90 C. M. CHILD. 



This possibility has already been suggested in connection with 

 certain observations on susceptibility in the lower animals 

 (Child, '13, pp. 118-119) and it seems worth while to call atten- 

 tion to it in this connection. Actually the death-level, whether 

 relative or absolute, must be a resultant of various component 

 factors such as aggregate condition of colloids, permeability, 

 rate of metabolism or of certain reactions, etc., and there can be 

 little doubt that different agents and conditions act primarily 

 or chiefly on different factors in this complex. But since these 

 are factors in determining the behavior of a system, i. e., are cor- 

 related, not independent, changes in one cannot proceed beyond 

 a certain limit without inducing changes in the others. This 

 being the case, the death-level as a quantitative level at which 

 persistence of the system as a whole becomes impossible, must 

 be for each particular case of death a more or less definite level 

 or region of the curve which represents the quantitative changes 

 in the activity of living protoplasm or its component factors. 

 On the other hand, the previous condition of the system and the 

 character and rate of action of the killing agent may play a role 

 in determining this level in different cases. If this suggestion is 

 correct, then regions or cells of different susceptibility in the hairs 

 do not necessarily attain exactly the same condition at the point 

 of death, and the condition in a particular cell or region may differ 

 according as it is killed rapidly or slowly. 



Since the hairs represent real physiological axes, similar to 

 other plant axes the question of their physiological relation to 

 other axes must be raised. Unquestionably the development of 

 a hair axis is a form of reproduction in a protoplasm specialized 

 in some way. As a reproductive process it must result from 

 physiological isolation (Child, 'n, '15^, Chap. IX., '156, Chap. 

 V.) and, as I have pointed out, physiological isolation and re- 

 production may occur, not only as the result of increase in size 

 but also under depressing conditions in consequence of decreased 

 dominance, i. e'., decrease or obliteration of a preexisting axial 

 gradient. Hairs frequently appear on young apparently vigorous 

 plants, but they are often very characteristic of advanced vege- 

 tative stages or of plants which seem to be in poor condition. In 

 Agardhiella, for example, I have frequently observed that parts 



