CHAPTER XXV 

 PLANT BREEDING 



Plant breeding has to do with the improvement of old plants 

 and the securing of new ones. Many and various are the aims 

 of plant breeding. The object may be to improve the yield, 

 increase the resistance to drought or disease, shorten the period 

 of development, or secure strains or varieties with new chara- 

 ters. The two important methods used are selection and hy- 

 bridization. 



In connection with plant breeding, the discoveries of De Vries 

 and Mendel have proven to be of inestimable value. The dis- 

 coveries of De Vries have resulted in a better understanding 

 of the nature of variations and have enabled us to improve plants 

 by selection much more efficiently. The introduction of Men- 

 del's methods of investigation and his discoveries concerning 

 the behavior of characters in hybrid offspring afford a scientific 

 foundation to the improvement of plants by hybridization. As 

 a result of Mendel's contributions, we now know much more 

 about how to proceed, what to expect, and how to interpret the 

 results obtained in hybridizing. 



Selection. Selection takes advantage of variations. Vari- 

 ations, as previously noted, are not only due to differences in 

 gametes and their combinations in fertilization, but also to 

 differences in temperature, moisture, soil conditions, and other 

 environmental factors. But only those variations due to factors 

 which can be transmitted by the gametes are inheritable through 

 the seed. A Corn plant may be larger or bear larger ears than 

 the ordinary type because of especially favorable conditions. 

 It may stand alone in the hill, thus having all of the water and 

 mineral supply to itself, or it may be growing on ground more 

 heavily fertilized. This increased size, due to external condi- 

 tions and not to any special factors in the cells of the plant, 

 is not transmitted to the offspring, and the embryos in the 

 kernels of this especially favored plant may have inherited no 



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