<52 PLANT GEOGRAPHY 



a single accidental introduction may suffice to establish 

 a species. Lepidium Draba L., for instance, is said to 

 owe its prevalence in the Isle of Thanet to the Walcheren 

 expedition of 1809, a farmer having used as manure the 

 stuffing from mattresses on which our fever-stricken 

 soldiers were brought home, and seeds of this species 

 being in it. 



CHAPTER IV 



DISPERSAL AND MIGRATION 



IT is obviously advantageous to the species that offspring 

 should be removed to some distance from the parent 

 plant, so as to have at least a chance of occupying new 

 ground free from parental competition, whether of root 

 or shade. The power which a species has of thus getting 

 away from its centre of origin is termed its mobility. It 

 depends partly on the number of the spores, seeds, or 

 other detached parts produced, and partly on the special 

 mechanisms and the agencies by which they are dis- 

 persed. In a few cases the entire plant is carried from 

 place to place. More often it is some vegetative organ, 

 such as detached bulbils, tubers, offsets, and most 

 frequently it is a reproductive structure, such as spore, 

 fruit, or seed, that is transported. 



The " wind- witches " or " steppe-witches " of the 

 centre of the Old World and the " tumble-weeds " of 

 America are sometimes whole plants whose tap-roots 

 are pulled out of the loose soil in which they grow by 

 the bending of the drying stems that bear the ripe fruit. 

 The plant is then rolled as a ball before the wind, many 

 often becoming entangled together in a large mass, and 

 over level plains they may thus travel great distances. 

 The seeds may be shaken out of the fruits en route, or, 

 in some cases, these latter only open when moisture is 

 reached. 



The little offsets of Sempervivum on mountain-ledges 

 become detached, and roll down, or are blown from 

 ledge to ledge, taking root where they rest; the tubers 

 of the Lesser Celandine (Ranunculus Ficaria L.) are 

 washed away by runnels of rain-water; while small 

 rounded branches of some Cacti, having barbed leaf- 



