Differentiation 215 



siventral character of the lateral branches persists in cuttings made from 

 these branches. All these cases of persistent differentiation are of particu- 

 lar interest in providing material for a study of the cause and character of 

 differential change. 



The origin of differences arising at different times in a repeated cycle, 

 rather than at a different place on the plant, Seeliger (1924) has termed 

 cyclophysis. 



DIFFERENTIATION IN RELATION TO ENVIRONMENT 



Most of the examples of differentiation thus far cited seem to be pri- 

 marily the expression of a developmental pattern controlled by the genetic 

 constitution of the individual. Obviously such a constitution cannot op- 

 erate except in an environment of some sort, for genes control specific 

 differences in reaction to specific environmental factors. It is therefore to 

 be expected that differentiation should be greatly influenced by the en- 

 vironment, both internal and external. 



The basis for differentiation itself is provided by the environment, for 

 the most important contribution that the physical environment makes, 

 morphogenetically, is to set up a gradient in the organism. This cannot 

 be done unless the environment itself displays a gradient in direction or 

 intensity. Fern prothallia, for example, grown in culture on a shaking 

 machine, and thus exposed equally to gravity on all sides, or on a revolv- 

 ing table, and thus exposed equally to light on all sides (p. 137), are in a 

 homogeneous environment which has no gradients, no single direction of 

 gravity or light. As a consequence the organism produces an amorphous 

 mass of tissue for it is without a polar axis, the basis for its differentiation. 

 Such an axis must be induced, at least at the very start, by an asym- 

 metrical environment. 



Environment and External Differentiation. The most obvious relation 

 between environment and differentiation is in the effect that external 

 factors have on the form and character of plant organs. Most of the final 

 part of this book will be concerned with the morphogenetic effects of such 

 factors. Light influences the differentiation of reproductive and other 

 structures by its intensity, its wave length, and the duration of its photo- 

 period. The amount of available water is important in the induction of 

 xeromorphic structures. Temperature, particularly in early development, 

 seems to affect the rate of certain processes that are precursors to flower- 

 ing. Chemical agents, notably growth substances, have a marked influence 

 on differentiation of all sorts. The discussion of these problems must wait 

 until later pages. There are a few conspicuous instances, however, where 

 differentiation obviously is dependent on environmental factors which can 

 best be described here. 



