72 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



causes a more pronounced development of cuticle in an adult leaf. 

 Frequently different influences produce similar results, and this fact is 

 often of economic value. Then a deficiency of water or food, and indeed 

 unfavourable conditions in general, induce or increase the tendency to peren- 

 nation, or to produce perennating organs, or the formation of reproductive 

 organs, such as flowers, spores, and the like. In this way reactions are 

 awakened which are directed towards the maintenance and preservation 

 of the species. Such reactions are purposeful in character in this respect, 

 as are also the various tropistic curvatures produced by external stimuli 

 in the preservation and maintenance of the individual plant. As might 

 be expected, radiant energy in the form of light is more effective in 

 producing changes of shape than is heat, for except as regards the 

 radiant heat directly received during insolation, no arrangement of the 

 parts can prevent the plant ultimately assuming a temperature approxi- 

 mating closely to that of the surrounding medium, even when transpiration 

 is active 1 . 



The amount of the reaction is mainly determined by the inherited 

 properties of the plant, and in most cases the range of the reaction is 

 very limited, either in regard to all or to single stimuli. Plants which 

 have only a slight power of adaptation are either unable to exist or 

 grow but feebly under conditions which permit of the luxuriant growth 

 of plants of more plastic character. Not all plants are, however, able to 

 reacquire this property of plasticity when once it has been lost, or to 

 transmit it to their descendants, and in such cases only a limited range 

 of distribution is possible. 



The energy for growth and movement is in all cases derived from the 

 external world, and external stimuli may also induce the local or general 

 utilization of this energy, or may retard or inhibit particular functional 

 activities, and call potential powers into play which were previously not 

 exercised. Since, however, the action of a particular stimulus is dependent 

 upon the nature of the organism or of the part of it affected, no complete 

 view of the processes involved in growth and in growth-reactions can be 

 given until the internal factors concerned are fully known. Hence such 

 terms as Chemomorphosis and Photomorphosis 2 simply indicate formative 

 changes resulting from particular agencies, and do not afford any explana- 

 tion as to how these changes are brought about. All such changes are 

 in fact automorphic in character, since they result from the plant's own 



1 Extremes of temperature induce spore-production by creating unfavourable conditions. 



2 The term heteromorphosis (Vol. I, p. 24) has been applied by Loeb (Unters. z. physiol. 

 Morph. d. Thiere, I. Heteromorphose, 1891 ; cf. Hertwig, Die Zelle u. d. Gewebe, 1898, p. 182) 

 to new growths occurring in unusual situations, or which assume peculiar shapes. Hence the terms 

 ' zenomorphosis ' or ' aitiomorphosis ' (curios, founder) may be used in the more general sense. 

 From the latter term, aitionomic, aitiogenic, aitiotropism, aitionasty may be derived. 



