STUDIES IN THE MORPHOGENESIS OF LEAVES

Abstract

In Part I of this paper the results of experiments are reported which show that ageing in fronds of Lemna minor is accompanied by diminution (due to a decrease in cell number) of the size of their successive daughter fronds; this is true, notwithstanding the fact that successive daughter fronds all come from the same meristem and develop under the same external conditions. By an examination of the growth curves of successive daughter fronds it was shown that the decrease in area from frond to frond is due either to a decrease in the duration of the logarithmic phase of cell division, or to a decrease in the size of frond initials. In this second part of the paper some of the possible causes for such decreases are examined. Preliminary experiments were carried out to decide between two obvious alternative explanations to account for the decrease in cell number from frond to frond in successive fronds, namely: (i) Late-formed fronds might be small because division of their cells depends on a supply of some growth-promoting substance from the mother frond which diminishes as the mother frond becomes older. Alternatively, (ii) late-formed fronds might be small because their growth is suppressed by some inhibitor from the mother frond which increases as the mother frond becomes older. These alternatives were examined by the simple procedure of removing immature daughter fronds from their mother fronds, and postulating that these excised daughter fronds will on the one hand fail to reach normal size if they depend on the mother frond for a growth-promoting substance, and on the other hand will become abnormally large if it is a growth inhibitor which a daughter frond derives from a mother frond. It was found that fronds with areas as small as I-2 mm.2 could be severed without injury by a very small cut with a cataract knife into the ventral side of the mother frond. Such severed fronds live independently in the culture solution for several weeks. They contain stored starch both before being severed from the mother frond and after several days of independent existence, and they develop roots and daughter fronds. In these preliminary experiments it was found that neither severed first-daughter fronds nor severed fourth-daughter fronds reached areas as large as those of controls left on the mother frond. Therefore the effect of the mother frond is not to inhibit but to promote growth, and it is reasonable to assume that a mother frond produces some substance which promotes the growth of its daughter fronds and which