

252 BOTAXICAL GAZETTE [october 







may occur in four ways: first, growth in size of the organism may 

 bring some part of it beyond the range of the dominant region; 

 second, since dominance is primarily dependent upon the metabolic 

 activity of the dominant region, a decrease in this activity, how- 

 ever brought about, will decrease the range of dominance and may 

 bring about physiological isolation in more distant parts without 

 increase in size; third, if physiological dominance is dependent 

 upon transmission of electric or other dynamic effects through 

 protoplasm, physiological isolation must result from blocking the 

 passage of such effects; fourth, the subordinate part may be 

 directly excited by external factors to such a degree that the action 

 of the dominant part upon it is no longer effective, for example, 

 stated in electrical terms, it may itself give rise to electric currents 

 in the opposite direction from those in the dominant region and 

 compensating them. 



The physiologically isolated part behaves essentially as it. 

 would if the dominant part had been removed, or it itself separated 

 from the dominant region. If its growth and development have 

 previously been inhibited, it begins to grow and develop. If it 

 represented a differentiated part of the body, as in many animals, 

 it reacts in the simpler forms by losing this differentiation and 

 may give rise agamically to a new individual. 



Physiological isolation and consequent development of new 

 parts or individuals as a result of growth is a familiar phenomenon 

 in both plants and animals. The experimental decrease in the 

 activity of the growing tip by inclosing it in an atmosphere lacking 

 oxygen, or in plaster, as well as many cases of the inhibiting action 

 of external factors in nature on growing tips afford numerous 

 examples among plants of the second kind of physiological isola- 

 tion. The fourth type of isolation appears in cases in which a 

 bud may be made to grow in spite of the inhibiting action of a f 



growing tip or other part, by subjecting it to external conditions 

 which increase its activity. Such isolation may be brought about 

 in some plants, particularly in the buds farthest away from the 

 dominant region. In Bryophyllum, for example, the buds in the 

 notches of the lower leaves will often develop under favorable 

 external conditions. t 









