It may be said that Nature protects tree- and plant-roots by shading in summer and surface-protection in winter. Under clean culture the bare surface is heated abnormally between the rows of cultivated trees to such extent that surface-roots and root-hairs cannot come near enough to the surface to reach the most plentiful supply of food-material, and in winter, at the North, the feeding-roots have no protection. Root-killing is quite common in the States west of the lakes and not unusual in open winters as far east as the Atlantic States. In Iowa, Wisconsin, and other western States even the forest-trees in great number are root-killed at times, where pasturing or other agencies have removed the undergrowth and leaf and leaf-mold protection. Hence the growing of cover-crops is now being considered across the continent, especially north of the 42d parallel. By "cover-crops" is meant early culture followed by the sowing of a crop of peas, beans, cowpeas, vetch, or other leguminous plant for soil-covering during late summer and autumn and for winter protection. This gives a near approach to forestry conditions and the needed supply of nitrogen and surface-soil humus. Some of the advantages of this system of culture are summed up by Professor Bailey as follows: "(1) Cover-crops improve the physical condition of the land ; (2) prevent hard soils from cementing or puddling; (3) hold the rains and snows until they have time to soak away into the land; (4) dry out the soil in spring, making earlier tillage possible; (5) sometimes serve as a winter protection from frost; (6) catch and hold some of the leaching nitrates, of which the roots of trees are in little need late in the season; (7) add humus to the soil; (8) render plant-foods available; (9) appropriate nitrogen if it is a leguminous crop."