[Chap. XXXVII HEREDITY IN PLANTS 439 



of the same parents, or even of one plant, some of the individuals may 

 differ greatly from others. All these variants among the progeny that 

 develop in the same environment are the results of heredity, and they 

 are referred to as heritable variations. Hereditary resemblances and dis- 

 similarities reappear in succeeding generations. Maple leaves continue 

 to reappear in opposite arrangement on maple trees growing in very 

 different habitats; peach fruits develop from the flowers of peaches but 

 not from apple blossoms; and white oaks grow only from the acorns of 

 white oaks. Some of the progeny, however, may resemble a grandparent, 

 or even a remote ancestor, more closely than the immediate parent. 



On the other hand, if the individuals of the progeny differ from each 

 other solely as a consequence of having developed in different kinds of 

 environments, these differences will not reappear in subsequent genera- 

 tions unless equivalent environmental conditions are repeated. These 

 differences are non-heritable, in contrast to those that are heritable. We 

 have already learned to refer to non-heritable variations as fluctuations. 

 They should also be recognized from now on as differences in degree of 

 expression of certain hereditary properties, or potentialities, of the plant. 



The expression of hereditary potentialities during the development 

 of a plant is, of course, dependent upon external conditions. The po- 

 tentiality of producing chlorophyll, for instance, is not expressed in most 

 "green plants" growing in the dark. Many other examples may be re- 

 called. Every plant has a great many hereditary potentialities, some of 

 which are expressed more than others in a particular environment 

 (Chapter XXXI). What any plant does during its lifetime depends, 

 therefore, upon its environment as well as upon its heredity. 



Now that we are familiar with the nature and causes of non-heritable 

 variations and with the sequences of processes and structures in sexual 

 reproduction and also in vegetative multiplication of plants, we may con- 

 sider some of the fundamental processes of heredity and heritable varia- 

 tions. What, for instance, is actually transmitted from parent to progeny? 

 Is it transmitted through cell division to all the cells of the roots, stems, 

 leaves, and other organs of the plant? May it be transmitted by any cell 

 of the plant? How is it transmitted? Is it a mysterious vital force, or is it 

 a material mechanism composed of discrete units of matter? How does 

 it increase and reproduce? What determines its constancy' from genera- 

 tion to generation over long periods of time? Is it alterable? 



These questions will appear even more significant if one begins to 

 recall the numerous ways in which all the plants he knows differ from 



