Nature and Rate of Development of Root System of Gonolobus laevis

Abstract

1. Plants of climbing milkweed, 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 25 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 laterals which continued to grow horizontally and on which arose roots that grew either downward directly, or did so after short horizontal growth, to become secondary vertical roots. 3. The plants spread horizontally by means of the 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 laterals of the first order. In a similar manner permanent laterals of the third order arose on those of the second, and those of the fourth order arose on those of the third, etc. Concurrent studies indicated that injury or too severe competition prevents extensive horizontal growth of the laterals of any order. 4. The plants spread radially 9 feet in undisturbed soil (10 feet where the last 4 feet penetrated soil which had been excavated and replaced two years earlier) and reached a depth of 31/2 feet in a growing season of 25 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 in the location of these adventitious buds on the permanent lateral roots. None of these buds was observed on secondary vertical roots. The shoot development of old plants was wholly from root-borne buds. 6. The type of development is similar to that of dogbane, and their common type of development 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-climbing milk-weed 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-climbing milkweed type, secondary vertical roots arise on the permanent lateral roots. The development of shoots appears to be the same in both types. Field bindweed and climbing milkweed were the only two of the five species to flower and fruit during the first growing season.