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PLANT GROWTH 



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organ or a whole plant. The definition is thus essentially irreversible increase in volume. 

 It must be made clear that the term "growth by cell division" is inaccurate, 

 for mere division of a cell does not in itself constitute growth. Usually division is 

 accompanied by enlargement, so that the daughter cells rapidly reach the initial 

 size of the parent, and commonly it is followed by much greater enlargement (see 

 above). But cell enlargement may go on for days without any division, giving rise 



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Fig. I. Examples of varied forms of the vacuole system. Fusiform initials of the cambium. 

 1-7: Ash; {Fraxinus americana): i, "Myelin" form, transitional; 2-5, four different types from 

 material collected in February; 6, transitional form; 7, highly vacuolated form; 8, similar 

 cell from Robinia Pseiido- Acacia, growing season. Cytoplasm stippled, vacuole and nucleus not 

 stippled. Originally from Bailey (1930); rearranged for "Contributions to Plant Anatomy"; 

 I. W. Bailey, Waltham, Mass., The Chronica Botanica Company, 1954. 



to cells 100 times their initial size. It is this which constitutes the growth as ordi- 

 narily understood. Many phenomena, such as accumulation of solutes, synthesis 

 of proteins and other polymers, and changes of physical properties may accompany 

 growth without being an essential part of it. The problem of the nature of growth, 

 indeed, is largely the problem as to which phenomena are essential to the growth 

 process and which are not. 



A growth hormone or phytohormone is an organic substance, produced naturally 



