Introduction to Factors 305 



reduced leaf size of microphyllous xerophytes, the nectaries in many 

 flowers, or the wound cork produced on an injured surface, are present 

 under almost all environments and are so deeply embedded in the 

 genotype, so to speak, that the only way they may be changed is 

 through genetic mutation. They have doubtless arisen by means of 

 natural selection, and their adaptive character is due to this fact. 



Other traits, such as the shape and structure of the leaf blade in 

 heterophyllous plants, the degree of development of vascular tissue in 

 the stem, or the place of origin of roots and shoots along a regenerating 

 axis, are often subject to very wide differences, depending on light, water, 

 auxin concentration, mechanical stresses, and other factors. Though they 

 can be greatly modified experimentally, these changes seem in most 

 cases clearly advantageous under natural conditions and are thus to be 

 regarded as adaptations. The adaptation here (doubtless also the result 

 of natural selection) is not a specific and unchanging structure but the 

 tendency to react developmentally in a favorable way as conditions 

 change. 



It seems clear, however, that in many other cases, where there is a 

 wide range of developmental differences induced by changes in the en- 

 vironment, these are not adaptive or favorable for survival but are 

 neutral in this respect. The degree of lobing in a leaf as affected by 

 temperature, the relative abundance of male and female flowers as 

 affected by nutritional factors, or the shape of the fruit as affected by the 

 size to which it is able to grow seem none of them to have significance 

 for survival. Such traits appear to be simply accidental developmental 

 results of the interaction between genetic constitution and environment. 

 Among these, particularly the ones induced by extreme environmental 

 changes, are some of the most interesting for morphogenesis. It is there- 

 fore necessary to divorce completely the problem of adaptation, which 

 is an evolutionary one, from that of the environmental induction of 

 characters, which is a morphogenetic one. 



In studying the various factors that are important in plant develop- 

 ment, emphasis in some cases is put on changes in the environment- 

 external and internal— and in others on changes in the genetic constitu- 

 tion. This is purely an arbitrary classification, however, and simply for 

 convenience. In the present treatment of the subject morphogenetic 

 factors will be discussed in several general groups. Some, such as light, 

 temperature, gravity, and some mechanical factors, originate chiefly in 

 the external environment though their effects, of course, are produced 

 internally. They may be grouped together as physical factors. Among 

 these is included water, since its morphogenetic effects (as opposed 

 to its physiological ones) are due not so much to its chemical composi- 

 tion as to the physical processes of its absorption and evaporation. 



