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THE AMERICAN NATURALIST [Vol. XL VIII 



ceases are known as the zero points. The plant does not 

 necessarily die at once, but passes into a dormant state. 

 If the intensity becomes still more unfavorable a point is 

 finally reached at which death occurs. The minimum de^ 

 gree of intensity of a factor at which the plant may re- 

 main active is known as the loiver zero point, while the 

 greatest intensity is called the upper zero point. With 

 some factors these points are wide apart, so that, other 

 conditions being favorable, the plant will continue to de- 

 velop after a fashion at any but the most extreme intensi- 

 ties of such factors. "With other factors the limits are 

 comparatively narrow. A plant will reach that degree of 

 development only which is permitted by that factor which 

 is in the least favorable degree of intensity. Such factors 

 are called limiting factors. In passing from one zero 

 point toward the other, a point is finally reached at which 

 any given function of a plant reaches its highest state of 

 activity. This point is known as the absolute optimum for 

 that function and may not correspond to the most favor- 

 able intensity of that factor for the performance of the 

 other functions of the plant. The point of intensity of a 

 factor at which all the functions of the plant are per- 

 formed to the best advantage is termed the harmonic 

 optimum. If each factor is of an intensity corresponding 

 to the harmonic optimum, the plant is in a condition of 

 equilibrium known as the ecological optimum and will 

 reach the highest state of activity of which it is capable. 4 

 As the life of a plant is made up of various functions, 

 so its structure is made up of a number of organs having 

 various characters. These characters are the result of 

 development, which in turn is dependent upon the per- 

 formance of the several functions of the plant under the 

 influence of those external conditions which make up the 

 environment. If a factor of the environment is modified 

 in its intensity, the balance of the functions of the plant 

 is disturbed and the plant reacts to its changed env 

 ment by a modification of its functions which may result 



* Schimper, A. F. W., "Plant Geography." 



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