Il8 OUTLINES OF PLANT LIFE. 



soil, etc.), result in the proper performance of all the func- 

 tions of the plant. By means of these powers it is brought 

 into relation to the world about it, being adapted to other 

 organisms in whose company it lives, and enabled to with- 

 stand 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.) 



153. Physical conditions set limits upon the discharge of 

 its functions. Varying amounts of light, of heat, of moist- 

 ure, 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 maxi- 

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

 which the performance of the function in question is im- 

 possible. Between these limits there lies some point at 

 which it proceeds most rapidly and effectively. This point 

 is known as the optimum. 



154. Summary. — Increasing size and complexity permits 

 an advantageous division of labor among different organs. 

 Physiology treats of the work of the plant as a whole; 

 ecology of its adaptations to external conditions and to other 

 organisms. All plant work depends on the living proto- 

 plasm. Its power of initiating and carrying on chemical 

 changes in itself and other substances provides for nutrition ; 

 its power of receiving impressions from the world about 

 enables it to regulate all its work and adapt itself to its sur- 

 roundings; its power of contractility enables it to move; and 

 its power of making and separating special parts of its own 

 substance secures a succession of like plants. All work is 

 limited by the physical conditions which surround the plant, 

 and may bring any or all of them to a standstill, because the 

 plant can only adjust itself to them within narrow limits. 



