11 THE STRUGGLE FOR EXISTENCE 29 



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

 Mr. AV. 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 (Eumex 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 wdth 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 Avith 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 Avith red clover, Avhich Avill also vanquish the 

 Polygonum aviculare. The most noxious Aveed in NeAV 

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

 coarse yelloAv-floAvered composite not uncommon in our 

 meadoAvs and Avaste places. This has been introduced Avith 

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

 stated that excellent pasture AA^as in three years destroyed by 

 this Aveed, Avhich absolutely displaced every other plant on the 

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

 drive out the Avhite clover, Avhich is usually so poAverful in 

 taking possession of the soil. 



In Australia another composite plant, called there the Cape- 

 Aveed (Cryptostemma calendulaceum), did much damage, andAvas 

 noticed by Baron Yon Hugel in 1833 as "an un exterminate 

 Aveed " ; but, after forty years' occupation, it Avas found to give 

 AA^ay to the dense herbage formed by lucerne and choice 

 grasses. 



In Ceylon Ave are told by Mr. ThAvaites, in his Emimera- 

 tion of Ceylon Plants, that a plant introduced into the 

 island less than fifty years ago is helping to alter the 

 character of the A^egetation up to an elcA'ation of 3000 feet. 

 This is the Lantana mixta, a verbenaceous plant introduced 



