THE OUTLOOK FOR PLANT BREEDING. 101 



If the breeder is dealing with only temporarily heritable characters, 

 as in the sugar beet, for instance, continuous selection is the price 

 of success and cannot be neglected. 



Should the scientific breeder finally demonstrate that there is a 

 cumulative action of selection the only change that it would make 

 in our methods of breeding would be that we would continue the 

 selection longer, probably in all cases continuing as long as exigen- 

 cies would permit. 



One of the great practical problems before breeders is to learn 

 how to produce variations, especially mutations, by artificial means. 

 They are evidently the result of changed conditions of some sort, 

 but what these conditions are which influence the changes should be 

 understood so that the breeder may force the variation and not be 

 compelled to await the slow and uncertain action of natural causes. 

 Experiments have indicated that mutations may be stimulated by 

 injections into the plant of chemical solutions such as zinc sulphate, 

 calcium nitrate, and the like, and many ex|Deriments on this method 

 are now in progress. 



Other experiments and observations indicate that severe changes 

 in temperature at certain periods in the life of the plant or animal 

 may, apparently, lead to the production of mutations. The writer's 

 attention was first called to this possibility by a consideration of 

 observations and studies which he made in the orange groves of 

 Florida following the great freeze of 1894-5. As a result of this 

 freeze, it will be remembered, practically all orange trees, large or 

 small, were killed nearly to the ground. In protected places the 

 trunks of the large trees were not killed but they were severely 

 shocked and all the limbs killed off. Large trees that were thus 

 severely injured continued to throw out sprouts and die back 

 farther and farther until finally the line of demarcation between 

 live and dead tissue became definitely established. Such old trunks 

 threw out numerous sprouts which under the abnormal conditions, 

 having an excessive and uninjured root system to supply them with 

 water and mineral salts in quantity, grew abnormally rapid. The 

 condition of all of the cells of such plants must have been for some 

 time very abnormal. During this early period, while the trees were 

 recovering, the writer had occasion to visit many groves and a very 

 large number of recovering trees were brought under his observation. 



