62 THE STORY OF THE PLANTS. 



one sort of food, and others another. That is 

 why farmers have recourse to what is called rota- 

 tion of crops, so as to follow up one sort of plant 

 in a field by another, whose needs are different. 

 Thus corn is alternated with swedes or turnips. 

 Virgin soil will produce crops for several seasons 

 together without the need for manuring ; but 

 when many crops have been cut from it in suc- 

 cession, the earth gets exhausted of nitrates and 

 phosphates, and then it becomes necessary to 

 manure and to rotate the crops in the ordinary 

 manner. 



But in nature crops are not, as a rule, removed 

 from the soil ; they die and wither, and return to 

 it for the most part whatever they took from it. 

 The dead birds and insects, and the droppings of 

 animals, are sufficient manure for the native wood- 

 land. Still, even in nature, certain plants more 

 or less exhaust the soil of certain valuable ma- 

 terials ; and therefore natural selection has se- 

 cured a sort of roundabout rotation of crops in a 

 way of which I shall have more to sa}^ hereafter. 

 Many plants, for example, which greatly exhaust 

 the soil, have winged or feathery seeds ; and these 

 seeds are carried by the wind to fresh spots, where 

 they alight and root themselves, in order to es- 

 cape the exhausted soil in the neighbourhood of 

 their mothers. Other plants send out runners, as 

 they' are called, on long trailing branches, which 

 root at a distance, and so start fresh lives in ex- 

 hausted places. Yet others have tubers, which 

 shift their place from year to year ; or they push 

 forth underground suckers, which become new 

 plants at a distance from the parent. All these 

 are different natural ways for obtaining what is 

 practically rotation of crops ; nature invented that 



