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 Eussian thistle,^ Pig. 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 



' Salsola Kali, var. tragus. 



