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TEXTBOOK OF BOTANY 



chromosomes per cell. Each vegetative cell of the plant, however, con- 

 tains all the hereditary potentialities of any other vegetative cell. It is 

 necessary to find, or to infer, smaller hereditary units of matter such as 

 the genes, which consist of one or, at most, of only a few molecules. 

 Numerous small bodies of matter are visible in the chromosomes, but 

 no one is certain that he has ever seen a gene; hence for the present 

 they are inferred units of matter. The best-known hereditary potentiali- 

 ties are dependent upon genes located in the chromosomes which in 

 turn are located in the nucleus of the cell.^ 



The genes are an-anged in a definite series from one end of the 

 chromosome to the other. The number of genes in a chromosome ma\' be 

 very large, perhaps a few thousand in some. The egg and the sperm of 

 corn each contains 10 chromosomes. The geneticists have alreadv recog- 

 nized hereditary potentialities of more than 350 pairs of corn genes all 

 of which are located in these 20 chromosomes. Moreover, thev can tell 

 us which genes are in the different chromosomes and the approximate 

 location of many of the genes within the chromosome. The total number 

 of genes in these 20 chromosomes of com is unknown, but there are 

 probably many thousand. 



Chromosomes are known to reproduce by dividing ( splitting ) length- 

 wise into two identical halves (Fig. 206). Hence when a chromosome 



Fig. 206. A diagrammatic representation of certain stages of the duplication of 

 chromosomes by longitudinal division, or "splitting," during the division of 

 vegetative cells. 



divides, each gene within it must also become duplicated by some means. 

 Plastids in the cytoplasm of plant cells ma\^ also pass bodih' from the 

 dividing cell to the two new cells formed. Thev are thus bodilv in- 

 herited and continue to multiply in the newly formed cells by simple 

 division. Any properties they possess are thus transmitted with them. 

 Plastids in the egg become the first plastids in the fertilized egg. Not 

 much is known about cytoplasmic inheritance or about the influence of 



^ Exceptions found in the cells of bacteria and blue-green algae will be noted in later 

 chapters. 



