Differentiation 205 



DIFFERENTIATION DURING ONTOGENY 



Differentiation, like most problems of morphogenesis, must be studied 

 not only as it is found in the structure of the mature plant but as it arises 

 during development. The mature plant is obviously very different from 

 the embryo and the seedling, but an important question, still far from 

 settlement, is whether the changes that take place here are simply the 

 result of increased size and the effects of environment or are internal 

 modifications arising during development and becoming manifest in the 

 progressive differentiation of the individual as its life cycle unfolds. 



It is obvious that environment is of great importance in determining 

 the differences that arise, and most of the experimental work in morpho- 

 genesis is concerned with a manipulation of environmental factors. It is 

 also clear that the specific response to an environment depends on the 

 innate genetic constitution of the individual. What is not so evident, how- 

 ever, is whether this response always remains the same or changes as the 

 organism grows older. 



There is a good deal of evidence that changes in the plant, independent 

 of environmental conditions, do indeed occur as development proceeds. 

 Juvenile stages are often very different from adult ones. That these are 

 real and often irreversible differences is proved by the fact that they can 

 be perpetuated by cuttings. Progressive changes in the shape and char- 

 acter of organs, especially leaves, at successive points along the stem have 

 often been observed and by some biologists are attributed to advancing 

 maturity or physiological aging. The onset of actual senescence has been 

 reported in some cases. A considerable school of physiologists believe that 

 the life history of a plant, particularly up to the time of flowering, consists 

 of a series of successive phases, each the necessary precursor of the next 

 but independent of the amount of growth attained. This concept has come 

 in part from the idea of vernalization ( p. 339 ) . 



In such phasic development the major change is the onset of the repro- 

 ductive period after one of purely vegetative development. This appar- 

 ently begins by a physiological change, the "ripeness to flower," as Klebs 

 called it. Only after this has begun do the floral primordia appear at the 

 meristem. They may not be the first visible evidence of the onset of repro- 

 duction. Roberts and Struckmeyer ( 1948 and other papers ) have shown 

 that the induction of the flowering phase is very early indicated by a 

 number of anatomical changes. Root growth is much reduced, cambial 

 activity almost ceases, and the vascular tissues tend rapidly to complete 

 their full differentiation. In other words, the plant structures become ma- 

 ture. Reproduction is a sign of maturity, and these anatomical changes are 

 evidence of a more general one that is about to take place. Many factors 



