Senescence and Rejuvenation in LemnZa minor

Abstract

STUDIES in this laboratory on the morphogenesis of leaves have disclosed that successive leaves of Ipomœa differ consistently in cell size, cell number and shape^1,2. The differences do not merely reflect changes in the external environment, but are clearly due to changes associated with the ageing of the apical meristem³. We now have evidence that in Lemna minor there is a similar ageing of the meristem. It is revealed by a progressive diminution in area of successive ‘daughter’ fronds from a ‘mother’ frond, even when the colony is grown in a constant external environment⁴. In the strain we are using, each frond produces on an average four or five ‘daughter’ fronds and then dies. In a typical experiment the mean area of the mother frond was 5·1 mm.², and its expectation of life was 36 days. The areas, and expectations of life, of its five daughter fronds were respectively 5·13 mm.² and 33 days; 3·84 mm.² and 30 days; 3·33 mm.² and 28 days ; 1·04 mm.² and 18 days ; and for the fifth, 1·10 mm.² and 10 days. Despite this evidence of senescence, the mean area of fronds in a colony of Lemna does not diminish. This is because senescence is accompanied by rejuvenation. An impoverished fifth daughter frond is about one-fifth the area of its mother frond ; but it in its turn produces daughter fronds much larger than itself, and so the mean area of fronds in the colony is maintained.