96 Plant-life of the Oxford District 



migration brings with it a long series of unconsciously imported aliens as 

 associated, rather than ' introduced ' weeds of pasture and arable land, as 

 also of waste-heaps and dung-heaps, which soon become the commonest 

 types of the new flora, evicting the previous natives from those stations, as 

 the conditions are the more divergent from those of the original formation. 



Such importations may also come into direct competition with indigen- 

 ous plants occupying similar stations ; and thus representative species of the 

 same general character, and derived from different districts, may be found 

 in increasingly mixed association. For example, where three or more closely 

 ' allied ', or apparently similar types of one genus occupy the same sort of 

 station, it is probable that two at least are aliens : cf. the remarkable ' sets ' 

 of Thistles, Chenopods, Polygonums, Veronicas, Euphorbias, and Poppies of 

 arable and waste ground. Much the same applies to the more definite 

 introductions of man for food or economical purposes. The Wheat of 

 Western Asia, cultivated as an annual, requires to be sown in the previous 

 autumn, and runs out before reaching the North of Scotland. In Orkney 

 the only cultivated cereals are Oats and the 4-rowed Barley (Bere), and this 

 in late seasons may be only harvested by November. 



Taking this long-continued northward drift of the plants of the Old 

 World area from Post-Glacial times, it is evident that no plant of S. Europe 

 is now ever likely to establish itself in open competition with indigenous 

 flora, however much it may apparently succeed with only slight assistance, 

 or in the milder climate of S. England by the sea, where frost is practically 

 eliminated. Thus Sycamore comes up freely as a weed in the West of 

 England, in garden-ground and underwoods, but less in natural woodland, 

 and not freely in gardens, in the Oxford District. Veronica Tournefortii 

 has established itself in all cultivated land, within the last hundred years, 

 but only in competition with other Veronicas of the same alien type 

 ( V. agrestis) ; as Crepis taraxacifolia, locally common in grassland, replaces 

 other alien forms of Crepis (as C. biennis) 1 in waste-places where unoccupied 

 ground may be artificially provided. 



From these standpoints aliens may be graded according to the degree and 

 amount of assistance gained from human occupation of the land as: 

 (i) Assisted (a), the case of plants just holding on at the limit of their dispersal 

 area, with the slight unconscious help of man in providing denuded ground, or 

 taking seeds (e.g. Clematis). (2) Assisted associates (aa), plants normally 

 indigenous, but owing their great and special development to the increased 

 area in which they find suitable conditions (Buttercups, Daisies, Dandelions). 

 (3) Assisted associate aliens (aaa), plants accompanying man in distant migra- 

 tions, now so common and abundant as to pass for indigenous (e. g. Chenopod- 

 ium album, Polygonum Persicaria), not establishing in normal pasture, nor in 

 woodland. (4) Assisted associate alien adventives (aaaa), including chance 

 casuals, coming in at any time, found only in cultivated or waste ground, 

 not establishing, and usually not found in the same spot in successive seasons. 

 It is also obvious that no sharp line can be drawn between such grades, but 

 the consideration of numerous special cases may be interesting on their own 

 merits. 



On the other hand, plants coming from a botanical district so remote 

 that there is no chance of dispersal by natural agencies, may be able to 

 compete with indigenous flora on their own ground, and even to become 

 invasive. The remarkable case of Elodea, brought from Canada about 1842, 

 and long a pest in streams and ditches, shows what can be done in this 

 direction, even by vegetative propagation alone, the introduction being a 

 carpellary plant. 2 Hence there is so far no reason why plants from the 



1 Cf. Dunn (1905), Alien Flora of Great Britain, pp. 113, 149. 



a Weeping Willow, carpellary only (1730) ; Populus serotina (1787), Lombardy Poplar (1758) 

 staminate, are all also the product of a single introduced individual. 



