CHAPTER V. 



PHYSIOLOGICAL CLASSIFICATION OF TISSUES. 



DIVISION OF LABOR IX THE PLANT. 



514. THE simplest plant, a green cell living in water, pos- 

 sesses all the appliances needful for the work of vegetation ; 

 namely, a protoplasmic body containing chlorophyll, and a cell- 

 wall protecting it. It finds in the water in which it floats, and in 

 the sunlight to which it is exposed, eveiTthing requisite for its 

 full activity. 



515. Its work is twofold : First, that which it does not share 

 with the animal, and which may therefore be called the proper 

 office of the plant, - - the production of organic matter out of 

 inorganic materials, under the agency of light. This work is 

 dependent upon the presence of chlorophyll in the cell, and is 

 known as Assimilation. Second, that which the animal like- 

 wise can perform, - - the conversion into various forms of ac- 

 tivity of the energy stored up in food. This takes place in the 

 protoplasm, whether chlorophyll be present or absent. 



516. In a spherical cell isolated from others and leading an 

 independent existence, floating free in the water, and therefore 

 presenting no one part exclusively to the light, there is very 

 slight if indeed any division of labor. One part of its cellulose, 

 protoplasm, or chlorophyll has the same work to perform and is 

 substantially under the same conditions as any other part. But 

 if the cell becomes one of many aggregated to form a mass of 

 tissue, its relations to its surroundings are not the same as be- 

 fore, for its exterior is no longer equally exposed either to water 

 or to light. The cells in the interior of such a mass must derive 

 their supply of material from without through the agency of the 

 neighboring cells ; hence division of labor begins. Inspection 

 of the mass shows that some of its cells have the office of ab- 

 sorption, others that of assimilation, others that of treasuring 

 up the products of manufacture, etc. With this incipient divi- 

 sion of labor there are also notable changes in the form of cells, 

 by which a more complete adaptation to a particular kind of 



