COLONISATION BY OTHER METHODS. 6g 



they enable them to utilise to the full, any chance 

 shower of rain, and in Britain to flower very early in the 

 season when the ground is bare of other plants. The 

 natural tendency to store up food has of course been 

 greatly modified by man, as one can readily see by 

 comparing a wild carrot with the cultivated form. He 

 has stimulated and developed a natural tendency and 

 in so doing has to some extent altered the character of 

 the plant rtself If, for instance, Helianthus tuberosus is 

 prevented from forming stem tubers, then unusual fleshy 

 swellings appear upon the roots or elsewhere. 



Thus the arrangements for colonisation possessed by 

 plants are often of a very unexpected nature. Exact 

 details of the ground over which they can spread in one 

 year are unfortunately very difficult to find. In the 

 American Tmnbleweeds, in which the root or flowerstalk is 

 torn out, the entire plant or \\.?> fi^uitifig pedtmcle forms a 

 sort of spherical ball of branches and stems (sometimes 

 ten inches in diameter) ; this may be driven fifteen 

 miles by a single hurricane. These Tmnbleweeds, such 

 as Salsola Kali, Corisperviuni, Hordeinn jubatuin, Brornus 

 sterilis, etc., have been very thoroughly studied in 

 America where the absence of hedges and trees permits 

 of a very wide distribution. But in Britain the dis- 

 tribution cannot be nearly as rapid except under very 

 exceptional circumstances. Still, if a grain of wheat 

 can be blown fifteen yards in a minute by a strong 

 wind, it is clear that it is not possible to give any exact 

 limit to the possible distribution of plants by these 

 methods. 



It is only necessary for a student to attempt to dig 

 up a complete plant of the Bisbop's Weed {Aegopodiuin 

 podagrarioi) to realise, both the complexity of its system 

 of underground rhizomes and roots, and also the very 



