26 PLANT GROWTH 



The counting of chromosomes is exceedingly difficult be- 

 cause of their small size. A few plant cells have chromo- 

 somes large enough to be seen clearly when magnified fifty 

 times, but in many others they are so small that they appear 

 as dust particles when magnified a thousand times. Most of 

 those counted fall within a range of from twelve to twenty- 

 four for each nucleus in the plant cell. In a few plants the 

 phenomenon of polyploidy has occurred naturally, and 

 recently artificial treatment has induced it in many others. 

 Polyploid plants are those which have more chromosomes 

 than is common for the species, often double the number, 

 but always a multiple of the number found in the sex cells. 

 How the identity of chromosomes, and the chromosome con- 

 tent can be maintained and transmitted from generation to 

 generation has been the subject of much study. Cell divi- 

 sion gives an idea of the mechanics of chromosome conti- 

 nuity but the physiology of control remains a secret. 



Chromosomes, believed to be that part of the sex cells 

 which transmits and gives to new organisms their character- 

 istics, are so important that their structure and behavior have 

 been the subject of extensive study. From this study a body 

 of facts has been deduced and certain hypothetical theories 

 have been projected, some of which are supported by con- 

 vincing evidence and are used in the study of inheritance. 

 Chromosomes are found to consist of numerous small units 

 of chromatin called genes which are arranged in a single row 

 similar to an orderly row of blocks or of popped corn, each 

 of which differs from the others in some way. This differ- 

 ence may be one of chemical arrangement in the protein 

 molecule. Since these units, thought to be the genes, have 

 been seen in the salivary gland cells of the common fruit 

 fly and, less clearly, in certain plant cells, they are believed to 

 exist, and further evidence substantiates the theory that 

 each becomes a determiner of, or a carrier for, a particular 

 characteristic. In other cases a number of genes together 



