INTRODUCTION xiii 



probably exceeds 75 million hundredweight. The 30 

 million from the U.S.A. come in clean, but the Russian 

 14 million is largely mixed with weed-seeds, and would 

 alone account for the constant recurrence of eastern 

 weeds around our large towns. 



More than 20 million hundredweight of barley are 

 imported into Britain annually. That from France and 

 Germany is usually in a clean condition, but Persian, 

 Danubian, and Turkish barley is usually much mixed 

 with other seeds ; the latter kind is that mostly used for 

 malting, and its siftings doubtless supply a large per- 

 centage of our grain-sifting aliens. The annual import 

 of oats is about the same as barley ; more than half of 

 it comes from Russia, and as the Russian consignments 

 are nearly always plentifully mixed with extraneous seeds, 

 they are probably a source of many Northern Russian 

 weeds. 



The same series of plants become introduced also by 

 the importation of agricultural seed containing weed- 

 seeds and the sowing of them unsifted in arable land. 

 Vetch crops are most frequently raised from German 

 seed, Rape seed is largely obtained from Russia, Clover 

 seed from U.S.A., France, and Germany. 



The extensive use of certain foreign seeds for feeding 

 cage birds is certainly responsible for several of our 

 common aliens. 



Species having adhesive fruits or seeds are imported 

 on all kinds of bales and other merchandise ; they thus 

 become established by reintroduction, if not by indivi- 

 dual permanence, about the quays and warehouses at 

 seaports, along railways, and on other trade routes. 



Certain species of this kind are especially imported in 

 wool, in consequence of their prevalence in the sheep 

 pastures of some of the wool-producing countries. Two 

 seeds, for instance, are so commonly met with in 

 Australian wool that they are well known to the English 

 wool manufacturers as "Burrs" and "Carrot Seeds." 

 The former are characteristic of Port Phillip (Victoria) 

 wool, being the fruits of Medicago denticulata ; the 



