ORIGIN OF THE BRITISH FLORA 215 



reason Ireland contains fewer species than Great Britain, 

 just as Great Britain contains fewer species than France 

 or Germany. 



Native Plants and Aliens. Plants which grow wild, and 

 which we have every reason to believe have always 

 formed part of our flora since the advent of man, are 

 called indigenous, or native. Those which have been 

 introduced since otherwise than by natural means 

 e.g., birds or wind are aliens. Now, there are aliens and 

 aliens. Some may be quite recent intruders, others came 

 very early when the first cultivators disturbed the soil ; 

 some have competed successfully for a long period of time 

 with the native flora, and have spread far and wide over 

 the land ; others spring up, flourish for a time, and then 

 die out ; others are mere casual strays, seen here and 

 there, and seldom in the same place two years together. 

 The following various kinds of aliens have been distin- 

 guished : 



1. Denizens. Plants which we know or have reason 

 to suspect are foreigners, but which have so successfully 

 established themselves in natural or wild and closed 

 habitats among the native flowers that they now form a 

 constituent part of the flora e.g., Elodea, Impatiensfulva, 

 Claytonia perfoliata and C. alsinoides, Crocus, cyclamen, 

 snowdrop, and probably the gooseberry and red and 

 black currants. 



2. Colonists. These are generally more recent immi-j 

 grants, and do not, as a rule, compete on equal terms 

 with the native vegetation. They are found in artificial 

 habitats that is, on ground disturbed by man, where 

 there are always bare spots open to settlement. Most 

 of the commoner weeds of cultivation are colonists in 

 this sense (see below). 



3. Casuals are aliens which appear here and there, but 

 never secure a permanent foothold even in artificial 

 habitats. They soon die out, either because they fail to 

 produce efficient seed, or because the conditions are against 

 them in the struggle for existence. Some are weeds 

 brought with seed from other lands, some arrive with 

 merchandise, some are relics and escapes from cultiva- 

 tion, others mere garden throw-outs. 



Weeds of Cultivation. This is a term applied to those 

 plants, generally annuals, which are found on cultivated 



