Annals of the New York Academy of Sciences, Volume 57,
Parental Age and Characteristics of Offspring pages 476–483, January 1954

E. Ashby and E. Wangermann

The process of aging in plants differs in certain respects from the analogous process in animals. In most animals, the development of the organs takes place at about the same time; and all the organs mature, grow old, and die together. The plant is different in that its cells mature successively. The oldest cells are at the junction of root and shoot. The cells are progressively younger in time as one moves towards root or shoot apex, and at the apex itself the cells remain permanently meristematic, adding new cells to the existing plant body. It is therefore impossible to speak of the age of a shoot as a whole. Every horizontal layer of the shoot is younger than the layers below it and older than the layers above it.

This adds complications to the study of aging in plants, and makes it necessary to introduce the concept of physiological aging. Can we assume, for instance, that successive leaves on a shoot are alike physiologically when they reach the same age? Does the meristem which produces the leaves remain as young physiologically as it does in time? Or do physiological changes occur in the meristem which are analogous to those occurring in other parts of the plant, i.e., changes which are summarized by the term "aging"? There is as yet no general agreement on this question.

It would appear that there is plenty of evidence for the view that the meristem does age. There is a progressive change from node to node in the sizes, shapes, and physiological characteristics of the leaves of many plants. Such changes might be regarded as symptoms of meristem aging. It is, however, possible that these changes are due to concurrent changes in the environment. A leaf produced by a six-weeks old plant may develop under conditions of light, temperature, day-length, etc., which might be very different from those under which a leaf produced by a one-week-old plant developed. And, indeed, the changes in leaf size or shape in a number of plants have been satisfactorily explained in this way. In a number of other plants, including Ipomoea, which we used in our own work, they cannot be explained by any observable change in the environment.2 It is notoriously difficult, however, to control the environment of a potted plant. Even if it could be done adequately, the meristem of a six-weeks-old plant might nevertheless be growing in a very different internal environment from that in which it was growing when the plant was one week old. The root/shoot ratio has changed, the meristem is now a great deal further away from the root system, and there are a great many more leaves between it and the root. Because of these complications, we have found it necessary to investigate the problem of aging at the meristem by means of a plant whose environment can be controlled adequately without difficulty and whose morphology remains approximately constant.

2. Ashby, E. and E. Wangermann, Studies on the morphogenesis of leaves, IV. Further observations on area, cell size and cell number of leaves of lpomoea in relation to their position on the shoot. New Phytologist 49: 23-35. (Mar 1950)