448 TEXTBOOK OF BOTANY 



longitudinally into two identical halves, or daughter chromosomes, which 

 separate and migrate to opposite poles of the cell, resulting finally in 

 two new cells identical with each other and with the parent cell in 

 chromosome number and composition (chromosome complement). In 

 other words, the important things to visualize as cells divide in the 

 growing regions of a plant are ( 1 ) the exact duplication of each chromo- 

 some and of its included genes, and (2) the definite manner of the 

 chromosome migration which results in the same chromosome comple- 

 ment in all the vegetative cells of the plant. Consequently all cells of 

 the vegetative tissues of a plant have the same chromosome comple- 

 ment and hereditary potentialities that were present in the fertilized egg 

 from which the plant developed. 



These facts help us to understand why heredity remains unaltered 

 when plants multiply vegetatively in nature or in cultivation. The cells 

 of cuttings, for instance, all have the same chromosome complement, 

 and it continues to be the same in the separate individuals that develop 

 from the cuttings. If the chromosomes do not behave regularly as de- 

 scribed above, mutations may occur in the vegetative tissues of the plant. 

 For example, the occasional white branch that is found in plants must 

 be the result of something that happened either in the plastids or in the 

 part of a chromosome containing the genes that condition some of the 

 processes involved in the synthesis of chlorophyll. 



Since all the cells of the vegetative parts of a plant ha\^e the same 

 chromosome complement and the same hereditary potentialities, one 

 may well wonder why all the cells of the plant are not exactly alike in 

 appearance. Their differentiation into the various tissues, such as epi- 

 dermis, cambium, xylem, and phloem, must be dependent partly upon 

 the influence of neighboring cells and partly upon an inheritance of 

 tissue patterns about which little is known at present. Furthermore, 

 certain hereditary potentialities are not expressed unless the plants at- 

 tain certain stages of development. Hereditary potentialities that influ- 

 ence flower color and form, for instance, are expressed only when flowers 

 develop; and potentialities that affect the color of endosperm of seeds 

 are expressed only when the endosperm tissue de\ elops from the triple- 

 fusion nucleus. 



Reduction division: meiosis. Cell division in the growing regions of 

 plants is often referred to as ordinary cell division in contrast to a 

 notable exception known as reduction division, or meiosis, which occurs 

 in the life cycle of a seed plant only when the microspore mother cells 



