586 FACTORS AFFECTING GROWTH 



ever, interaction of the environmental factors with the hereditary mechanism 

 will occur in such a waj^ as to induce the process of chlorophyll synthesis in 

 the leaf cells. As this example illustrates, while environmental conditions 

 rarely have a direct influence upon the genetic makeup of an organism they 

 often exert a profound influence upon the expression of its heredity. 



So far, however, we have considered only one side of the story. Certain 

 varieties of maize do not carry all of the genetic factors necessary for the de- 

 velopment of chlorophyll. This trait is inherited in such strains of corn as 

 a Mendelian recessive and hence is apparent only in plants homozygous for 

 this factor. Even if all the environmental factors necessary for chlorophyll 

 synthesis are present such seedlings cannot make chlorophyll and they develop 

 as "albinos." As soon as the food stored in the seed is exhausted such albino 

 seedlings die. 



The genetic constitution of a given organism is a constant quantity and 

 sets definite ultimate limits to the types of development and the reactions of 

 which that organism is capable, beyond which no environmental condition can 

 carry it. A potato plant, for example, remains unmistakably a potato plant in 

 any environment in which it can survive. 



The specific environment to which a plant is subjected also sets limits 

 upon its development. For example under "short day" conditions, as dis- 

 cussed later in this chapter, a radish plant continues to grow vegetatively for 

 an indefinite period of time. Although radish plants possess the hereditary 

 capacity for reproductive development such an environment imposes a barrier 

 to the expression of this particular potentiality. On the other hand, under 

 long day-lengths, if other environmental conditions are favorable, a radish 

 plant will flower and produce fruits within the course of a few weeks. In- 

 numerable other examples of the environmental limitation of the expression 

 of hereditary factors can be cited. 



The full gamut of the hereditary potentialities of a species can never 

 be realized until individuals of that species have been observed growing 

 in each of a wide range of environmental complexes. Since most observa- 

 tions of the behavior of plants are made while they are growing under natural 

 or cultural conditions which represent only a rather narrow range of variations 

 in the environmental complex the many possible developmental reactions of 

 a given species of plant are not always appreciated. 



Environmental Factors Influencing Plant Growth. — The environment 

 of living organisms is so complex as to defy any completely logical analysis. 

 However, the important physical factors of the environment which ordinarily 

 exert a more or less direct effect upon the growth and development of terres- 

 trial plants can be recognized and are enumerated in the following list: 



