120 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Steps on to an aircraft in one continent and off it in another, often 

 with Httle movement meanwhile to brush off adhering seeds and 

 fruits. Much the same may be true of transported animals, which 

 are always apt to carry seeds and fruits in their wool and fur, or 

 otherwise about or in their bodies, and evidently account for many 

 plant introductions. Moreover, there is practically no limit to the 

 number and diversity of seeds and fruits that adhere to men and 

 women when they fall or merely walk in mud and clay, to be brushed 

 or picked off later, or are transported by them for food or as curios— 

 often to be discarded at a distance. And to the abilities of some 

 seeds to pass through the human digestive tract unharmed, the 

 ' spontaneous ' growth of Tomato plants in sewage farms bears 

 ample testimony. Finally, with their hosts are frequently carried 

 parasitic (and also saprophytic) species. That such spread of plant 

 diseases can be very serious is indicated by the rigid restrictive 

 measures adopted by many governments against the importation of 

 living plants. 



The great differences often observed in the actual migration of 

 thus ' artificially ' introduced plants are, however, probably due 

 more to the adaptability of the species to local environments than 

 to the dissemination itself, essential though this is. As we shall see 

 in Chapter VI and elsewhere, plants tend to be adaptable when they 

 are variable — in habitat requirements as well as in form. Accord- 

 ingly, some familiar European weeds, such as Shepherd's-purse 

 {Capsella bursa-pastoris), Common Chickweed {Stellaria media agg.), 

 and the little grass Poa annua, have become practically world-wide 

 without having any special adaptations for long-distance dispersal, 

 whereas other plants the disseminules of which are doubtless more 

 commonly, and sometimes more widely, carried, are still relatively 

 restricted in their geographical area. Often the climatic, soil, or 

 other local conditions are unsuitable ; or the competition of native 

 plants is so severe that ' open ' habitats have to be found — ^especially 

 by weeds. Such open habitats are commonly due to Man's acti- 

 vities, soon becoming closed over with vegetation when abandoned, 

 so that the colonizing aliens become restricted or often ousted. In 

 time the signs of human interference may virtually disappear, 

 though, as has already been emphasized, such interference is nowa- 

 days so widespread and drastic in various ways as to constitute the 

 most active agent of vegetational change in the world. Moreover, 

 as compared with earlier times, the barriers to dispersal by human 

 agency are greatly diminished now that men can (and frequently do) 



