2 PLANT PHYSIOLOGY 



mands unusual ability to detect the differences between 

 them. The differences disappear as we descend the scale of 

 development, and the likenesses become more and more evi- 

 dent. The bacteria and diatoms, which have been repeat- 

 edly regarded first as animals, then as plants, and the 

 Myxomjcetes (slime-moulds), which are still believed by 

 some to be animals {Mycetozoa), illustrate the difliculty 

 of determining to which "kingdom" these organisms belong. 

 It is important to the students of systematic botany and 

 zoology to know where to place these organisms in their 

 systems of classification. This enables other naturalists to 

 go further. To the physiologist it is a convenience rather 

 than an essential to know whether the organism which he 

 is studying is called an animal or a plant, but the results 

 of his work have often been useful to the systematists in 

 confirming or correcting their classifications. From the 

 classifier's point of view the differences which enable him 

 to make an orderly arrangement of the objects of his study 

 are of the utmost importance; from the physiologist's 

 standpoint the likenesses are most important. This wiU be- 

 come plainer when we state the aim of physiology. 



THE AIM OF PHYSIOLOGY 



According to Pfeffer,* "the aim of physiology is to study 

 the nature of all vital phenomena in such a manner that, 

 by referring them to their immediate causes, and subse- 

 quently tracing them to their ultimate origin, we may ar- 

 rive at a complete knowledge of their importance in the life 

 of the organism." Physiology is a study not merely of 

 structure, though to its successful pursuit a knowledge of 

 structure is indispensable ; nor of organized bodies, though 

 a knowledge of the laws which govern their organization 

 (structure and form) is important. It is the study of the 

 living organism. Crystals have definite and characteristic 

 structures and forms, they increase in size and in number in 

 accordance with a few laws common to all and a few laws 



* Handbuch der Pflanzenphysiologie, 2te Auflage, Bd. I., p. 7, 1897. 

 English translation by Ewart, Physiology of Plants, vol. I., p. 8, 1900, 



