Experiments on growth and inhibition. Part III—Inhibition and growth promotion

Abstract In a Leguminous seedling, and probably in most young Dicotyledons, the rapidly growing leaves near the apex of the shoot inhibit the growth of axillary buds (except of those that are very close bebeath them), and they also tend to inhibit the growth of other shoots, if any are present, and subsequently to kill them. But at the same time they promote the elongation of their own shoot below them, and also stimulate its growth in thickness. The question therefore arises how it is that the growing leaves tend to affect other shoots in the opposite way to that in which they affect their own shoot. This question was brought to notice very clearly by experiments previously reported (Snow 1931, b). For it was found that in a young pea or broad bean plant that possessed two equal shoots springing from the same level, if one of the shoots was deprived of its growing leaves, it was quickly inhibited in growth by the other shoot, and was killed after about 4 weeks. The question therefore arose how it was that the defoliated shoots were inhibited and killed by the intact shoots, of which the rapidly growing leaves are the inhibiting members, whereas the growth of the intact shoots themselves was not inhibited, but increased, by their own growing leaves above them. It was made clear that the explanation must be that the defoliated shoots were differently situated in relation to the growing leaves of the intact shoots; and it was pointed out that they were differently situated in that, firstly, any influence entering them from these leaves must be travelling upwards in them instead of downwards, and, secondly, they were out of the line between the growing leaves and the roots. But there was also a third difference not mentioned previously, for the stimulus for cambial growth, proceeding from the growing leaves of the intact shoots, travelled down those shoots, but did not spread up into the defoliated shoots, since it is unable to travel in the morphologically upward direction.