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TEXTBOOK OF BOTANY 



ing them, and that concerted eflForts are now being made to check the 

 rapid deterioration of one important part of their environment, the soil. 

 New kinds of plants are obtained only by changes in hereditary factors, 

 not through fluctuations. 



Heritable variations. Heritable variations may be further distinguished 

 as hybrid variations and mutant variations. Further illustrations of mu- 

 tants are given in Plates 5, 6, and 7. 



Plate 5. Double and crested flowers of cosmos (right) from mutations in the 



parent stock (left). 



The cause of hybrid variations is easily recognized. Cross-fertilization 

 results in combining a part of the hereditary mechanism and potentiali- 

 ties of two genetically diflFerent plants. The chromosomes and their con- 

 tained genes are brought together in the fertilized egg during cross- 

 fertilization and become segregated in subsequent generations of the 

 hybrid. 



The causes of mutations are not as easy to recognize as are the causes 

 of fluctuations and hybrid variations. In spite of the prevalent popular 



