PART II. PHYSIOLOGY. 



CHAPTER XII. 



INTRODUCTION. 



144. Division of labor. — The study of the external form 

 and internal structure of plants may be carried on as well 

 upon dead as upon living material. Even the observation 

 of the course of development requires only the examination 

 of the plant as it exists at a particular moment. But the 

 plant may also be studied as a working organism. For this 

 purpose living material is indispensable. The work that 

 plants do, by which they are distinguished from non-living 

 bodies, is extremely varied, and the more complex the plant 

 the more varied it is. In the preceding part the aim has 

 been to show that there exists great variety of form, and that 

 from the smaller to the larger plants there is gradually in- 

 creasing complexity by differentiation into tissues and 

 members. 



Nutrition, respiration, growth, movement, and reproduc- 

 tion are all executed by the single cell of the simplest plant. 

 But with specialization in structure there occurs division of 

 labor. Each kind of physiological work is known as a 

 function, and each part of the organism which does a par- 

 ticular work is called an organ. 



145. Physiology and ecology. — Physiology proper treats 

 of the plant at work, discussing the different functions and 

 the way in which these are affected by external forces, such 

 as light, heat, etc. In its broadest sense it also treats of the 



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