Chapter III 



PHYSIOLOGICAL REACTIONS, 

 'ADAPTATIONS', AND LIFE-FORMS 



Plants can grow only where the conditions are reasonably suitable 

 for them, and different species have often entirely different needs. 

 It follows that local conditions are a primary factor in limiting the 

 distribution of any particular kind of plant. Tropical plants cannot 

 survive in arctic conditions, nor aquatic plants in a desert. The 

 same holds good to varying degrees in less obvious instances, down 

 to examples where the balance is so fine that the difference between 

 success and failure, or actual life and death, is struck by some 

 barely perceptible difference in local conditions. Very often a com- 

 plex of interacting factors will be found operating, whose differences 

 may be extremely small but nevertheless sufficient to determine 

 whether or not a particular plant can grow successfully in a given 

 situation. 



What actually determines the reactions of a plant to the conditions 

 making up the environment in which it finds itself ? Fundamentally 

 it is the general physiological make-up of the kind of plant involved, 

 although the state of development of the individual and its degree 

 of adaptation to local conditions may also come into play. Plant 

 physiology deals primarily with the internal workings of plants, 

 whether biological, chemical, or physical. The main physiological 

 characteristics that are found in a particular species are usually 

 inherited and, taken together, largely determine the conditions under 

 which it can grow and the places where it can survive. 



Physiological Make-up 



Later in this chapter we shall give an account of special features 

 which enable plants to offset or at least limit the effects of unfavour- 

 able conditions. Such so-called ' adaptations ' include physiological 

 acclimatization, and are commonly responses to external conditions. 

 When not inherited they seem best considered as mere temporary 

 modifications in make-up. Their real value to the plant can be 



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