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.