xii INTRODUCTION 



rows, and other habitats will occur to every field botanist 

 which, offering the same conditions, receive the same 

 flora. Cultivated fields, again, with their abundance of 

 plant-food, harbour all sort of weeds, but only those 

 gain permanence which by quick seeding can withstand 

 the frequent ploughings. Many of the short-lived spring 

 annuals of the Mediterranean region have thus found 

 their way northwards. 



Introducing Agents. A study of habitat is only useful 

 in the case of the better established aliens ; there remains 

 a large class of plants which owe their presence in 

 Britain more to their adaptation for frequent artificial 

 introduction than to any power of spreading along the 

 lines offered by human operations. These are indivi- 

 dually of short duration, but in consequence of constant 

 reintroduction maintain their presence in our flora* 

 They can be most easily reviewed by a consideration of 

 the various means of plant introduction from the outside 

 world to Britain now taking place. 



By far the most important agent of plant introduction 

 at the present time is the importation from foreign 

 countries of the kinds of grain which are most largely 

 used for making flour and for distilleries. In every 

 sack countless seeds of the cornfield weeds of the country 

 of origin come mixed with the grain. Before the grain 

 is used these seeds are sifted out, and are either thrown 

 away with other rubbish on waste ground or sold for 

 feeding domestic fowls and game. In the former case 

 astonishing crops of exotic weeds may be produced in 

 a small area, and some of them will possibly survive and 

 become established there for a time. In the second case 

 the aliens will spring up here and there around cottages,, 

 along roadsides, in coppices, or wherever the birds are 

 fed. All the species introduced in this way must be 

 cornfield weeds. It should be remembered that corn 

 has been continuously imported since the fourteenth 

 century at least, and that some of our oldest recorded 

 weeds may be due to this source. 



The total foreign wheat imported annually into Britain. 



