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THE STRUGGLE FOR EXISTENCE. 205 
the attacks of their enemies prevents any hardy kind of plant 
from covering all suitable portions of a whole continent, to 
the exclusion of most other vegetable life. New Zealand and 
the pampas of La Plata and Paraguay, in South America, have, 
during the present century, furnished wonderful examples of 
the spread of European species of plants over hundreds of 
thousands of square miles of territory. The new-comers 
were more vigorous, or in some way better adapted to get on in 
the world than the native plants which they encountered, and 
so managed to crowd multitudes of the latter out of existence. 
- In our own country, a noteworthy case of the kind has 
occurred so very recently that it is of especial interest to 
American botanists. The so-called Russian thistle,’ Fig. 184, 
which is merely a variety of the saltwort, so common along 
the Atlantic coast, was first introduced into South Dakota in 
flaxseed brought from Russia and planted in 1873 or 1874. 
In twenty years from that time the plant had become one 
of the most formidable weeds known, over an area of about 
25,000 square miles. 
HOW PLANTS PROTECT THEMSELVES. 
248. Protection from Weather.— Several allusions have 
been made in earlier chapters to the means by which plants 
defend themselves from excessive cold, moisture, or drought. 
The varnish and the woolly coating of bud-scales very likely 
serve the double purpose of preventing sudden changes from 
heat to cold, and of keeping the tender interior of the bud 
from becoming watersoaked. 
The corky layer of the bark, whether of the stem above- 
ground, of the underground stem, or of the root, prevents loss 
of water, as was proved by Exp. 20, § 99. 
The waxy coating on the under side of leaves keeps the 
1 Salsola Kali, var. tragus. 
