INTRODUCTION 21 



unwelcome immigrants, and one finds the common weeds of most 

 localities very far away from their original homes. 



The thistles, burdocks, plantains, mulleins and other common 

 weeds of the eastern United States are of European origin. In 

 California, where the long summer is not favorable to these weed-, 

 other species — black mustard, "filaree" (Erodium) bur clover, 

 wild oats — have been introduced, probably from Spain, where 

 the climate is much like that of California. 



Many of these introduced plants seem to grow with increased 

 vigor in their new home, and may largely replace the native plants. 



In Australia some of the common weeds come from South 

 Africa, Brazil, India, and one of the most troublesome, the prickly- 

 pear cactus, is of American origin. The latter has become a very 

 serious pest, occupying extensive tracts, especially in Queensland, 

 and rendering them quite useless for agriculture or grazing. 



Man has also been responsible for the introduction of many 

 animal pests — injurious insects, rats and mice, rabbits, etc., which 

 may do great damage to vegetation, and greatly affect the flora 

 of large areas. Destructive fungi have also been introduced by 

 human agencies, and are responsible for extensive damage to 

 crops, or native species. A recent example of the ravages of such 

 a fungus, is seen in the complete extermination of the native chest- 

 nut over a large part of the Atlantic states through the ravages 

 of the fungus causing chestnut blight. It is supposed to have been 

 introduced from China or Japan. The blister-rust of the white 

 pine is another extremely injurious fungus which has come into 

 the United States from Europe. 



While man has been responsible for the destruction of many 

 plants, he must also be credited with having added many new 

 forms to those already existing. The thousands of new varieties, 

 the result of the labors of plant breeders, are witness to man's 

 success as a creator of new plant types. 



