In the previous chapter four questions about leaf origin and development were posed. The first of these, which asked why any outgrowths of the shoot meristem occur at all, was left essentially unanswered. The second, which dealt with the location of outgrowth, was dealt with in that chapter. The last two, which questioned the nature of the influences that cause an outgrowth to become a leaf and the response of the outgrowth to these influences, could be phrased in another way: If outgrowths are initiated, why do some become leaves and others branch shoots? The reason for phrasing the question in this way is that in addition to the regular formation of leaves, it is characteristic of the shoots of all but a few vascular plants to give rise to a succession of branches such that the whole shoot becomes a ramifying system. Clearly the difference between a determinate and dorsiventral leaf and a branch that is a replica of the main axis is a striking one, and it is important to seek an explanation for this difference in the initiation or early development of both types of appendages. This chapter is devoted to a consideration of these questions.

LEAF DETERMINATION

In many ways, one of the most revealing approaches to the study of leaf development is that in which the partially developed organ is removed from the plant and allowed to continue its development on a culture medium of known composition in complete isolation from the parent organism.