150 COLLEGE BOTANY 



The plant differs from the ordinary machine which is the 

 product of man's skill in that it is a living, growing machine and 

 is capable of making its own repairs and of reproduction ; i.e., 

 of producing new individuals like itself. It is an automatic 

 machine and its workings are very imperfectly understood. In 

 order to grow and reproduce itself it must take in water, carbon 

 dioxide, minerals, nitrogen, etc., and transform them into parts 

 of itself. This involves very complex physical, chemical and 

 physiological activities. 



Although we are accustomed to think of these activities in 

 connection with the plant as a whole, we should remember that 

 they are the properties of the individual cells ; that the cell is the 

 physiological as well as the structural unit of the plant. The 

 most important of these are: (a) Absorption, which involves the 

 taking in of water and gases the water-soluble materials are 

 usually taken in through the roots (page 155) and the gas is 

 taken in mostly through the leaves (page 193) ; (6) photosyn- 

 thesis, which is the making of carbohydrates (page 195) ; 

 (c) transpiration, which is the giving off of water and gases, 

 mostly through the leaves (page 170) ; (d) metabolism, which 

 involves the digestion of foods ; the breaking down of parts of 

 the plant (or machine); the elimination of waste; transloca- 

 tions of food materials, true foods and waste ; and growth (Chap- 

 ter XVIII) ; (e) contraction, expansion and response to external 

 stimuli; and (/) reproduction, which involves the formation of 

 new individuals. 



Plant Foods. We are accustomed to speak of water, mineral 

 salts and carbon dioxide as plant foods, but this is not strictly 

 true. We cannot conceive of the animal living on these com- 

 pounds. The animal lives on carbohydrates, fats, oils and pro- 



