v THE SPKEADING OF FOOD-PLANTS 57 



plants which they have brought from Spain or 

 from Africa, they have brought insect pests or plant 

 diseases. They must study these when they 

 exist, and be able to give the farmers advice how 

 to prevent or to cure plant disease in valuable 

 plants. They must also know the different kinds 

 of the same plant, for some are suitable for one 

 climate and some for another. 



The men who are busy with this work form the 

 Bureau of Plant Industry, which includes many 

 different kinds of workers. Some must travel all 

 over the world, looking everywhere for useful 

 plants, studying everywhere the methods by which 

 the plants are best grown, and then sending home- 

 wards their seeds, or fruits, or cuttings, as the case 

 may be, and also notes on the way to grow the 

 plants, the kind of soil, the manure to use, the 

 season to plant, and so on. Then, the people at the 

 Central Office, who receive the plants, must first 

 settle where and how they should grow them ; then 

 they must grow them, and sometimes there are 

 great difficulties in the way of this. Then, if they 

 succeed, they must supply plants or seeds to the 

 farmers, and encourage them to make experiments. 

 After all this work is done, much still remains. 

 Often the desperate struggle against disease only 

 begins when the plants are grown on the large 

 scale, and it may be so severe that it requires 



