The Growth of the Isolated Root in Culture

Abstract

SUMMARY This paper presents a discussion of the conditions that control changes in the rate of division in the apex of the root. Data published earlier indicate three phases in the change in the total number of cells in a cultured root; a lag phase, a phase of exponential increase, and a final phase in which numbers are approaching a stationary state. It is suggested that this succession is determined by change in metabolic state that occurs in cells in different parts of the root with age. It is a fact that has frequently been recorded that even when environmental conditions are constant the absolute growth-rate of any plant system changes with time. This phenomenon may be observed in a variety of situations, and this contribution is concerned with the origin of the changes in rate in the isolated cultured root. In many respects this is admirable material, since it provides a considerably simpler system than that of the whole plant, and while it represents a fully differentiated and organized unit its growth is not affected by that of other organs. Further, since the root grows fully immersed in a liquid medium the experimental situation is not complicated by nutrients being available only to certain regions. In the culture nutrients are equally accessible to all parts of the root. The discussion is based on certain measurements of growth in terms of changes in numbers of cells, and it involves a revaluation of data already published by Brown and Wightman (1952). It may be emphasized that change in total number of cells is a particularly sensitive and instructive measure of growth. It has been used of course with conspicuous success in analysing the growth of cultures of unicellular organisms. It has not hitherto been used with multicellular organisms and the data given by Brown and Wightman represent the first attempt to measure the growth of a multicellular system in terms of change in total number of cells. At the same time it may be urged that the method is likely to be as successful with multicellular as it has been with simpler systems, since it is a measure of the basic biological process on which all other aspects of growth must depend and from which they must proceed. The data of Brown and Wightman show with a variety of conditions the change in the number of cells in an average root of pea over a cultural period which varied from 7 to 9 days. They also show in certain cases the number of non-vacuolated cells available in the apical meristem and in lateral root primordia at different stages of culture. The values for non-vacuolated cells are important since from them changes in the absolute increment may be analysed in terms of rates of division.