160 ROTATION OF CROPS. 



that the organic kingdom was developed from water alone ; then 

 it was considered that both water and air were necessary ; and 

 now we know, with the greatest certainty, that the soil furnishes 

 other important conditions, which must be added to the former, 

 otherwise plants will not obtain the power of propagating and 

 of multiplying themselves. 



The quantity of food for plants in the atmosphere is limited, 

 but still it must be sufficient to cover the surface of the earth with 

 a rich vegetation. 



In the tropics, and in those regions of the earth where a favor- 

 able soil, moisture, light, and an elevated temperature— the usual 

 conditions of fertility — are combined, the vegetation is scarcely 

 confined by the space on which it grows ; there, when the soil is 

 deficient, the bark and branches of dead plants soon form soil for 

 succeeding ones. It is obvious, therefore, that there is no 

 deficiency of atmospheric food for the plants of these regions, 

 and there can be none for our own cultivated plants. 



The constant movement to which the atmosphere is subjected, 

 causes an equal distribution of the gaseous food necessary for 

 the growth of plants, so that the tropics do not contain more 

 of it than the cold zones ; and yet, how different appears 

 to be the power of production of equal surfaces of land in these 

 regions ! 



All plants of tropical regions, such as the sugar-cane, the 

 palms bearing wax and oil, contain, in comparison with our own 

 cultivated plants, only a small quantity of the constituents of 

 blood necessary for the nourishment of animals. The produce 

 in tubers, of an acre of potatoes, growing, as in Chili, to the 

 height of a tall bush, would scarcely suffice to prolong the life 

 of an Irish family for a day (Darwin). The nutritious plants 

 which are the objects of culture, are only a means for generating 

 the necessary constituents of the blood. If the ingredients of the 

 soil indispensable to their formation be absent from it, the consti- 

 tuents of the blood cannot be formed in the plants, although it is 

 possible that wood, sugar, or starch, might be produced under 

 such circumstances. If we desire to produce from a given sur- 

 face more of these constituents of the blood, than the plants 

 growing on it could receive from the atmosphere or from the soil 



