178 TILLEES OF THE GROUND CHAP. 



extensive scale, and can be used to make the 

 improvement of cultivated plants much more rapid. 

 Grafting means the fastening of a little branch 

 of a valuable tree to a branch of a less valuable 

 tree. If it is properly done, the graft, or scion, 

 joins completely to the tree or stock, and grows as 

 if it were part of it. It is usual to cut away all 

 the branches of the stock, so that all the food 

 absorbed by its roots goes into the graft, which 

 then produces flowers and fruit. 



CHAPTER XV 



THE STRUGGLE WITH DISEASE 



WE have seen that, ages and ages ago, man picked 

 out certain plants which were worth cultivation, 

 and began to grow these with special care. We 

 saw next that this was done only at certain places 

 on the surface of the earth, and that the people in 

 other parts were for the most part content to 

 borrow these plants, which in this way gradually 

 spread over the earth. We saw also that this 

 spreading process is now going on at perhaps a 

 faster rate than ever it did before, with the result 

 that the earth is year by year producing more food 

 for man. 



