Nature and Rate of Development of Root System of Apocynum cannabinum

Plants of dogbane, grown from seed on a typical upland loam soil at Manhattan, Kansas, under known temperature and precipitation conditions and not subject to competition, were studied at various ages, from the seedling stage through 28 weeks of growth, to determine the nature and rate of development. 2. The root system of well-established plants consisted of the original root (primary vertical) and one to many permanent lateral roots which continued to grow horizontally and on which arose roots that either grew downward directly, or did so after short horizontal growth, to become secondary vertical roots. 3. The plants spread horizontally by means of these permanent lateral roots. The permanent laterals of the first order arose on the primary vertical root. Branch lateral roots (laterals of the second order) arose on the permanent lateral roots of the first order. In a similar manner permanent lateral roots of the third order arose on those of the second, and those of the fourth order on those of the third, etc. Concurrent studies indicated that injury or too severe competition prevents extensive lengthwise growth of the lateral roots of any order. 4. The plants spread radially 112/3 feet and reached a depth of 7 feet in a growing season of 28 weeks. 5. The source of shoot development, other than that arising from the plumule, was from root-borne buds which produced shoots directly (if at the ground line), or rhizomes (if below ground) which in turn gave rise to leafy shoots. There was no regularity to the location of these adventitious buds on the permanent lateral roots. None of these buds was observed on primary or secondary vertical roots. The shoot development of old plants was wholly from root-borne buds. 6. The general type of development of dogbane exhibits certain similarities to, and certain differences from, the type common to field bindweed, hoary cress, and Russian knapweed. The horizontal spread in both types is by means of permanent lateral roots, but in the dogbane type the lateral roots continue to grow horizontally and do not bend downward to produce vertical roots (secondary verticals) as they do in the other type. In the dogbane type, secondary vertical roots arise on the permanent lateral roots. In general, dogbane and field bindweed appear to grow more rapidly, at least in radial spread, for the first growing season of 28 weeks than do hoary cress and Russian knapweed, when all are free from competition. Dogbane had the most rapid vertical penetration in this interval. Dogbane has the coarsest and bindweed the finest roots. The development of shoots appears to be the same in both types. Bindweed was the only one of the four species considered in these studies which flowered during the first growing season.