A COMPARATIVE-STUDY OF THE ANCHORAGE SYSTEMS OF HIMALAYAN BALSAM IMPATIENS-GLANDULIFERA AND MATURE SUNFLOWER HELIANTHUS-ANNUUS

The anchorage systems of Himalayan balsam Impatiens glandulifera and mature sunflowers Helianthus annuus were investigated by combining morphological and anatomical study of the root systems with mechanical tests on roots and with studies in which mature plants were pulled over. The root system of balsam is dominated by large numbers of fleshy tapering adventitious roots which point downwards from their origin at the wide stem base. Sunflowers, in contrast, have a tapering tap-root from which 20-30 well-branched laterals emerge, pointing radially outwards and downwards. Roots of each species have contrasting anatomy: those of balsam resemble stems, having a central watery pith and being strengthened peripherally by lignification of vascular tissue; roots of sunflowers are strengthened by a solid woody stele. Roots of both species are rigid in tension and, towards the base, in bending. Both species exhibited similar behaviour to that known for trees such as Sitka spruce; when pulled over they rotated about a hinge leeward of the stem base and a root-soil ball was pulled out of the surrounding soil. Anchorage was resolved into three components which, in order of decreasing magnitude, were (i) the resistance to pulling of the roots on the windward side of the plant (and, for sunflower, the tap-root); (ii) the resistance of roots and soil at the leeward hinge to rotation; and (iii) the weight of the root-soil ball. Sunflower had stronger anchorage but achieved it at a greater cost in terms of the dry mass of its root system. In each species, the morphology, anatomy and mechanical properties of the root system can be related to those of the stem. The wide stem base of balsam allows large numbers of mechanically efficient fleshy roots to be attached whereas in sunflowers a woody tap-root system is necessary to anchor the much narrower stem.