38 TYPES OF BRITISH PLANTS 
principle, and there seems scarcely any limit to its appli- 
cation, although one is rather startled by Virgil’s remark 
in the Georgics that some kinds of fruit trees are best 
grown from “four-cleft stakes or poles of sharpened 
wood.” But perhaps the poet’s exigencies of metre pre- 
vented the clearer explanation one would have liked. 
The artificial interference of mankind with the distri- 
bution of plants is too wide a subject to enter on here, 
but one may note that it has had very great effect. There 
is an American pondweed called Anacharis, which in the 
seventies was brought over to England, and for some years 
drove all those responsible for the state of canals and 
slow-running rivers almost to despair. It spread at a 
tremendous pace, and despised all efforts to keep it down. 
Happily it was a plant of the advanced kind which 
requires two individuals to make a fertile seed, and only 
one had come over, but by means of the offshoot method 
it choked up canal after canal. After some years, partly 
owlng to more energetic weeding of the rivers, and partly, 
perhaps, because it had exhausted the kind of food which 
it liked best, it was got under, and order reigned again in 
Warsaw. 
This American invasion was bad enough, but English 
plants have succeeded in making themselves unpopular 
in other countries. Someone introduced watercress into 
New Zealand as a table dish, but the watercress refused 
to retain its humble form out there and has grown into 
a large and vigorous water-bush, stopping up streams and 
getting in the way generally. 
In Australia a Scotch farmer planted a few thistles in 
his garden to remind him of home, but unfortunately 
they refused to stop in the garden, and they went forth 
and occupied the earth promptly, much to the disgust of 
