II 
TIIE STRUGGLE FOR EXISTENCE 
29 
tenax), a large plant with iris-like leaves 5 or G 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 olcraceus) grows all over the country up to an 
elevation of G000 feet. The water-cress (Nasturtium officinale) 
grows with amazing vigour in many of the rivers, forming 
stems 12 feet long and § 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 Hypochseris 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 
