Physical Basis of Heredity and Evolution 23 



subject of his investigations. This chapter, therefore, 

 must be regarded simply as the presentation of a teacher, 

 to explain a subject that belongs logically in the series. 

 Another restriction is that this presentation deals chiefly 

 with structures that are visible by means of laborator}- 

 technique, and not with the results of experiment upon 

 these structures. A final restriction is that the statements 

 deal with plants, a limitation necessary to the writer, and 

 offset by the fact that the corresponding facts in animals 

 will be stated in one of the later chapters. 



Some conception of what is meant by the power of repro- 

 duction will be useful. Among the simplest plants, every 

 cell has this power; in fact, some plants are so simple that 

 the adult body consists of a single cell. As plant bodies 

 came to be made up of numerous cells, some of them lost 

 the power of reproduction ; and as the body became increas- 

 ingly complex, the number of cells retaining the power of 

 reproduction became relatively smaller. This means that 

 in the complex plant body, the relati^'ely few reproductive 

 cells are not so much "cells set apart for this special func- 

 tion," as cells that have not lost this primary power. The 

 specialized cells are not those that reproduce, but those 

 that cannot. The loss of reproductive power is usually 

 not complete, for most cells can reproduce their own kind, 

 even if they cannot reproduce the whole body. 



This leads to a consideration of what is included in full 

 reproductive power. Without including confusing details, 

 it ma}' be said that such reproduction as one has in mind 

 in connection with heredity involves four general things. 

 First, there is cell multiplication, the fertilized egg initiating 

 a sei-ies of cell divisions that may result in a multitude of 

 cells. It is evident, however, that a complex plant body is 



