CHAPTER 16 



Various Physical Factors 



There are a number of other morphogenetic factors which may be grouped 

 together as physical ones in a general sense, notably such clearly me- 

 chanical factors as external compression, tension, bending and swaying; 

 gravity and inner tissue tension, together with absolute size and bio- 

 electrical factors, each with its bearing on development. 



Those factors which may be called mechanical in the strict sense are 

 relatively simple in character in comparison with light, electricity, and 

 many chemical ones and evidently produce most of their morphogenetic 

 effects indirectly through modifying physiological processes in the living 

 cells. Much of the early work in this field was done by Schwendener 

 ( 1878, 1898 ) . Among other problems, he emphasized the importance of 

 such factors in determining the arrangement of leaf primordia at the 

 apical meristem. 



Mechanical effects are in many cases much like the ones produced by 

 other factors, suggesting that both are acting upon the same proto- 

 plasmic mechanisms. Such parallel effects are familiar to the student of 

 morphogenesis and emphasize again the importance of the complex re- 

 acting system rather than that of the relatively simple stimulus or evo- 

 cator. There has been a good deal of disagreement as to experimental 

 results in this field, much of which is probably due to the fact that the 

 reactivity of the plant is very different at different stages of its develop- 

 ment and under different environmental conditions. 



In studying these effects it is sometimes difficult to separate various 

 plant movements and tropisms from more strictly form-producing and 

 morphogenetic phenomena. Changes in the positions of parts, as in the 

 leaves of Mimosa, the fly-traps of Dionaea, the stamens of various plants, 

 and other structures, are due chiefly to changes in turgor brought about 

 by specific substances. This is essentially a problem in plant physiology 

 and offers opportunity to study the mechanisms of stimulus and response, 

 with their various chemical and electrical correlates. 



Tropistic responses to gravity and light, however, are usually due to 

 more rapid cell elongation on one side of the axis than on the other and 

 are thus, in part, growth reactions. The various thigmotropisms, or 



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