Comparative Studies of Plant Growth and Distribution in Relation to Waterlogging: X. Differential Formation of Adventitious Roots and their Experimental Excision in Epilobium Hirsutum and Chamerion Angustifolium

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(1) Response to soil waterlogging was experimentally compared in two closely related species. Epilobium hirsutum was almost unaffected but Chamerion angustifolium was seriously harmed by 31 days of waterlogging which reduced dry weight production and total leaf area of adult plants. (2) The first symptom of damage in C. angustifolium was total wilting which occurred within 12 hours of flooding; it was followed by the slow recovery of the upper leaves and death of the lower leaves. (3) Within a few days of flooding, E. hirsutum commenced adventitious root formation from primordia already present near the stem base. (4) Once the first adventitious roots appeared, the stem base swelled consequent on callus formation in the cortex, medulla and probably vascular ray parenchyma. Later-formed adventitious roots arose from this callus which became so massive as to produce vertical, callus-filled cracks on the stem base. Both adventitious roots and stolon buds emerged from these cracks. (5) C. angustifolium produced no adventitious roots and showed no stem base hypertrophy. (6) A few lateral roots of C. angustifolium and many more of E. hirsutum emerged from the soil surface and floated in the floodwater. (7) Wild E. hirsutum formed adventitious roots and showed stem hypertrophy in wet sites but this was never observed in C. angustifolium. (8) Neither species showed significant accumulation of iron or manganese in leaf tissue during 31 days of waterlogging; this contrasted with many plants previously described in this series. (9) Pruning of adventitious roots limited growth of waterlogged plants but had no effect on unflooded plants in which adventitious rooting was stimulated by burial of the stem base. (10) Waterlogged plants with pruned adventitious roots were unstable and easily felled by wind or flooding. (11) Pruning of superficial adventitious roots limited the depth of the primary root system.