ii THE STRUGGLE FOR EXISTENCE 29 



tenax), a large plant with iris-like leaves 5 or 6 feet high. 

 Mr. W. L. Travers has paid much attention to the effects of 

 introduced plants in New Zealand, and notes the following 

 species as being especially remarkable. The common knot- 

 grass (Polygonum aviculare) grows most luxuriantly, single 

 plants covering a space 4 or 5 feet in diameter, and send- 

 ing their roots 3 or 4 feet deep. A large sub-aquatic 

 dock (Rumex obtusifolius) abounds in every river-bed, even 

 far up among the mountains. The common sow-thistle 

 (Sonchus oleraceus) grows all over the country up to an 

 elevation of 6000 feet. The water-cress (Nasturtium officinale) 

 grows with amazing vigour in many of the rivers, forming 

 stems 12 feet long and f inch in diameter, and completely 

 choking them up. It cost £300 a year to keep the Avon 

 at Christchurch free from it. The sorrel (Rumex acetosella) 

 covers hundreds of acres with a sheet of red. It forms a 

 dense mat, exterminating other plants, and preventing cultiva- 

 tion. It can, however, be itself exterminated by sowing the 

 ground with red clover, which will also vanquish the 

 Polygonum aviculare. The most noxious weed in New 

 Zealand appears, however, to be the Hypochaeris radicata, a 

 coarse yellow -flowered composite not uncommon in our 

 meadows and waste places. This has been introduced with 

 grass seeds from England, and is very destructive. It is 

 stated that excellent pasture was in three years destroyed by 

 this weed, which absolutely displaced every other plant on the 

 ground. It grows in every kind of soil, and is said even to 

 drive . out the white clover, which is usually so powerful in 

 taking possession of the soil. 



In Australia another composite plant, called there the Cape- 

 weed (Cryptostemma calendulaceum), did much damage, and was 

 noticed by Baron Von Hugel in 1833 as "an unexterminable 

 weed " ; but, after forty years' occupation, it was found to give 

 way to the dense herbage formed by lucerne and choice 

 grasses. 



In Ceylon we are told by Mr. Thwaites, in his Enumera- 

 tion of Ceylon Plants, that a plant introduced into the 

 island less than fifty years ago is helping to alter the 

 character of the vegetation up to an elevation of 3000 feet. 

 This is the Lantana mixta, a verbenaceous plant introduced 



