If the study of structural patterns in the shoot apices of vascular plants does not lead to an understanding of this region in functional terms, other methods of investigation must be employed to attain such an understanding. A considerable body of research has attempted to analyze more precisely the activities of the shoot tip and its component parts and has produced some new information and brought some of the more challenging unsolved problems into sharper focus. This chapter will discuss some of the significant contributions that analytical studies have made.

GROWTH OF THE SHOOT APEX

One essential function of the shoot meristem is that of producing cells. If the region is treated as a population of dividing cells, some interesting quantitative estimates of cell production can be derived. For example in Pisum sativum (garden pea) Lyndon (1968) has determined the increase in cell number in the shoot apex during a plastochron, the interval between the initiation of two successive leaves. Immediately after the initiation of a leaf primordium the apex contains 900 to 1,000 cells. While the next primordium is being formed, approximately 1,600 new cells are added so that there is nearly a threefold increase in cell number. Since the duration of a plastochron can be determined, in this case forty-eight hours, it is possible to know the average rate at which the cells are dividing. This average rate is often expressed in terms of a mean cell generation time, the average time required for all of the cells to double, that is to divide once.