SEED-GERMINATION AND SEEDLING-GROWTH. 17 



most plentiful supply of food-material, and in winter, at 

 the North, the feeding-roots have no protection. Koot- 

 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 condi- 

 tion 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." 



20. Root-division. The common practice of growing 

 trees and some shrubs and plants from root-cuttings well 

 illustrates some of the varied peculiarities of root-struct- 

 ure. It may be said that all trees and plants which 

 naturally sucker from the surface-roots, or which may be 

 made to sucker by breaking with the plough, or cut with 

 the spade, may be grown from root-cuttings. But this only 



