Regulation of Branching in Decussate Species with Unequal Lateral Buds

In the decussate plants Alternanthera philoxeroides and Hygrophila sp. the opposite axillary bud primordia are of unequal size from the time of their inception; the larger or + buds lie along one helix and the smaller or − buds along another (helicoidal system). In decapitated plants of Alternanthera both buds grew out, but unequally; if the node was vertically split growth of the two shoots was more equal, and if the + buds were excised growth of the − shoots approximately equalled that of control + shoots. In decapitated shoots of Hygrophila grown in sterile culture only one bud, the + or larger one, grew out at each of the upper nodes. In excised cultured nodes, also, only the + bud grew out; but if the nodes were split longitudinally both buds grew out, initially rather unequally. These experiments support the view that the regulation of branching in these species has two components, apical dominance and the dominance of the larger (+) bud over the smaller (−) bud at the same node. The restriction of growth potentiality imposed on the − bud is not permanent but can be modified. Further correlative effects on bud outgrowth include those of the subtending leaves and of buds at other nodes.