CHAPTER 12 



Introduction to Factors 



In earlier chapters various morphogenetic phenomena were discussed, 

 but relatively little was said as to the factors that produced them. There 

 now remains the task of relating these phenomena to changes in the 

 outer or inner environment of the plant and attempting to account for 

 their origin. This is really a part of the broader field of plant physiol- 

 ogy, and no sharp line can be drawn between the two. Much of physiol- 

 ogy, particularly those parts of it that deal with the various metabolic 

 processes, is not of primary interest for morphogenesis. Other parts of 

 it, however, such as photoperiodism, vernalization, auxin action, water 

 relations, and the carbohydrate-nitrogen ratio, for example, have much 

 significance for the morphogenetic phenomena of polarity, differentia- 

 tion, regeneration, and others. To present these morphogenetic implica- 

 tions adequately would mean going more deeply into plant physiology 

 and its vast literature than can be attempted in the present volume. No 

 discussion of the problems of plant morphogenesis would be com- 

 plete, however, without some mention of the physiological factors which 

 influence development so powerfully. The purpose of this final section 

 of the book is to introduce the reader to the more important of these 

 factors and to provide him with an entry into the literature of the sub- 

 ject. No attempt will be made to discuss them thoroughly from the point 

 of view of plant physiology. 



It is first necessary to consider the relation between the two chief 

 sorts of factors— environmental and genetic. A living plant is an or- 

 ganized system maintaining itself in a complex and changing environ- 

 ment. Its genetic constitution (or genotype) remains unaltered save for 

 occasional doubling of the chromosomes in local areas or the rare oc- 

 currence of somatic mutations. Despite this, the plant does not remain 

 unchanged. Its appearance (or phenotype) is often greatly modified as 

 the environment is altered, and we commonly say that this change is 

 the result of an environmental factor. So, in a sense, it is, but there is 

 often difficulty in disentangling the effects of heredity and environment 

 in morphogenetic changes. One should remember that both are always 



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