28 PLANT GROWTH 



are thought to be responsible for a single characteristic. The 

 number of genes to a characteristic chromosome is variable, 

 but usually there are more than one hundred and in some 

 cases perhaps many hundreds. The exact location of genes 

 in the chromosomes has been attempted as is shown in the 

 study of the fruit fly (Fig. 7). It should be borne in mind 

 that most of the evidence for genes and all the evidence that 

 they carry hereditary characteristics is experimental and 

 not something seen with a microscope. This will be de- 

 scribed further under the title of "the hybridizing of plants." 



Cell division must be understood to appreciate plant 

 growth and to deal with plant breeding. Cells divide only 

 where growth is taking place. These growing places are 

 the meristematic regions found at the tips of all roots and 

 all stems, in the cambium region of dicotyledonous plants 

 and at the base of the internodes of monocotyledonous plants. 

 A few specialized regions of division might be added such as 

 in very young growing leaves, in the developing flower parts, 

 and in fruits and seeds. It should be remembered that 

 every organism begins as a single fertilized egg cell and by 

 the process of cell division, growth and specialization a single 

 cell becomes a plant or animal with many millions of cells. 

 Reduction division, in which the number of chromosomes is 

 reduced in the formation of sex cells, is described in Chap- 

 ter 18. 



The diagrams of Figure 6 show certain stages of divi- 

 sion in a continuous process which can be better shown with 

 motion pictures. It should be noted in numbers 2 and 3 

 that in the resting cell the chromatin of the nucleus forms a 

 thread. This thread splits longitudinally in number 4, and 

 in number 5 is broken to form the numerous chromosomes 

 each of which contains a portion of every gene. These 

 newly formed chromosomes grow to normal size and arrange 

 themselves across the equator or center of the cell as shown 

 in number 5 and then pull apart and begin to move toward 



