Regulation of sizes of leaf primordia by older leaves

Abstract

Previously we found that in Lupinus albus when the summit of the stem apex was weakened with a prick, the leaves arose subsequently from smaller areas; and we concluded that normally the summit of the apex exerts some inhibiting influence on the leaf-forming zone, which needs to be overcome before a new leaf can be determined. Here it is shown that if several of the leaves of the bud are cut down repeatedly rather low, the youngest leaf and the new leaves are found subsequently to cover larger arcs of the circumference, so that normally the leaves of the bud must somehow decrease the arcs of the youngest and of the new leaves. In order, therefore, to compare more closely the effects of the leaves with those of the summit of the apex, we repeated the experiment by which we previously estimated the primary arc over which a leaf is determined, splitting with a vertical cut the presumptive area of I1 or I2, the next 2 leaves due to arise, and later measuring the angular distance from the wound of the morphological center of a new leaf determined at 1 side of the wound and abutting on it. But this time we combined the operation with cutting down the leaves of the bud either very lightly or more severely. No difference was found to be made to the primary arc of the new leaf by the severity with which the leaves of the bud were cut down, although the variation was very small. So it follows that normally the leaves of the bud decrease the secondary lateral extension of the bases of the new leaves, whereas the summit of the apex increases the area of the apical surface which is needed for their determination. The primary leaf arc estimated from the present experiments is 90[degree], which is much less than the previous estimate of 124[degree] or 122[degree]. This is apparently because a quite different race of Lupinus was used, and other evidence confirms that in this race the primary leaf arc is much smaller.