Nature and Rate of Development of Root System of Centaurea picris

Abstract

1. Plants of Russian knapweed, 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 from the seedling stage through 72 weeks of growth. 2. The root system of well-established plants consisted of the original root (primary vertical), one to many permanent lateral roots, and their vertical extensions (secondary vertical roots). 3. The plants spread horizontally by series of permanent lateral roots. The permanent laterals of the first order arise on the primary vertical root. Unless injury or too severe competition prevents, successive orders of permanent laterals arise at or near the bend where a permanent lateral of the preceding order turns down to become a vertical (secondary vertical root). 4. The plants had spread radially 61/2 feet in one growing season, and 11-12 feet by the end of the second season, at which time several vertical roots had reached a depth of 101/2 feet. 5. The source of shoot development, other than that arising from the plumule, was from root-borne buds which produce either leafy shoots directly (if at the ground line), or rhizomes (if below ground) which in turn give rise to leafy shoots. These buds arose in greatest abundance at or near the bend separating the permanent lateral of any order from its vertical phase. The shoot development of old plants is wholly from root-borne buds. 6. The general type of development is the same as that of field bindweed and hoary cress. The rate of development of knapweed appeared to be about the same as that of hoary cress for the first 72-75 weeks. After the first 8-10 weeks following emergence, both plants developed at a somewhat slower rate than bindweed. Bindweed flowered during the first season of growth, while hoary cress and knapweed did not flower until the second season.