Page:Popular Science Monthly Volume 13.djvu/133

This page has been validated.

western Iowa, offers a very ingenious theory, and one that certainly appears to account for the phenomena observed by him in the above-mentioned region. He finds that—1. In the hill-regions where the slopes are inclined from 5° to 10°, timber occurs mostly on the northern slopes, just south of creeks flowing east or west; it occurs a little less frequently on western slopes, east of creeks flowing north or south; 2. In the bluff-region, where the slopes are from 10° to 45°, just east of the bottom-lands of the Missouri, timber is found over most of the surface. This belt of timber-land is usually bounded on the west by the crest of the most western ridge of the bluffs, leaving the slopes facing the bottom-land bare, except when a lake, slough, or stream, comes close to the base of the bluffs, or where the bluff-side is deeply furrowed by ravines; 3. In the low alluvial valleys, timber is found along the streams, usually in narrow strips, and generally wider on the east and north banks; the rest of the bottom-land is destitute of trees and bushes. According to the author, constancy of moisture is the condition sine qua non of forest-growth; and, 1. This constancy of moisture must be in one or both the media in which the trees are to exist—the soil or the air; 2. It is plain that moisture of soil will be more constant on northern slopes than southern, the former being less exposed to the sun's heat. In the spring, and after showers, the northern slopes dry up more slowly, and, at certain degrees of humidity of the air, the moisture given off by the southern slope of a hill may be condensed by the northern. These and other like considerations may perhaps account for the timber occurring on northern slopes, while it is nearly absent from southern; 3. The fact that the prevailing winds of Southwestern Iowa in spring and summer are westerly may perhaps explain the preponderance of timber-areas on the east banks of the streams flowing south; and this, combined with the increased roughness of the surface, may also go far toward explaining the timber-belt of the bluff-region; 4. It remains to explain the distribution of timber and prairie in the alluvial valleys. Here layers of clay prevent the ready drainage of many parts; these conditions render much of the surface too wet (for trees) at all times, while other places are too wet in spring and too dry in summer. On the other hand, the occurrence of trees along the streams and on ridges along old channels may be explained partly by the inequality of surface, making the drainage of surplus water possible, so that moisture around the roots is more constant than elsewhere on the bottoms.


More about the Agricultural Ant.—While visiting Texas last summer, the Rev. H. C. McCook attentively studied the habits of the agricultural ant (Myrmica molefaciens). His observations are, for the most part, strongly confirmatory of the statements made by the late Dr. Lincecum; but he also adds to our knowledge of these interesting insects a number of new and interesting facts. Mr. McCook has published, in the "Proceedings of the Academy of Natural Sciences of Philadelphia," a general preliminary statement of his results, intending soon to treat the subject more fully. He carried on his observations in the vicinity of the city of Austin, where the soil is black and sticky, varying in depth from three feet to a few inches. The formicaries of the agricultural ants are commonly flat, circular clearings, hard and measurably smooth, aptly called "pavements" by Lincecum. A few of them had in the centre low mounds, a few inches in height, and two or three in diameter. The formicaries vary in width from twelve feet to two or three feet. They are invariably located in open sunlight. The process of making a clearing strongly suggested the modes of pioneers in a forest—spires of grass taking the place of trees. The chain of evidence that determines these ants to be true harvesters in as follows: 1. Workers were seen gathering seeds and carrying them into the formicaries through the central gates; 2. The same seeds were found in granaries within the opened formicaries; 3. The seeds, with outer shell removed, were found in other granaries; 4. The ants were found carrying out shells to the refuse-heaps. The author's opinion is, that these ants do not plant seeds on purpose, but that they carefully preserve on the outer margin of the clean space the growths which arise from seeds dropped accidentally. To the question