THE AMERICAN BOTANIST. 21 



bark of the limien tree readily sug-gested the namo it now 

 bears. The tree is also called lime tree and this name may 

 be traced to the same source as the others. It was formerly 

 Ime-tree. 



Helplessness of Cultivated Plants. — We rarely real- 

 ize how helpless man has rendered the plants he cultivates by 

 the centuries of protection from their weed enemies that he 

 has given them. An experiment that well illustrates this point 

 was made on one of the Government farms some time ago. A 

 field about one acre in extent upon which wheat had been 

 grown for forty years in succession was not harvested but 

 allowed to stand and shed its seeds as it would. The next 

 year a fair crop of wheat came up but the weeds were gaining 

 the ascendency and by the fourth season all the wheat had 

 disappeared from the field and the weeds held full sway. If 

 man should suddenly disappear from the earth it is certain that 

 his cultivated crops would soon follow him. And yet these 

 very plants held their own against their competitors before 

 man took them under his care. The reason they can no longer 

 persist in the face of competition is not alone because they 

 have grown weaker, but in a measure because the weeds have 

 grown stronger. In protecting his crops man has constantly 

 killed out the weak and least persistent weeds and only those 

 wer'". left to perpetuate their kind, that were able to elude man 

 himself. One species, the self heal (Prunella vulgaris), which 

 ordinarily grows a foot or more high has produced a variety 

 so low that it is able to thrive on a closely mowed lawn. Dar- 

 win never considered the lawn-mower as one of the factors 

 of evolution but undoubtedly this yankee invention has played 

 its part in the great struggle. 



