220 The Phenomena of Morphogenesis 



cell occupies in the whole developmental pattern and in the ontogeny 

 of the individual and especially that it depends upon the point that the 

 cell has reached in its own cycle of maturity. The developmental expres- 

 sion of the genetic constitution of a plant and of its various parts is there- 

 fore not fixed and constant but is continually changing. 



PHYSIOLOGICAL DIFFERENTIATION 



All differentiation, of course, has its basis in the physiological activities 

 of living substance, but it can usually be recognized most readily when 

 these activities result in the production of visible differences in structure. 

 It is such differences that have chiefly been considered in the preceding 

 pages. Physiological differentiation itself, however, can often be demon- 

 strated, and experiments in this field offer hope for the solution of many 

 developmental problems. A few typical examples will be discussed briefly 

 here and others in later pages. 



The diversities in structure between root and shoot are doubtless the 

 expressions of fundamental physiological differences. One of the most con- 

 spicuous of these is in vitamin synthesis. By culture methods it is possible 

 to grow roots indefinitely from a bit of root tip. Such root cultures must 

 be provided with the necessary mineral salts and also with a source of 

 carbohydrates (usually sucrose). These alone prove to be not enough to 

 secure indefinite growth, and they must be supplemented by small 

 amounts of certain vitamins, in most cases thiamin. It is clear, therefore, 

 that such roots are unable to synthesize this vitamin. Since thiamin is 

 known to be present in the shoots of plants, this is evidently the region 

 in which it is produced. In nature, roots must obtain their supply from 

 the shoots. Just when this physiological differentiation first occurs is not 

 known, but it is probably at the time when root and shoot are set apart 

 in early embryology. 



It has been shown that root and shoot also differ in their ability to 

 synthesize certain alkaloids. Tobacco shoots can be grafted onto tomato 

 roots, and leaves and stems of such shoots are free from nicotine (Daw- 

 son, 1942). If tomato shoots are grafted on tobacco roots, however, the 

 tomato tissues contain large quantities of this alkaloid. It is therefore 

 obvious that in such cases the capacity to synthesize nicotine is confined 

 to the tobacco root and is not possessed by its leaves, as has commonly 

 been assumed. Certain other alkaloids (as quinine) can be shown by such 

 experiments to be synthesized in both roots and shoots. The fact that a 

 substance occurs in a certain part of the plant is evidently no proof that 

 it is formed there. This technique of reciprocal grafting provides a useful 

 tool for the demonstration of physiological differentiation of this sort. 



