2. The next object of inquiry is, the influence which this saline air has upon vegetable life. Independently of the facts already stated, there are many others which prove its deleterious agency upon the vegetable creation. Dr. Mitchill informs me, that in some parts of the south side of Long-Island fruit trees do not thrive well, except at a distance of thirty miles from the sea, and even the sturdy oak does not extend its branches towards the ocean.[61] If I am correctly informed, it was with great difficulty, that the trees on our Battery were made to accommodate themselves to a situation so near the salt water. It is also well known, that when plants are taken to sea, they speedily perish, if exposed but a short time to a wind, which is sufficiently strong to turn over the tops of the waves into white caps, as they are called by the sailors.
In order to ascertain positively, whether these effects were to be attributed to the operation of salt, I made a solution of muriate of soda in common rain water; with this I watered for a couple of days the leaves of different plants. In a short time they began to dry up, and in a few days were completely dead.
It appears from Volney, that the Egyptian air is strongly charged with salts. The evidences of it are to be found even at Cairo.[62] It is this property of the air, which this philosophical traveller considers, as one of the causes of the rapid vegetation in that country. He mentions, however, that exotic plants will not thrive there. It is found necessary to renew the seeds of them every year. May not this be occasioned by the saline quality of the air? The native plants are doubtless accustomed to its action, and do not so sensibly feel its injurious effects. And if the Egyptian air is so very penetrating from this very cause, as to produce ophthalmia, may we not rationally conclude, that its influence must be equally injurious to plants not accustomed to it.
Another illustration of the influence of salt on vegetation is to be found in the Dead Sea, or Lake Asphaltites. "In Lake Asphaltites," says Volney, "there is neither animal nor vegetable life. No verdure is to be seen on its banks, nor fish to be found within its waters; but it is not true, that its exhalations are pestiferous, so as to destroy birds flying over it. It is not uncommon to see swallows skimming its surface, and dipping for the water necessary to build their nests. The true cause which deprives it of vegetables and animals is the extreme saltness of the water, which is vastly stronger than that of the sea. The soil around it, equally impregnated with this salt, produces no plants, and the air itself, which becomes loaded with it from evaporation, and which receives also the sulphureous and bituminous vapours, cannot be favourable to vegetation; hence the deadly aspect which reigns around this lake."[63]
3. In what way does the salt operate in producing its deleterious effects on the leaves of vegetables? It is by no means easy to answer this question. It cannot be by shutting up the pores of the leaf, and thus obstructing its perspiration. It is well known that when the surfaces of leaves are covered with oil, they will soon die.[64] But salt water is certainly not sufficiently viscid to act in a similar way.
Nor can it be satisfactorily attributed to the difference of structure between maritime and land plants. There is some difference indeed between many of these, maritime plants being generally covered by a pubescence, of which most land plants are destitute. It is idle however to suppose that the object of this covering is to protect maritime plants from the action of the salt air, as there are many of them which do not possess it. Besides, is it not rational to conclude, from the large quantities of soda which are always found in sea plants, that this saline atmosphere is rather propitious than otherwise to their growth, and that it only proves injurious to plants accustomed to the unadulterated air of the land.
Again, I do not think that it can be explained by supposing, that the salt is absorbed into the plant, and thus acts as a poisonous substance. We know, that in land plants which are cultivated in the neighbourhood of the sea, salt is absorbed through their roots.[65] It must of course circulate with the juices through the whole plant; and yet in these cases the leaves are not destroyed by it.
The most plausible method of explaining it appears to be this: that the salt, by its irritating or corrosive power, destroys the small vessels in the leaf which are necessary for the circulation going on in it during health.
Dr. Darwin has ingeniously shown the analogy between the functions of the leaves of plants, and the lungs of animals. If this be admitted, it will not be difficult to account for the action of salt upon leaves. This substance, when taken into the stomach, proves not merely innocuous, but wholesome; but when accidentally introduced into the lungs, irritation, inflammation, and death are the consequences. So with plants—when admitted into them in combination with their juices, it may be harmless; but when applied to the lungs or leaves, death ensues.