154. Torrid Zone.
A curious theory concerning the climate of the torrid zone, is to be found in the Problemas[1] of Dr. Cardenas, published at Mexico, in 1591 "That climate, (he says,) is hot and moist, and were it not for the moisture the heat would render it uninhabitable, as the ancients supposed it to be. But the soil of these Indies has this strange property, that it is full, or as it were every where, undermined with tremendous caverns, by which the frequency of earthquakes is explained. These hollows are full of water, or of air, which would be the same thing, for the intense cold of the abyss would convert it into water;— and the rays of a vertical sun acting continually upon the earth, draws up continual vapours from these immense reservoirs. So far therefore from the Indies being in the same proportion hot and dry, the hotter they are the moister they must needs be. This mighty force of heat acts upon the crust of the earth also, dissolving all which is capable of solution, and leaving it full of hollows, like an over-baked loaf: the more subtile particles which are thus sublimed rush upward with great force, and by their conflict with the air, occasion those tremendous winds to which these countries are subject: the more gross and earthy parts are the materials from which metals are concocted.
"The trees here are green and flourishing when the herbs are dried up, because their roots reach down to the moisture. But the reason why all trees here spread out their roots horizontally instead of striking them down as in Europe, is to be found in the climate, which necessarily occasions this defect. For in Europe the cold of winter drives back all vegetative virtue into the root, which then pushes out its fibres and strikes deep, growing and strengthening itself, till the warmer season comes, and draws the sap upward, and then the branches grow in their turn. But here the upper stratum of earth being hard and as it were burnt, and what lies immediately below weak and porous, the roots rest in that which affords them a firm hold, and seek not to penetrate deeper. Neither indeed is the weather ever cold enough for the vegetative power to accumulate in the roots; thus as they have no season for strengthening themselves, they have no strength to impart in their turn, and hence it is that the foliage of the tropical trees is usually pensile, because there is not vigour of vegetation enough for the branches to shoot upward."
A more philosophical hypothesis to account for the manner in which the American trees spread their roots horizontally is given by Volney.
"I must not omit, he says, a singular fact in natural history, which is well established in Kentucky, that many of the streams have become more abundant, since the woods in their neighbourhood have been cut down.
"I have discussed the causes of this phenomenon on the spot with witnesses deserving of credit; and it appeared to us that in times past the leaves of the forest trees, accumulating on the ground, formed there a thick compact bed, retaining the rain water on its surface, gave it time to evaporate, particularly in summer, before it could penetrate the ground. At present this bed of leaves not existing, and the bosom of the earth being opened by the plough, the rain, which is enabled to sink into it, establishes in it more durable and abundant reservoirs. This particular case, however, does not overturn the more general and more important doctrine, that cutting down woods, more especially on heights, in general diminishes the quantity of rain, and the springs resulting from it, by preventing the clouds from stopping and discharging their waters on the forests. Kentucky itself affords a proof of this, as well as all the other states of America; for a number of brooks are pointed out, which were never dried up fifteen years ago, and now fail every summer. Others have totally disappeared, and in New Jersey several mills have been relinquished on this account. It must be observed too, that formerly, the beds of the rivers being incumbered with trees blown down, and reeds, detained their waters more, which, now they are cleaned, they suffer to run off too fast.
"Another phenomenon observed in America, may perhaps be explained by means of the fact I have just mentioned. You cannot cross any forest in this continent without meeting with fallen trees; and it is remarkable that the root is only a superficial tuft, in the shape of a mushroom, and scarcely eighteen inches deep for a tree seventy feet high. If the trees put out no tap-root, was it not that they might avail themselves of the superficial humidity that covered them, and the rich mould arising from the decayed leaves, in which they found a substance much preferable to the interior strata, that remained dry, and consequently more hard to penetrate? And now, as they have contracted this habit through a lapse of ages, ages are requisite to change it."
- ↑ Ch 2, 3, 5.