94 Plant-life of the Oxford District 



what they can do. The subject of aliens in the widest sense is of special 

 interest, as it opens up views of the migration of plants, and the complex 

 factors involved in determining the chances of their failure or success in 

 becoming established. 



Bearing in mind the original condition of the country, as covered with 

 dense forest, bare heaths, and downs, populated with the slowly returning 

 representatives of the deteriorated post-glacial flora of Central Europe, it 

 would appear that all special pasture-plants, all the weeds of arable land, all 

 crops, garden-plants and flowers, as well as many valuable timber-trees, are 

 open to the suspicion that they have been at some time intentionally or 

 unconsciously introduced by man. The difficulty will consist in isolating 

 such forms when well-established, and the records of the historical epoch are 

 of more definite value. On the other hand, historical references are often 

 merely negative, as suggesting that the plant was not commonly known to 

 the writer before a certain time, and usually to one whose botanical know- 

 ledge of the country as a whole may have been extremely incomplete. 1 



The simple lines of alien migration may be checked from the behaviour 

 of common garden-plants of known introduction. Thus Euonymusjaponicus, 

 a common evergreen, does not flower locally, although it does so freely in the 

 S. of England by the sea ; other plants are cultivated because they flower 

 freely (Jasminum offidnale^ J. nudiflorum, Forsythid), but never, or only 

 very exceptionally bear fruit. Other plants appear to fruit, but the seed is 

 imperfect or wanting (Populus nigra, Liriodendron). All such plants are 

 evidently introductions which will never make good ; they remain dependent 

 on the hand of man, and require to be propagated vegetatively, unless grown 

 from imported seed. All trees and plants similarly non-flowering, non- 

 fruiting, or producing only occasional seeds, even if found growing remote 

 from cultivation, must have been similarly introduced at some time. Even 

 in the case of plants occasionally fruiting, there is the further problem of 

 seed-wastage, and the question whether the amount of fertile seed produced 

 is sufficient to cover such wastage of dispersal and germination under com- 

 petition ; that is to say, in any indigenous plant seed must be produced in 

 great profusion. A chance for long-continued survival is afforded to plants 

 which have already attained a method of ready vegetative propagation, 

 which will multiply individuals without affecting the race. Thus the 

 Common Elm and the Gray Poplar (Populus canescens)^ which flower freely, 

 but do not produce fertile seed, may maintain their station practically 

 indefinitely by copious growth of suckers. Other herbaceous perennials 

 which flower but never fruit, of known introduction, may in virtue of an 

 efficient rhizome- system, defy extirpation (Aristolochia Clematitis), or carry 

 on indefinitely where not interfered with ( Vinca> Helleborus foetidus, Acorus) ; 

 and this applies particularly to ' bulbous ' plants as the Snowdrops, Crocus, 

 and Narcissus (sp.) of garden-cultivation. All plants which fail to produce 

 seed at all under natural conditions, are clearly of alien origin ; since they 

 could not have come to the country by the ordinary agencies of seed- 

 dispersal, except as individuals rather than as a race, and for these the case 

 is so far clearly recognized. 2 But the subject takes on a wider range, and 

 the main principle may be approached from another standpoint. 



Great Britain over a northward extension of some 600 miles presents 

 a marked range of climatic dispersal. Many types are fully successful in 

 establishing themselves by seed, as apparently ' indigenous ' in the South, 



1 Thus Caesar recorded that neither Beech nor Fir grew in Britain (54 B. c.), De Bello Gall., 

 v. 12. 



* Dunn (1905), Alien Flora of Great Britain. 



