236 THE PLANT WORLD. 



iniental methods used are fairlv simulated bv natural forces. 

 Likewise Dr. Tower has been able to induce similar changes in 

 the germ cells of potato beetles bv the application of such stimuli 

 as variations in temperature and moisture. It is to be said, 

 therefore, that in taking plants from their native habitats to the 

 uttermost extent of their possible ranges, we possibly subject 

 them to forces which may be a most potent factor in the origina- 

 tion of new qualities and new lines of heredity. 



GEjSTERAL coi^siderations. 



A more direct application of the ideas elaborated in the 

 foregoing may be possible by a brief restatement of the more im- 

 portant generalizations. 



The forces or factors affecting vegetation are simple phys- 

 ical properties capable of ready apprehension and easy measure- 

 ment. Much is known as to the mode of response, reaction, ac- 

 commodation and adaptation to such single factors or to a com- 

 plex of them, and further experimentation upon the problems in- 

 volved mav be readilv organized. 



The thermal requirements of two plants, as indicated by the 

 records of a season, have been tested, and with reasonable allow- 

 ance for variation may be taken as characteristic of the forms in 

 question. The method used consists in the measurement of the 

 number of hour-degrees exposure, beginning with the winter sol- 

 stice or with the germination of seeds. 



The difference between localities only a few hundred feet 

 apart at the same altitude may be sufficiently large to make exist- 

 ence in one impossible to a form which may find its optimum in 

 the other. 



The stimulative reactions of plants to sudden changes in 

 environmental conditions form the basis of many important gar- 

 dening practices. On the other hand the capacity of the plant 

 for accommodation to conditions ^^'idely different from the aver- 

 age, following gradual changes, is very great and is perhaps the 

 most important phase of the subject with respect to acclimatiza- 

 tion. The more extensive studies in this problem have been con- 



