PART II: PHYSIOLOGY. 



CHAPTER XI. 



INTRODUCTION. 



176. 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 various stages of development requires only the examina- 

 tion 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 which 

 plants do and 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 

 increasing complexity by differentiation into tissues and 

 members. 



Nutrition, respiration, growth, movement, and reproduction 

 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. 



177. Physiology and ecology. — Physiology proper treats 

 of the plant at work, discussing the different functions and 



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