DISTRIBUTION OF WEEDS . n 



in the weed floras of many districts, and the difficulties of agri- 

 culturists have thereby in many cases been greatly augmented. 

 As is shown below, weed seeds are transported from one 

 country to another in various ways, and it frequently happens 

 that a weed that is of little or no account in its country of 

 origin finds the conditions of a foreign land so congenial that 

 it spreads and becomes a veritable pest, often needing legis- 

 lation for its suppression. Sometimes, too, with change in 

 methods of cultivation, an alien weed flora may crowd out the 

 native weeds more or less completely. In the settled por- 

 tions of New Zealand and Australia, 1 where clearings have 

 been made and introduced crops like wheat are grown, the 

 alien weeds are most conspicuous and dominate the situation. 

 Apparently the native plants are adversely affected by the in- 

 terference due to the new methods of cultivation, and are un- 

 able to withstand the comp tition of the foreign plants ot 

 which the seeds are introduced with the crop seeds, and as a 

 result the intruders have been able to gain a firm foothold. 

 A considerable intrusion of Northern plants into the New 

 Zealand flora has occurred. When a party of the British 

 Association visited the country in 1914 much watercress was 

 noted in one place, and in other localities Geranium molle and 

 G. robertianum (the former a typical arable weed) were 

 completely naturalised. 



The extent to which weed seeds can be carried from one 

 country to another in cargoes of grain and other crop seeds 

 has been well shown by Stapledon, 2 who has proved that it is 

 possible to trace the country from which samples of commercial 

 oats have originated by means of the quantity and varieties of 

 weed seeds that are present. For instance, oats from Russia 

 usually contain an abundance of corn cockle, and wild vetches 

 and field bindweed are also often plentiful. Turkey seed 

 is characterised by the presence of a considerable amount of 

 Rapistrum rugosum, an unidentified Medicago, and a fair 

 quantity of darnel and sweet clovers ; Canadian origin is 

 shown by excess of ball mustard (Neslia paniculata] and by 

 blue bur (Lappula echinata) and prairie sunflower, while in 

 British seed black bindweed (Polygonum convolvulus} and 

 charlock (Brassica spp.~] are often the only weed seeds, 



1 Rendlc, A. B. (1915), " The British Association in Australia," Jour. Bot., 

 LIII, No. 625, pp. 23-34. 



- Stapledon, R. G. (1916), " Identification of the Country of Origin of Com- 

 mercial Oats," jfour. Bd. Agric., XXIII, No. 2, pp. 105-116. 



