PAET IV. 

 THE PHYSIOLOGY OF PLANTS. 



1. 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 are, 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 (see pp. 2, 92). In these higher plants each 

 member, and each tissue, is adapted to the performance of one 

 or more functions, and is the organ (p. 2) by which these special 

 kinds of physiological 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 in- 

 tensity. 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 performing 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. 

 GENEEAL PHYSIOLOGY. 



2. The Functions. In entering upon the analysis of the 

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

 phenomena all depend upon the living protoplasm ; that the vital 



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