Half shade favored the growth of seedlings of black walnut, buckeye, red oak, shellbark hickory, and hard maple but retarded growth of honey locust, boxelder, and sycamore. 2. Root systems were deeper and more branched in all cases where the seedlings grew in full insolation. 3. Transpiration rates were consistently higher in full insolation. Hard maple transpired 2.5 times more rapidly but boxelder only 20per cent more than in the shade; other species were intermediate. 4. The root systems of seedling honey locusts were 1.5 times as deep as the height of the top, and those of shellbark hickory 10 timesas deep; other species were intermediate. 5. Root penetration of all seedlings, except red oak, was greatest in loess soil, where depths of 36 to 65 inches were attained. 6. Total lateral spread of seedling roots varied from 6 to 18 inches, except those of black walnut which spread 4 feet in clay, 3 feet in alluvial soil, and 3.5 feet in loess. 7. Roots were most poorly developed in the alluvial soil by midsummer, owing to deficient aeration in spring; by September honeylocust and black walnut had penetrated deeper here than in the clay soil. 8. Honey locust has a generalized root system readily modified byenvironment. In upland soil taproots of saplings penetrated to 5 feet, but on the floodplain they penetrated only to 2 feet, and laterals extended outward 10 to 17 feet. 9. The generalized root system of boxelder is very plastic. On the upland the taproot and nearly all the major branches penetratedeeply. In alluvial soil the root system is much shallower but extends widely in the surface foot. 10. The taproots of 6-year-old sycamore saplings penetrated upland soil to about 7 feet, branched widely at all depths, and were also furnished abundantly with deeply penetrating laterals. 1. Seven- to 12-year-old black walnut saplings developed strong taproots 4-6 feet long with many oblique branches spreading 3-6 feet and often reaching similar depths. In addition numerous strong horizontal laterals extended outward to distances two or three times the diameter of the crown. 12. The root systems of 5- to 8-year-old red oaks were deep, abundantly supplied with mostly horizontal laterals in their upper course and obliquely descending ones at greater depths. A column of soil 9 feet square and 5 feet deep was usually well occupied and a few strong branches extended 6 or more feet laterally. 13. The widely spreading root system of shellbark hickory thoroughly occupied the soil within a radius of 6 feet from the treeand to a depth of 5 feet when only 6 to 8 years old. 14. Hard maple has a relatively shallow root system, that of a 10- to 16-year-old sapling extending to depths of only 2.5 to 3 feet. Large laterals were fairly numerous; a few spread beyond 3 feet, but the smaller branches were relatively few. 5. The sturdy taproot of bur oak penetrates upland clay to 15 feet in 8 years. Laterals from the upper portion were abundant and afew spread far outward and in addition gave rise to sinkers that extended deeply. Deeper laterals usually pursued an obliquely downward course. 16. The shallow rooted cottonwood of the floodplains sends its strong taproot deeply in upland soil, at the rate of 1 foot or more each year. It is well furnished with laterals throughout but these do not spread widely for 6 or more years.
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H. H. Biswell (1935) studied this question.