Minimum areas and leaf determination

Abstract

In shoot apices of Lupinus albus the greater part of the presumptive area of $I_{2}$, the leaf due to arise next but one, was confined between vertical radial cuts as nearly as possible 100 degrees apart. No leaf was formed between the cuts except in apices near to flowering, in which the leaves are always much smaller. But when, in addition, the growing-point of the apex was destroyed by a shallow split, then $I_{2}$ always arose between the confining cuts. In Euphorbia lathyris the greater part of the presumptive area of one of the $I_{2}$'$\text{s}$ was similarly confined between cuts intended to be 70 degrees apart. In nine apices in which the arcs between the cuts were found afterwards to range from 58 to 88 degrees, no leaf was formed in the confined region. In one apex in which the arc was 99 degrees, an $I_{2}$ was formed between the cuts. In eleven apices of Lupinus albus the central part of $P_{1}$, the youngest visible leaf, was removed by two sloping cuts or destroyed by a prick. The angles $P_{2}-I_{1}$ $\text{and}$ $I_{1}-I_{2}$ were not changed at all by the operation. The angle $I_{2}-I_{3}$ diminished by 3 degrees.4, indicating a slight shift of $I_{3}$ towards the wound which is interpreted as due to an increase of available space. It is concluded, in agreement with previous conclusions, that in Lupinus, Euphorbia, and probably in most dicotyledons, a leaf is determined only when there is available a space on the apex far enough below the growing-point and covering some minimum arc. This arc is estimated at 122 degrees in non-flowering lupin apices with intact growing-points. It is also concluded that in Lupinus the central parts of young leaves do not repel the centres of leaves about to arise. Leaf determination in these plants is further discussed in comparison with plants of other groups.