Page:Popular Science Monthly Volume 80.djvu/220

216 in poorly defined bunches are, however, conspicuous wherever they are found.

Within the bunch-grass association there are a number of secondary types that are quite well defined. But as one views the vegetation of the Sand Hills in a general way these are lost in the great monotony of the bunch-grass association. However, the vegetation and general life conditions of the uplands, the home of the bunch-grasses, are very noticeably different from those characters on the river flats or in the wet valleys. The exposure to both the drying and the mechanical effects of the wind is most keenly felt on the uplands, composed of the hill tops and upper slopes. Sometimes the wind sweeps over the crests of the hills with such terrific force that one can not stand in its path and endure the sting of the sand blast. During a bright day with a high temperature and such a wind, life on the hills is well nigh impossible. During the hottest days of summer the surface sand in such situations is frequently heated to a temperature of 140° F. Such conditions with a low water content of the soil and a high saturation deficit are the factors that plants must meet. One can not but admire the vegetation that possesses the power of successfully resisting such a combination of conditions. That Sand Hill vegetation has been very successful in meeting these conditions is fully attested by a glance at the region as we find it to-day and a comparison with the dismal waste of bare sand dunes that once wandered over this same area.

Aside from the bunch-grasses, the most characteristic plant of the uplands is the dagger weed (Yucca glauca) which often occurs m great abundance on the upper slopes. In certain portions of the region over restricted areas this species really becomes dominant and the bunchgrasses then play only a subordinate part in the floral covering. The dagger weed reaches its best development on the south and west exposures, although it is by no means confined to these slopes. The sand is often blown away from the roots for many inches beneath the rosette of bristling leaves, and yet the plant continues to thrive. Frequently it puts out new shoots from the exposed roots and develops new rosettes of leaves beneath the old.

The so-called "cat steps" formed on steep slopes in the Sand Hills owe their origin to the grazing habits of cattle and very frequently to the presence of dagger weeds. On such slopes the cattle, avoiding the sharp-pointed leaves of the Yucca, follow angling paths which eventually become netted and worn into the sand in such a way as to cover the hillside with a network of trails. Clumps of dagger weed often fill in the more or less diamond shaped meshes of this network. From a distance, such a slope bears a close resemblance to the "cat steps" so commonly seen on steep slopes in the loess region. The origin is, however, very different. One may find in the Sand Hills a great many stages in