Differentiation 183 



whereas another photoperiod will stimulate the differentiation of re- 

 productive structures ( p. 315 ) . 



When the cycle of differentiation is complete, growth usually ceases. 

 Thus, after the formation of reproductive organs in one of the higher 

 plants has begun, growth of the plant as a whole is reduced and finally 

 stops. When a fruit is fully differentiated, its growth in volume ceases, 

 though dry weight may continue to increase for some time. The two 

 processes of growth and differentiation may go on at different rates, and 

 therefore their relative rates are important in determining differences in 

 size. Where growth is relatively rapid, a large size will be attained before 

 the completion of the cycle of differentiation stops; where it is slow, the 

 cycle will be complete before much growth has occurred and the struc- 

 ture will be much smaller. This is well illustrated by the analyses of 

 inherited size differences in gourd fruits (p. 20). The balance between 

 these two major processes in development— the addition of new material 

 and its differential distribution— is of much significance. 



The process of differentiation and the problems it presents may be ex- 

 amined from several different points of view. 



1. Differentiation may be studied in plant structure, for it is here that 

 differences can most readily be seen. For purposes of convenience we 

 may distinguish between external differentiation, which involves the out- 

 ward structure and configuration of the plant, and internal differentiation, 

 which involves the cells and tissue systems of which the plant body is 

 composed. 



2. Differentiation may be considered in its ontogenetic aspects. It is not 

 a static process, evident in mature structures alone, but often changes its 

 expression during development. Differentiation in a young plant is unlike 

 that in a mature one, and these changes proceed in an orderly cycle 

 of development both of the plant as a whole and of each of its com- 

 ponents. Such differences are not in structure alone but in the reactivity 

 and developmental potency of its parts. 



3. What course differentiation will take is determined not only by the 

 genetic constitution of the plant but by the particular environment in 

 which development takes place. External factors of many sorts affect the 

 character of the structures which arise in the process of differentiation. 



4. The ultimate basis of differentiation must be in physiological 

 changes in the living material itself. Most of these express themselves 

 sooner or later in visible structural diversity, but there are many cases in 

 which cells, structurally alike, can be shown to differ physically, chemi- 

 cally, or in physiological activity. 



Examples of these four aspects of the process of differentiation, and the 

 problems they involve, will be considered in the present chapter. 



