CHAPTER XXVII 

 GENERAL REGIONS AND PROCESSES IN STEMS 



The facts already learned about processes and structures in cells and 

 tissues of leaves may be applied at once to an understanding of stems. 

 Moreover, we should by this time more fully appreciate and apply four 

 generalizations concerning the relations of structures and processes : ( 1 ) 

 that all structures are the consequences of certain processes; (2) that 

 these structure-forming processes are conditioned by heredity, and influ- 

 enced by environmental factors; (3) that differences in structure, 

 whether they are dependent upon differences in heredity or in environ- 

 ment, were preceded by differences in processes; and (4) that after 

 the structures are once formed they in turn influence all the processes 

 that continue to occur in them. In the light of these generalizations, 

 reference to any structure as having a "function" leads to a misinterpre- 

 tation of all plant phenomena. 



The more minute differences in stem structure, like those of leaves, are 

 so numerous that one might study them for a lifetime without learning 

 all about them. Much useful and interesting information about processes 

 and structures in stems may be obtained, however, in a short time, and 

 much of it without even a microscope. All one needs at first is a sharp 

 knife and the stems of several kinds of plants. The familiar trees, shrubs, 

 and herbs of any locality are suitable material. In some of them certain 

 stem structures are more prominent than in others, and there are particu- 

 lar structures that are not present in the stems of all kinds of plants. A 

 hand lens will be useful in these preliminary observations, and a micro- 

 scope will be needed to obtain answers to some of the problems that 

 arise. 



Woody stems. If small pieces of woody stems, such as those of apple, 

 ash, and grape, are cut crosswise and split lengthwise, the bark, the 

 wood ( xylem ) , and the pith are at once evident. The relatively greater 

 hardness of the wood cylinder may be detected by pressing on each tissue 

 with the edge of the thumb nail. These three regions are characteristic of 

 all woody stems. 



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