exception, they resisted the salt-water badly; seven species of the allied orders, Hydrophyllaceæ and Polemoniaceæ, were all killed by a month’s immersion. For convenience’ sake, I chiefly tried small seeds, without the capsules or fruit; and as all these sank in a few days, they could not have been floated across wide spaces of the sea, whether or not they were injured by the salt-water. Afterwards I tried some larger fruits, capsules, etc., and some of these floated for a long time. It is well known what a difference there is in the buoyancy of green and seasoned timber; and it occurred to me that floods would often wash into the sea dried plants or branches with seed capsules or fruit attached to them. Hence I was led to dry the stems and branches of 94 plants with ripe fruit, and to place them on sea-water. The majority sank quickly, but some which, whilst green, floated for a very short time, when dried floated much longer; for instance, ripe hazel-nuts sank immediately, but when dried they floated for 90 days, and afterwards when planted germinated; an asparagus-plant with ripe berries floated for 23 days, when dried it floated for 85 days, and the seeds afterwards germinated; the ripe seeds of Helosciadium sank in 2 days, when dried they floated for above 90 days, and afterwards germinated. Altogether, out of the 94 dried plants, 18 floated for above 28 days; and some of the 18 floated for a very much longer period. So that as 64/87 kinds of seeds germinated after an immersion of 28 days; and as 18/94 distinct species with ripe fruit (but not all the same species, as in the foregoing experiment) floated, after being dried, for above 28 days, we may conclude, as far as anything can be inferred from these scanty facts, that the seeds of 14/100 kinds of plants of any country might be floated by sea-currents during 28 days and would retain their power of germination.”
We have thus sufficient evidence before us to prove that salt or sea water does not totally destroy the vitality of seeds when they are in a dry state, that some of them will float for 90 days, and when planted subsequently will germinate; but that when not dry they will sink immediately. We may, therefore, justly conclude from the result of these experiments that salt is not noxious to vegetable life, neither does it destroy the latent principle of procreation which exists in them; and that though the process of germination may be retarded, and kept in a state of abeyance, it is not virtually annihilated, as one would feel inclined to predict, by the prolonged immersion of seeds in salt-water, be they dried or fresh.
Darwin’s experiments were afterwards verified, for he states that subsequently M. Martens tried “similar ones, but in a much better manner, for he placed the seeds in a box in the actual sea, so that they were alternately wet and exposed to the air like really floating plants. He tried 98 seeds, mostly different from mine; but he chose many large fruits and likewise seeds from plants which live near the sea; and this would have favoured both the average length of their flotation, and their resistance to the injurious action of the salt water. On the other hand, he did not previously dry the plants or branches with the fruit; and this, as we have seen, would have caused some of them to have floated much longer. The result was that 18/98 of his seeds of different kinds floated for 42 days, and were then capable of germination. But I do not doubt that plants exposed to the waves would float for a less time than those protected from violent movement as in our experiments. Therefore it would, perhaps, be safe to assume that the seeds of about 10/100 parts of a flora, after having been dried, could be floated across a space of 900 miles in width, and would then germinate. The fact of the larger fruits often floating longer than the small, is interesting; as plants with large seeds or fruit which, as Alph. de Candolle has shown, generally have restricted ranges, could hardly be transported by other means.”
Darwin’s experiments show us that salt or sea water does not entirely extirpate the life which is dormant in seeds, and those of Martens prove that seeds may be immersed in sea-water itself and yet retain the power of germination; and that when dry they may even float for 900 miles, and germinate when planted; developing into plants at the usual period of time allotted by nature!
In Cheshire it is a custom to let out the water of the salt-springs after rain, in order to improve the character of the soil and make it more productive. If we call to mind the preservative properties of salt and the purifying action which it possesses, with regard to animal and vegetable substances, we need not at all be surprised at the above use to which it is put by the agriculturists of Cheshire. The reader, perhaps, would like to know why it is used after rain. After a heavy shower, and more especially in the country, every insect leaves its little secluded habitation: the bee is once more on the wing; the spider resumes his usual central position in his web; flies of all sizes buzz here and there in search of food or for more secure homes; every bush is alive with its usual occupants; the lofty tree is once more the tenement of song; the caterpillar crawls on his solitary way; the ant trudges along on the gravel-path; the snail emerges from his retreat and plods slowly to another home; and the earth-worm raises itself on the lawn; all with one accord hail the reappearance of sunshine, and show signs, however feeble, of joy that the rain-cloud has passed and that the landscape has resumed its beauties, and the sky its gold and azure. The earth after rain, and particularly in spring and summer, teems with almost reanimated life, both with that which is harmless and with that which is hurtful, so that the Cheshire custom is one which cannot be too highly recommended, for when the soil is saturated with moisture, a soluble salt like the chloride of sodium, already in a state of solution, sinks in more rapidly, and permeates it more thoroughly than if it were merely sprinkled over the surface; and such insects as are associated with or which live in the earth are speedily eliminated, or are forced to seek shelter at a greater depth, where they ultimately die by reason of their inability to obtain their proper sustenance or the unsuitableness of their new abode.
There is a plant called Halimodendron which only grows in the dry, naked salt-fields by the river Irtysh, in Siberia; it is a genus of the Leguminosæ, and has purple flowers. Saltwort, or Salsola, (salsus, salt) is chiefly maritime, and the kelp of our shores is principally obtained from it. At one time the carbonate of soda was derived from this kelp or barilla, the ashes being obtained from burning sea-weeds and a species of Salsola; but now it is almost invariably made from common salt, by adding sulphuric acid, and so converting the chloride of sodium into a sulphate, and afterwards, by combustion with chalk and small coal, resolving it into a sulphide, and then into a carbonate. It is manufactured on a very large scale, and is an important staple of commerce. From it is obtained a most important drug, the bicarbonate of soda, the efficacy of which everyone, more or less, has once in a lifetime experienced.
This kelp has been put to a fraudulent use, for Sir Robert Christison tells us that disease has been traced to an impure kind of salt, in which, when investigated, the hydriodate of soda was detected, resulting, he says, from an inferior salt obtained from kelp.[52]
In all those districts which are intersected by salt marshes, there is almost a complete absence of miasmatic effluvia, though, as a natural consequence, the vegetation is not of that rank luxuriance which is invariably to be seen in other marsh lands; because, whenever the soil is in a state of moisture, it is always covered with all kinds of weeds and useless plants, which altogether stop the growth of those which are of utility to the agriculturist.
In the case of salt marshes it is the reverse, and the neighbourhood is perfectly free from those endemic diseases which are prevalent in such localities as the fen-country, and other similar districts; for the atmosphere is pure, and the soil comparatively dry, and intermittent fever is unknown.