20 INTRODUCTION 



secretions. Such is the origin of the cell-wall. It is 

 secreted by the protoplast which it encloses. Sugar (as 

 in the sugar-cane), starch (as in corn and nearly all 

 plants), fats (as in Brazil nuts), and green coloring 

 matter and other pigments, are among the substances 

 secreted by protoplasm. 



26. Complexity of the Cell. — A recent writer, after 

 describing the minute details of cell-structure, states 

 that, ''The vital processes exhibited by a cell indicate a 

 complexity of organization and a minuteness in the 

 details of its mechanism which transcend our compre- 

 hension and bafHe the human imagination, to the same 

 extent as do the immensities of the stellar universe." 



27. Value of the Cell-theory. — It is hardly possible to 

 overestimate the value of the cell-theory to botany, and 

 to all biological science. By means of it we are led to 

 see that all the vital activities of any living thing have 

 their seat in the protoplasts of the individual cells. If a 

 plant or an animal grows, it does so because the individual 

 cells of its body multiply and grow; if it respires, it is 

 because every living cell of its body respires; if a wound 

 heals, it is because the adjacent cells reproduce them- 

 selves and form new tissue to replace that destroyed by 

 the wound; sickness results because certain cells behave 

 abnormally, or perform their normal functions out of 

 place; reproduction is the setting free by an organism of 

 one or more of its cells, which become the starting point 

 of a new individual. In fact, all that a plant or animal 

 does, physiologically speaking, is the sum total of what 

 the cells that compose it do. Thus the cell-theory gives us 

 a necessary, basic idea of all life-processes. 



