Differentiation 189 



(1931). In inflorescences the leaves may be reduced to bracts. Foliage 

 leaves, bracts, and cataphylls are presumably equivalent morphologically, 

 and the development of a primordium into one or the other depends on 

 its function. Primordia at a meristem are thus multipotent (Foster), 

 since they may form several kinds of structures. 



How a given structure differentiates is closely related to the position 

 that it occupies in the developmental pattern. Not all morphologically 

 equivalent organs develop alike. In Ginkgo the axis is differentiated 

 into short shoots and long shoots (Gunckel and Wetmore, 1949). All 

 buds form short shoots but some of these will grow into long ones. The 

 ratio of the two affects the form of the tree. A somewhat similar situation 

 occurs in Cercidiphyllum (Titman and Wetmore, 1955). 



The vertical axis of a tree and the lateral axes ( branches ) that it bears 

 also may differ markedly, the former being radially symmetrical and 

 orthotropic, the latter more or less dorsiventral and plagiotropic. Conifer- 

 ous trees offer familiar examples, where the lateral branches are much 

 flattened and branches of the second order occur in two lateral ranks. In 

 Abies and Picea this is evidently related to gravity, for if the terminal 

 bud or branch is removed, a lateral branch will bend upward and replace 

 it. In Araucaria excelsa, however, this difference is so deeply seated that 

 if cuttings are made from the lateral branches, the flattened character 

 now persists in the new plant even if the cutting is oriented vertically. 

 Not only structure but physiological behavior may be permanently 

 altered, for such lateral branches will grow horizontally in whatever posi- 

 tion they may now be placed. Carvalho, Krug, and Mendes (1950) 

 report a similar behavior in Coffea. There are many other examples of 

 such topophysis (p. 212). 



The differentiation of particular organs— root, stem, leaf, flower— 

 during development is markedly influenced by growth substances of 

 various sorts (Chap. 18). 



Internal Differentiation 



Visible differentiation involving external diversity in organs and their 

 parts is accompanied in most plant structures by a high degree of internal 

 diversity. This involves differentiation among various types of cells and 

 tissues. 



Histological differentiation presents two chief problems : ( 1 ) How do 

 cells become different from one another and (2) what is the origin 

 of the various tissue patterns found in the internal structure of plant 

 organs? 



Origin of Cellular Differences. Cells differ from each other in many 

 ways. Frequently it is possible to determine the exact cell division at 

 which such a difference becomes evident. In young roots of certain grasses 



