232 



BREEDING CROP PLANTS 



variety can be rejuvenated by planting under a favorable environ- 

 ment tubers from diseased plants is as yet an unanswered question. 

 The rapid deterioration of varieties when University Farm 

 tubers are used for their propagation is believed to result 

 from these transmissible diseases which are now called " mosaic 

 dwarf." Degeneracy can apparently be prevented by covering 

 that part of the field in which tubers are to be saved for the next 

 year's planting by a cheese-cloth cover. The following data 

 with the variety Green Mountain seem sufficient authority for 

 this statement: 



TABLE LVIII. DEGENERACY PREVENTED BY USING TUBERS OF VINES WHICH 

 WERE COVERED WITH A CHEESE CLOTH COVER 



All seed stock was obtained from the same grower at the North 

 Central Experiment Station, Grand Rapids, Minn. Whether all 

 degeneracy is due to such transmissible diseases is as yet un- 

 answered. Possibly unfavorable cultural conditions may also 

 affect the development of the tuber so that the yield of the fol- 

 lowing year's crop may be modified. 



Another explanation of degeneracy has been commonly 

 mentioned. This is the hypothesis that continued asexual 

 propagation causes senility or degeneracy. Perhaps this question 

 may be answered for the potato by the consideration of a fact 

 reported by Heribert Nilsson (1913). In a report of yields of 

 67 varieties, as tested in Sweden, he emphasized the fact that a 

 variety "Hvit Jamtlandspotatis" which has been cultivated 

 more than 100 years proved to be the highest yielder. This is 

 given as a refutation of the theory of senility. 



It has not been the intention in this discussion to lead the 





