156 



BOTANY: PRINCIPLES AND PROBLEMS 



those inherited characteristics which enable it to thrive in a 

 particular environment and which have become implanted in the 

 species dm-ing the course of its evolution. Adjustments of this 

 kind, either of structure or function, we usually speak of as 

 ada'ptations. The natural adaptations of cacti for desert life and 

 of orchids for insect fertilization may be cited as examples. 



We have called attention to the complexity of the environment 

 in which the plant grows. The first step in an analysis of the 



55 

 .§90 



I-^ 



\ 



/ 



/ 



ll 14- 16 18 20 21 24 76 28 JO 32 34 56 38 40 42 

 Degrees Cenfi.qrade 

 Fig. 76. — Growth in length of maize shoots per hour, when exposed for twelve 

 hours to a constant temperature. The minimum temperature for growth was in 

 this case found to be 12° the optimum 32° and the maximum 43°. {After 

 Lehenbauer). 



relations between this environment and the plant is to isolate 

 the separate environmental factors and to study the specific effect 

 of each. Of the wide range of such factors a few are particularly 

 important and deserve consideration here, notably temperature, 

 -light, gravity, moisture, and various chemical substances, which 

 constitute the inorganic environment ; and the surrounding plants 

 and animals, or the organic environment. 



Temperature. — It is characteristic of all vital processes that 

 their maintenance is possible only within a comparatively narrow 

 range of temperatures, and temperature changes therefore elicit 



