CHAPTER 8 



Differentiation 



At the beginning of its development the young plant, as it grows from a 

 fertilized egg or from some larger embryonic mass, is relatively simple 

 and homogeneous. A characteristic feature of the developmental process, 

 however, is the origin of differences in the amount, character, and loca- 

 tion of growth which lead to differences between the various parts of an 

 individual. Such structural or functional differentiation and its origin in 

 development constitute one of the chief problems of morphogenesis. 



Differentiation is the manifestation of that "division of labor" which is 

 so conspicuous a characteristic of living things. Organs are differenti- 

 ated. Tissues in their development become unlike each other. Cells grow 

 very diverse in character. Even the contents of a single cell are divided 

 into nucleus and cytoplasm, and each of these possesses a considerable 

 diversity of its own. There is evidence that even the clearest cytoplasm 

 possesses submicroscopic differentiation. Strictly speaking, there is 

 probably no really undifferentiated structure in a plant. Protoplasm is 

 an organized system, not a homogeneous material, and this implies a 

 degree of physical and chemical diversity. Furthermore, because of the 

 dynamic quality of protoplasm, differentiation in living cells can never 

 be entirely stable but is subject to change under changing conditions. 



Differentiation occurs wherever a true development is taking place and 

 may be expressed in many ways. At a terminal meristem like that of a typi- 

 cal shoot, the primordia of leaves, buds, and flowers early become dis- 

 tinguishable. From cambium cells, uniform in character, there differen- 

 tiate sieve tubes, fibers, tracheids, vessels, and other cell types. In the 

 primordium of a fruit, where growth is diffuse and determinate, internal 

 differences of many sorts begin to manifest themselves throughout the 

 mass. In regenerative development a single cell or group of cells may 

 dedifferentiate (p. 232) and become meristematic, and from this embryonic 

 center a new series of structures then differentiates. Many differences have 

 no visible expression in structure but involve physical, chemical, or 

 physiological distinctions only. During ontogeny the course of dif- 



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