I46 PLANT LIFE. 



inherent in it that a new organism similar to that of which it 

 forms part may be produced. 



185. Adaptation.- -The interrelation of these powers, 

 their harmonious coworking and their variation to suit the 

 varying conditions of the surrounding media (air, water, soil, 

 etc.), result in the proper performance of all the functions of 

 the plant. By means of these powers it is brought into re- 

 lation to the world about it, being adapted to other organisms 

 in whose company it lives, and enabled to withstand the 

 adverse conditions by which it is frequently threatened. 

 Every organism, indeed, must adjust itself first to the external 

 physical conditions, and, second, to other organisms. (See 

 Part IV.) 



186. Physical conditions set limits upon the discharge of 

 its functions. Varying amounts of light, of heat, of moisture, 

 determine more or less rigidly how rapidly, or to what extent, 

 each function may be discharged. Every function of the 

 plant is adapted, therefore, to an upper limit, the maximum, 

 and to a lower limit, the minimum, above or below which 

 the performance of the function in question is impossible. 

 Between these limits there lies some point at which it pro- 

 ceeds most rapidly and effectively. This point is known as 

 the optimum. 



