THE CAUSES OF SPECIFIC SHAPE 137 



lower plants, but instead they are subjected to various forms of control by 

 neighbouring or far-removed parts, and their potential powers so regulated 

 as to render harmonious co-operation possible. 



These general conclusions are justified, even in the absence of any close 

 insight into the complex interactions which take place in the living organism. 

 The reactions involved bear a general similarity to those produced by 

 changes in the external conditions, and they may be compared to those 

 arising from the various forms of symbiosis, whether conjunctive, disjunctive, 

 antagonistic, or reciprocal. 



Any change in the external conditions which directly or indirectly 

 modifies one or more of the internal conditions must unavoidably produce 

 some change in the vital activity of the organism, or even in its structure also. 

 The changed activity is, however, the direct result of the altered internal 

 disposition so long as the new external conditions remain constant, and to 

 indicate the primary importance of the hereditary properties of the organism 

 it is permissible to use such terms as automorphosis, self-differentiation, 

 autoplasy, autotropism, and autonasty, although there is no form of vital 

 activity which is independent of the external conditions. By the words 

 aitiomorphosis, heteromorphosis, aitiotropism, and aitionasty, we merely 

 indicate certain reactions which changes in the external conditions 

 produce by modifying the vital activity, while the terms photomorphosis, 

 chemomorphosis, photonasty, heliotropism, indicate the external agency 

 which produces the reaction. If the external conditions are kept constant, 

 it becomes possible to determine whether particular phenomena are, in the 

 above sense, of autonomic (internal) or aitionomic (external) origin. 



It must, however, be remembered that the properties of the entire 

 organism or of a single organ may undergo automatic alteration, so that it may 

 react differently to the same external conditions. Thus light is necessary 

 for the normal development of the leaf-primordia formed in darkness ; the 

 induction or alteration of the geotropic sensibility may produce special 

 curvatures during development ; and, lastly, the appearance of dorsiventrality 

 or of a localized irritability produces the conditions for response to 

 unaltered external conditions or to diffusely applied stimuli. In general, 

 the importance of the external conditions always depends upon the condition 

 of the organism at the time, and every physiological effect may be due to 

 a change in the properties of the organism, or to an alteration in the 

 external conditions, or to both combined. 



External factors are more readily controlled and their influence deter- 

 mined than is possible in the case of internal ones. Indeed it is frequently 

 impossible to determine whether the reaction is due to one or to several 

 internal factors, or to decide the part played by changes in the properties 

 of the protoplasm. When, however, a result can be definitely ascribed to the 

 action of a tension of internal origin, or to that of some metabolic product, 



