104 



BIOLOGY OF PLANTS. 



These laws, by their wise constitution and their constancy, aid 

 in fulfilling the decree of heaven, that while the earth remains, 

 seed time and harvest, summer and winter, cold and heat, 

 shall not fail. 



Finally. The distribution of plants over the surface of 

 the earth is governed more by temperature than by any other 

 circumstance. It is well known that each species of plants 

 has its natural limit. This is particularly true of the food- 

 bearing plants; and their agricultural limits are principally 

 determined by temperature and moisture. The northern 

 agricultural limits are bounded by lines passing through places 

 of equal summer heat and are called isotheral lines. 



The southern agricultural limits are bounded by similar 

 lines of equal winter heat, and are called isochimenal lines. 

 These lines are exceeding tortuous in their course around 

 the earth. Thus barley, the grain which has been cultivated 

 farthest north, has its extreme limit in the Shetland Islands, 

 61° N. ; in the Feroe Islands, 61°— 62^^° N. ; Lapland, 70° N. ; 

 near the White Sea, between 67° and 80° N. ; in Eastern Rus- 

 sia, about 66° N. ; and in Central Siberia, between 58° and 

 59° N. In North America the line is probably similar. The 

 limit for the potato extends a little farther north. Now it 

 is found that this line passes through places of equal summer 

 temperature, the mean of which is from 46° to 47°. 



Or if a similar examination of the northern limits of wheat 

 be made, it will be found, that equal summer temperature is 

 the condition of the limit. This is in latitude 64°, and the 

 average summer temperature 57.4° F. This limit coincides 

 with that of fruit-trees, apples, pears, and also of the oak. Other 

 grains have similar isotheral lines which limit their northern 

 cultivation. 



If now we turn to the equatorial regions, we shall find that 

 the limits are governed by equal winter temperature ; for it is 

 found that extreme heat arrests the cultivation of the grains. 

 The temperature for germination must not exceed 120 de- 



