| Abstract: | The anchorage systems of Himalayan balsam Impatiens glanduliferaand mature sunflowers Helianthus annuus were investigated bycombining morphological and anatomical study of the root systemswith mechanical tests on roots and with studies in which matureplants were pulled over. The root system of balsam is dominated by large numbers of fleshytapering adventitious roots which point downwards from theirorigin at the wide stem base. Sunflowers, in contrast, havea tapering tap-root from which 20–30 well-branched lateralsemerge, pointing radially outwards and downwards. Roots of eachspecies have contrasting anatomy: those of balsam resemble stems,having a central watery pith and being strengthened peripherallyby lignification of vascular tissue; roots of sunflowers arestrengthened by a solid woody stele. Roots of both species arerigid in tension and, towards the base, in bending. Both species exhibited similar behaviour to that known for treessuch as Sitka spruce; when pulled over they rotated about ahinge leeward of the stem base and a root-soil ball was pulledout of the surrounding soil. Anchorage was resolved into threecomponents which, in order of decreasing magnitude, were (i)the resistance to pulling of the roots on the windward sideof the plant (and, for sunflower, the tap-root); (ii) the resistanceof roots and soil at the leeward hinge to rotation; and (iii)the weight of the root-soil ball. Sunflower had stronger anchoragebut achieved it at a greater cost in terms of the dry mass ofits root system. In each species, the morphology, anatomy and mechanical propertiesof the root system can be related to those of the stem. Thewide stem base of balsam allows large numbers of mechanicallyefficient fleshy roots to be attached whereas in sunflowersa woody tap-root system is necessary to anchor the much narrowerstem. Key words: Impatiens, Helianthus, roots, anchorage |
