Introduction

Growth is one of the most fundamental and conspicuous characteristics of living organisms, being the consequence of increase in the amount of living protoplasm. Externally this is manifested by the growing system getting bigger, and growth is therefore often defined as an irreversible increase in the mass, weight or volume of a living system. The size increase must be permanent; the swelling of a cell in water is not growth, being easily reversed by returning the cell to a solution of lower Ψ. It is, however, possible to consider as growth developmental changes not immediately involving an increase in size. An amphibian embryo, or a Selaginella female gametophyte, for a long time utilizes the nutrient store with which it was released from the parent, to produce many new cells without any increase in overall size, yet growing in the sense that living protoplasm is increasing at the expense of stored nutrients. Again, if dry mass is measured, a flowering plant seedling loses dry mass while utilizing reserves and growing.

Growth is an exceedingly complex process. Every reaction associated with the synthesis and maintenance of living protoplasm is associated with it, which makes it complicated enough at the cellular level. At the organismal level, it means the coordinated multiplication, size increase and specialization of millions of cells, all arranged in precise positions. Growth processes are also synchronized with seasonal changes, plants responding to appropriate environmental stimuli to achieve this synchronization.