WEEDS AND TIIEIIl CHARACTERISTICS. 199 
siou, because tliey know that the newly imported succeed better than 
their own. The law is further illustrated by our system of rotation of 
crops, in which one kind of plants is most advantageously replaced by 
another; and here, at last, we get at a chemical explanation of the 
advantages enjoyed by new-comers, and why, in a straggle for exist- 
ence between them and the natives of the soil, they must ever come off 
victorious. 
A weed, then, in our language, signifies a naturalized herb, which 
has a soft and membranaceous look, grows fast, propagates its kind 
with great rapidity, and spreads, to the prejudice of endemic or culti- 
vated plants, in places in some way or other disturbed by the agency 
of man. 
Ihere are, in various parts of the world, trees and shrubs, though 
their number is not so great as at first sight might be imagined, that 
may be said to occupy the same position amongst woody plants as 
weeds do amongst herbs. Psidium Guayava, Melia Azedarach, Acacia 
lamesiana, and Crescent m Cujete, may serve as instances of such trees ; 
the Brazilian Tecomaria Capensis at the Cape of Good Hope and 
other parts of the Old World, and Vlex Europmts at St. Helena, as 
examples of such shrubs. In most places, where we find them, they 
are exotics, out-elbowing the indigenous vegetation by their rapid 
growth. But in most cases we know that their geographical range 
has been extended by their being taken under man's fostering care. 
The word " Tree-fern " having gained an easy admission in our lan- 
P»ge, it might be advisable to speak of these plants as tree-weeds 
and shrub-weeds, without being charged with unnecessary innovation. 
Whence do different countries derive their weeds? is a question 
that naturally suggests itself. Off-hand, one would be inclined to 
*aswer that all countries indiscriminately, having a climate similar to 
that of Europe, would be the sources whence Europe derived its weeds. 
And to a great measure this is true. Many European weeds have an 
undoubted Asiatic and African origin ; but if any part of the world 
m, ght be expected to have supplied its due share, it would be the 
temperate parts of the North American continent, where many Euro- 
pean plants, such as Thistles, have multiplied to such an extent as to 
have become a perfect pest. From the constant intercourse between 
Eu, ope and North America, and the number of North American plants 
,ltiv ated in our gardens, one would have expected a great many 
