PART III 

 PHYSIOLOGY 



37. Introductory. The province of physiology is the study 

 of those phenomena which, taken together, constitute the life of 

 the plant ; in other words, whilst morphology is concerned with 

 what plants arc, and histology with their structure, physiology 

 deals with what they do. These phenomena may be classified, 

 according to their nature, into functions, or different kinds of 

 physiological work. 



The body of the plant, whether it be unicellular or multicellular, 

 is one physiological whole. In the lower and simpler plants the 

 various functions are equally discharged by all parts of the body ; 

 but in more highly-organised plants the functions are distributed 

 among the members and tissues, that is, there is physiological 

 division of labour. In these higher plants each member, and 

 each tissue, is adapted to the performance of one or more functions, 

 and is the organ (p. 1) by which these special kinds of physio- 

 logical work are done. 



The performance of their functions by the organs of the plant is, 

 however, materially affected by various external conditions. For 

 instance, the activity of the assimilatory function of green leaves is 

 altogether dependent upon exposure to light of adequate intensit3 r . 

 Hence the object of physiology is not only to distinguish and study 

 the various functions, and to demonstrate the relation between them 

 and the internal structure and the external form of the organs per- 

 forming them, but also to determine what are the external conditions 

 by which the performance of the various functions is affected, and 

 the modes in which these conditions exert their influence. 



CHAPTER I 



GENERAL PHYSIOLOGY 



38. The Functions. In entering upon the consideration of 

 the vital phenomena of plants, it must be clearly understood that 

 these phenomena all depend upon the living protoplasm ; that the 



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