INVESTIGATIONS ON THE ROOT HABITS OF PLANTS

Ten years ago, while studying the vegetation of semi-arid eastern Washington, interest in root studies was aroused because of the extensive root systems of the native species. Balsamorrhiza sagittata, the first plant excavated, is representative. The strong tap root reaches a depth of six feet, and very numerous branches run laterally to distances of about two feet. Further investigations in several other states showed that extensive root systems are the rule among grassland species. A ndropogon haliji, with exceedingly well branched fibrous roots seven feet deep, is representative of many of the dominants. About ninety percent of the more important species are rooted well below the two-foot level, and not a few, such as Liatris punctata, extend to depths of fifteen to twenty-two feet. The great extent of roots in relation to above-ground parts is often very striking. Gleditsia triacanthos, thirteen weeks after seed-germination, although reaching a height of only nine inches, produced a remarkably widespread root system that extended well into the fourth foot of soil. The roots of Solidago oreophila, a small species only about a foot high, spread about two feet on all sides of the plant while some penetrate downward to a distance of two and one half feet. Nor is the great extent of roots in comparison to tops confined to native species. Maize (Zea mays indentata) has a wonderfully developed root system which occupies rather thoroughly over two hundred cubic feet of soil. The root system of the sugar beet (Beta vulgaris) is likewise very extensive, branching widely and extending downward to a depth of five or six feet. The rapidity of root growth is quite as remarkable as root extent. Spartina cynosuroides reaches a depth of over four feet at the age of eleven weeks. This growth rate of over half an inch a day is not unusual among our native grasses. The widely spreading roots of potatoes, when they begin their vertical descent, elongate at the rate of an inch a day for a period of two or more weeks. When the main vertical roots of corn begin to develop, they sometimes penetrate downward at the remarkable rate of 2 to 2.5 inches per day during a period of 3 or 4 weeks, sometimes reaching depths of more than eight feet. The rate of growth of the primary root system of winter wheat during ioor Is-day intervals is of interest (text fig. i). An average growth rate of over half an inch per day is maintained for a period of 70 days. Of course, during this time abundant laterals and also roots of the secondary system are developing.