304 Morphogenetic Factors 



operating. A visible trait is the developmental reaction of a specific 

 (and constant) genetic constitution to a specific environment. Every 

 trait is therefore inherited since it will always be produced if the en- 

 vironment is of a certain sort. In some traits the expression of the genetic 

 constitution is essentially the same under a wide range of environments. 

 The relative position of the floral parts, for example, the arrangement 

 of the leaves, or the character of the pitting on the side walls of the 

 vessels in the wood is usually quite unchanged under various condi- 

 tions of light, moisture, temperature, or auxin concentration. Such 

 traits, for this reason, are especially useful in taxonomy. Others, such 

 as the height of the plant, the thickness of the cuticle on its leaves, and 

 whether it flowers or not, may be very different under different condi- 

 tions of nutrition, water supply, and photoperiod. Such traits 

 are usually said to be determined by environmental factors. Actually, 

 both types of traits are inherited and both are environmentally deter- 

 mined. In the former, the repertoire of responses of the genetic constitu- 

 tion to changes in the environment is relatively meagre whereas in the 

 latter it may be very wide. Under a given length of day, for example, 

 salvia plants will flower but lettuce plants remain entirely vegetative. 

 What promotes or inhibits flowering is not simply the day-length but 

 the different inherited responses of these two plants to this day-length. 



Where the developmental response of a plant varies widely under 

 different environments as it often does when such factors as light or 

 water or auxin concentration are changed, the obvious way to study the 

 morphogenetic processes concerned is to use genetically uniform ma- 

 terial but to change one or another of the environmental conditions 

 under experimental control. This method has proved very successful and 

 has yielded a great body of information as to the relations of environ- 

 ment to plant development. This has been by far the most fruitful 

 method of morphogenetic analysis since it lends itself so readily to ex- 

 perimental attack. 



Traits in which environmental changes have little effect on the de- 

 velopmental expression of the genetic constitution can be studied by the 

 usual techniques of genetics. These consist primarily in maintaining a 

 constant environment, crossing genetically pure stocks that show dif- 

 ferent aspects of the trait to be studied, and analyzing the results in 

 subsequent generations. There is much less opportunity here to modify 

 the variables, for the genes themselves can be altered only with great 

 difficulty and in an unpredictable fashion. The rise of biochemical 

 genetics, however, is providing a much wider basis for experiment here. 



A question often raised in the discussion of these environmentally in- 

 duced characters is whether they are adaptations and thus may serve to 

 maintain the life of the plant. Many structural traits, such as the much 



