CONCLUSIONS AND SUGGESTIONS 195 



brings about an extension of dominance and so inhibits 

 or retards physiological isolation (see pp. 149-51), and 

 it is probable that sooner or later with advancing 

 senescence a decrease in conductivity occurs in at least 

 some cases. It is also probable that decrease in con- 

 ductivity occurs in the lower organisms under external 

 conditions which decrease metabolic rate in the organism 

 in general. Such changes, where they occur, may play 

 a part in determining physiological isolation and repro- 

 duction. 



Local external conditions undoubtedly assist in the 

 physiological isolation of subordinate parts in many 

 cases. In various plants local conditions very favorable 

 to metabolic activity and growth may determine the 

 development of buds in spite of the inhibiting influence 

 of the dominant region. We have seen how in pieces 

 of Tubularia stem the presence of the wound at the 

 basal end assists in estabHshing the new gradient, even 

 in spite of the presence of the old (see pp. 132-37). This 

 is a good case of physiological isolation by the action of 

 local factors. 



Further analytic investigation along these lines is 

 greatly needed to enable us to determine the part played 

 by the various factors in different cases of reproduction, 

 but the mere observation of various reproductive 

 processes — such, for example, as the production of a new 

 plant by a strawberry runner, after it has attained a 

 certain length — will enable us to learn much concern- 

 ing the range of dominance and its changes under 

 different conditions. 



The redupHcation of parts in an organism, such as 

 leaves and roots in the plant and segments and various 



