6 PLANT PHYSIOLOGY AND ECOLOGY 



as a subdivision of physiology. A plant that is more or less hin- 

 dered in carrying out its usual functions by the presence of a fungus 

 exhibits abnormal adjustment due to a biotic factor of the habitat. 

 A sun plant that finds itself placed in the shade has likewise to 

 adjust itself to light stimuli that are abnormal to it. During the 

 period of adjustment it also is in a pathological condition. In 

 both cases the adjustment must be successfully carried out or the 

 plant dies. Consequently normal functioning is physiological and 

 abnormal functioning is pathological. There is clearly no hard- 

 and-fast line between the two, since any plant is acted upon by 

 abnormal stimuli while it is getting established in a new habitat, 

 but these same stimuli become entirely normal when the plant has 

 become adapted to them. In studying the behavior of plants, it 

 is both illogical and inconvenient to separate the normal and the 

 abnormal. In the practical study of specific plant diseases, such 

 separation is a matter of convenience, but in an elementary treat- 

 ment it is undesirable to distinguish pathology from physiology. 



