CHAPTER XXXIII 

 FACTORS AFFECTING GROWTH 



Regardless of the habitat in which it is growing, whether a greenhouse, 

 cultivated field, forest, prairie, mountain top, or lake bottom, a plant is 

 continuously subjected to the variabilities of a complex, more or less inter- 

 dependent set of environmental factors. The environment is the foster parent 

 of every plant and animal and plays as indispensable a role in its development 

 as do hereditary factors which have been transmitted to it from its biological 

 parents. 



The development and reactions of an organism are the result of the coordi- 

 nated interplay of the hereditary factors and environmental conditions upon 

 the internal physiological processes of that organism, as indicated in the fol- 

 lowing diagram: 



Genetic Constitut}ons^j^^^j.j^^j Processes _^ Organic Development 



T. . / and Conditions "^ and Behavior 



hnvironment-^ 



Under the term "internal processes and conditions" are included all of the 

 manifold variations possible in the physico-chemical conditions within cells 

 and tissues, as well as the relative rates of the various fundamental physio- 

 logical processes. With our present state of knowledge we are able to visualize 

 only some of the grosser aspects of these processes with any great clarity. 

 Most of the preceding chapters of this book have been devoted to a discussion 

 of the internal processes and conditions in plants. The intermediate stages 

 between genetic constitution and environmental factors on the one hand, and 

 the development or reaction of the organism on the other, is a large and 

 intricate one and is far from being bridged in terms of present day knowledge. 



As a specific example of this principle we will recall the process of chloro- 

 phyll synthesis as it occurs in corn (maize). The usual varieties of this 

 species contain the genetic factors which ordinarily induce chlorophyll forma- 

 tion. Certain environmental conditions, including light, are also necessary 

 for its synthesis. A corn seedling developing in a dark room is devoid of 

 chlorophyll, even if all the other environmental conditions necessary for 

 chlorophyll formation are present. In a seedling growing in the light, how- 



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