452 COMPETITION AND INVASION 



American plants, on the other hand, have in many cases 

 overrun other countries. Modea, a common water weed with 

 us, introduced into Great Britain about 1847, now chokes up 

 many pools and water courses in England and Scotland. The 

 prickly pear cactus (Opuntia Ficus-indica) and the century 

 plant, both emigrants from North America, are now the most 

 conspicuous plants along many cliff sides all over southern Italy. 

 A prickly pear has become such a nuisance in New South Wales 

 that large rewards are offered for its extermination. 



429. Weeds. Any flowering plant which is troublesome to 

 the farmer or gardener is commonly known as a weed. Though 

 such plants are annoying from their tendency to crowd out 

 others useful to man, they are of extreme interest to the bot- 

 anist on account of this very hardiness. The principal charac- 

 teristics of the most successful weeds are their ability to live 

 in a variety of soils and exposures, their rapid growth, resist- 

 ance to frost, drought, and dust, their unfitness for the food 

 of most of the larger animals, in many cases their capacity to 

 accomplish self pollination, in default of cross pollination, and 

 their ability to produce many seeds and to secure their wide 

 dispersal. 



Sometimes the seeds have great vitality ; those of shepherd's 

 purse and purslane are capable of germinating after fifteen 

 years or more. Many of the worst weeds, such as sow thistle, 1 

 sorrel, 2 witch grass, 3 nut grass, 4 and field garlic, 5 have creeping 

 rootstocks or bulbs or tubers. Not every weed combines all of 

 these characteristics. For instance, the velvet leaf, or butter 

 print, common in cornfields, is very easily destroyed by frost; 

 the pigweed and purslane are greedily eaten by pigs, and the 

 ragweed by some horses. The horse-radish does not usually pro- 

 duce any seeds, but is propagated by vegetative methods. 



It is a curious fact that many plants which have finally 

 proved to be noxious weeds have been purposely introduced 

 into the country. The fuller's teasel, melilot, horse-radish, wild 



1 Sonchus. 2 Bumex. a Agropyrum. * Cyperus. 6 Allium. 



