CHAPTER XII 



THE PLANT AS A WORKING MACHINE 

 The Plant is a Living Machine. We have learned that the 

 cell is the structural and physiological unit of the plant. In the 

 lower forms of plants the cell is undifferentiated and capable of 

 performing all the functions of plant growth and reproduction. 

 But as we study the higher plants we find a differentiation of 

 the cells into tissues, systems and organs accompanied by a cor- 

 responding physiological division of labor. We have studied the 

 cells, tissues, systems and organs in the preceding chapters and 

 have learned that the higher plants are very complex mech- 

 anisms. We will now study the very complex workings of 

 this mechanism. This phase of the subject is known as 

 plant physiology. 



The Physiology of the Cell. The functions of the cells in 

 the various parts of the plant vary with its structure. The cells 

 of the stem serve primarily for support and transportation of 

 foods, those of the roots for absorption of water, and those of 

 the leaves for absorption of carbon dioxide, transpiration of 

 water and photosynthesis. Transportation of food depends on 

 the character of the cell wall and the contents of the cell ; absorp- 

 tion depends on the character of the cell contents ; photosynthesis 

 depends on the presence of chlorophyll in the cell. But there are 

 many other characters and functions, in addition to these well- 

 known functions, such as the organization of starch grains by 

 the amyloplasts or leucoplasts ; the making of proteins, fats and 



oils ; the processes of metabolism ; growth and reproduction. 



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