Comparative Functional Morphology of the Anchorage Systems of Annual Dicots

Studies of root anchorage have shown that the mechanical function of roots is restricted to strengthened basal areas, whereas delicate distal roots have a role largely in absorption. Analysis further shows that to most economically withstand the upward grazing forces to which procumbent and climbing plants are subjected a fibrous root system is optimal, while tall free-standing plants need to invest in a further stiff element such as a tap-root to resist toppling. Relative investment in the fibrous roots of procumbent and climbing plants should fall with plant size but investment in the tap-root of upright plants should be size independent. These predictions were tested in three surveys in which the aboveand below-ground morphology of several species of full-grown annual dicots were examined and the relative dry mass investment in the shoot, tap-root, fibrous roots and absorption roots were measured. Below-ground morphology varied with above-ground morphology both between and within species and with plant size as predicted: free-standing plants had tap-root systems containing a relatively constant proportion (3-O/6 of total dry mass) in the tap-root and 3-5% in the fibrous roots; procumbent plants had fibrous root systems containing 15-12% of the total dry mass, larger plants having relatively smaller root systems. Multi-stemmed and rosette species had root systems of intermediate morphology. There was no relationship between above-ground form and the investment in absorption root. Mechanics has helped us explain below-ground differences in the morphology of annual dicots and may help explain growth responses to the environment.