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WATER RESERVOIRS OF PLANTS.
of essential value to each species; we do not know why white terriers are more subject than darker-coloured ones to the attacks of the fatal distemper; why yellowfleshed peaches in America suffer more from diseases than the white-fleshed varieties; why white chickens are most liable to the gapes; or why the caterpillars of silkworms, which produce white cocoons, are not attacked by fungus so much as those that produce yellow cocoons ? Yet in all these cases, and many others, it has been shown that immunity from disease is correlated with some slight difference in colour or structure, but as to the cause of that immunity we are entirely ignorant.
At last we reached the summit of the range, which is probably not less than three thousand feet above the sea, and entered on the district of Libertad. Rounded boggy hills covered with grass, sedgy plants and stunted trees, replaced the dry gravelly soil of the Juigalpa district. The low trees bore innumerable epiphytal plants. on their trunks and boughs. Many of these are species of Tillandsia, which sit perched up on the small branches like birds. They have sheathing leaves that hold at their base a supply of water that must be very useful to them in the dry season. Insects get drowned in this water, and the plants may derive some nourishment from their decomposing bodies, but I believe the principal object is to obtain a supply of moisture, as the roots of the plants do not hang down to the ground, like those of many other epiphytes in the tropics, nor are they provided with bulbs like the orchids. Some plants that hold liquids in cup-shaped leaves are simply insect traps, many of them growing in bogs, where the supply of moisture is perennial and constant. Such is the Indian-cup (Sarracenia) that grows in the bogs of Canada, and the Californian pitcher-plant (Darlingtonia californica), which also grows in bogs, and is such an excellent fly-trap, that there is generally a layer of from two to five inches of decomposing insects lying at the bottom of the cup.* The different species of Drosera, or sun-dews, possess quite a different apparatus for catching insects, and they also live in bogs, which supports the inference that plants growing in such situations have some especial need to obtain nutriment, which they cannot draw from the decaying vegetation on which they live. Possibly they obtain the salts of potash in this way. I did not notice any provision in the leaves of the Bromeliaceous epiphytes of Chontales to ensure the capture of insects, but often saw their dead bodies in the water held at the base of the leaves, and any that came to drink would be very liable to slip into the water from off the nearly perpendicular side of the leaf and be drowned. It is not impossible that the small supply of mineral salts required for the organisation of these plants that do not draw any nutriment from the earth may be obtained from dead insects, but, as I have already stated, I believe that the principal object is to lay up a store of water. to carry them safely through the dry season. Incidentally, the further advantage has been gained that insects fall into the receptacles of water and are drowned, affording in their decomposition nourishment to the plants.
Our road now lay over the damp grassy hills of the Libertad district. It edged away from the Amerrique
* See “Nature,” vol. iii. pp. 159 and 167.
SHARP DELIMITATION OF FOREST.
range on our right. To our left, about three miles distant, rose the dark sinuous line of the great forest of the Atlantic slope. Only a fringe of dark-foliaged trees in the foreground was visible, the higher ground behind was shrouded in a sombre pall of thick clouds that never lifted, but seemed to cover a gloomy and mysterious country beyond. Though I had dived into the recesses of these mountains again and again, and knew that they were covered with beautiful vegetation and full of animal life, yet the sight of that leadencoloured barrier of cloud resting on the forest tops, whilst the savannahs were bathed in sunshine, ever raised in my mind vague sensations of the unknown and the unfathomable. Our course was nearly parallel to this gloomy forest, but we gradually approached it. The line that separates it from the grassy savannahs is sinuous and irregular. In some places a dark promontory of trees juts out into the savannahs, in others a green grassy hill is seen almost surrounded by forest. When I first came to the country, I was much puzzled to understand why the forest should end just where it did. It is not because of any change in the nature of the soil or bed-rock. It cannot be for lack of moisture, for around Libertad it rains for at least six months out of the twelve. The surface of the ground is not level on the savannahs, but consists of hill and dale, just as in the forest. Altogether the conditions seemed to be exactly the same, and it appeared a difficult matter to account for the fact that the forest should end at an irregular but definite line, and that at that boundary grassy savannahs should commence. After seeing the changes that were wrought during the four and a half years that I was in
the country, I have been led to the conclusion that the forest formerly extended much further towards the Pacific, and has been beaten back principally by the agency of man. The ancient Indians of Nicaragua were an agricultural race, their principal food then, as now, being maize ; and in all the ancient graves, the stone for grinding corn is found placed there, as the one thing that was considered indispensable. They cut down patches of the forest and burnt it to plant their corn, as all along the edge of it they do still. The first time the forest is cut down, and the ground planted, the soil contains seeds of the forest trees, which, after the corn is gathered, spring up and regain possession of the ground, so that in twenty years, if such a spot is left alone, it will scarcely differ from the surrounding untouched forest. But it does not remain unmolested. After two or three years it is cut down again and a great change takes place. The soil does not now contain seeds of forest trees, and in their stead a great variety of weedy-looking shrubs, only found where the land has been cultivated, spring up. Grass, too, begins to get a hold on the ground; if it prevails, the Indian, or Mestizo, does not attempt to grow corn there again, as he knows the grass will spoil it, and he is too indolent to weed it out. Often, however, the brushwood has been cut down and burnt, and fresh crops of corn grown several times before the grass has gained such an advantage that the cultivator gives up the attempt to plant maize. There is then a struggle between the weedy shrubs and the grass. The leaf-cutting ants come to the aid of the latter. Grass they will not touch, excepting to clear it away from their paths. Ch. X.]
RETROCESSION OF THE FOREST.
The thick forest they do not like, possibly because beneath its shade the ground is kept too damp for their fungus beds. But along the edge of the forest, by the sides of roads through it, that let in the air and sunshine, and in clearings, they abound. They are especially fond of the leaves of young trees, many of which are destroyed by them. Should the brushwood ultimately prevail, and cover the ground, the Indian, or Mestizo, comes again after a few years, cuts it down, and replants it with maize. But as most of his old clearings get covered with grass, he is continually encroaching on the edge of the forest, beating it back gradually, but surely, towards the north-east. As this process has probably been going on for thousands of years, I believe that the edge of the forest is several miles nearer the Atlantic than it was originally.
In this way many acres in the neighbourhood of Pital were taken from the forest, and added to the grass-lands, whilst I was in the country. The brushwood-land does not yield such good crops as the virgin forest, but it is nearer to the huts of the cultivators, who live out on the savannahs, so that whenever the weedy shrubs gain possession of a spot sufficiently large for a clearing, and choke off the grass, these places are again cut down and burnt, and thus the forest is never allowed to establish outposts, or advanced stations, in the disputed ground. What would be the result if man were withdrawn from the scene, I do not know, but I believe that the forest would slowly, but surely, regain the ground that it has lost through long centuries. The thickets and dense brushwood that always spring up along the edge of the forest, and consist of many