INTRODUCTION 13 



cassava to the Euphorbiacece ; Mexican clover to the 

 Rubiacece; and chufas to the Cyperacece. 



7. Aggressiveness necessary in perennial forage crops. 

 Aggressiveness, or ability to hold the soil against weeds 

 and other competitors, is an exceedingly important char- 

 acter in all perennial meadow or pasture crops, and often 

 important in annuals. Plants of Old World origin are 

 in this respect far superior to those of- American origin, 

 at least for cultivation in the New World. With but a 

 single exception, every perennial hay plant cultivated in 

 America is of Old World origin, and among perennial 

 pasture plants there are but few exceptions. Many of 

 the native American grasses are equal to Old World 

 grasses in yield and nutritiousness, but with perhaps a 

 few exceptions they lack in ability to retain possession 

 of cultivated land against the competition of weeds. 

 The reasons for this are not apparent, but the fact scarcely 

 admits of doubt. Grasses are not alone peculiar in this 

 respect, as most of our troublesome weeds are also of 

 Old World origin. Some of these weedy grasses and other 

 plants furnish good forage, and are therefore welcome. 

 Among such are Japan clover, white clover, narrow- 

 leaved vetch, bur clover, alfilaria, sweet clover, crab-grass, 

 Bermuda- grass and blue-grass. A few temperate Ameri- 

 can plants have shown similar aggressive propensities 

 when transplanted to the Old World, such as water- weed, 

 Elodea canadensis, and horse-weed, Erigeron canadensis. 

 Many tropical American plants introduced into the Orient 

 have prospered amazingly, where they behave as weeds, 

 in a manner exactly analogous to European plants in- 

 troduced into the United States. All this points to aggres- 

 siveness as being in part at least a phenomenon of plants 

 transferred to a new but similar environment. 



