SELF-FERTILIZATION OF PLANTS. 995 
18. Naturalized abroad, self-fertilising plants may acquire great vigour and even 
replace the native vegetation. 
This has been the case in New Zealand. Mr. W. T. Locke Travers, writing to 
Sir J. D. Hooker, from Canterbury, in 1864, thus speaks of some of our British weeds :— 
* You would be surprised at the rapid spread of European and other foreign plants in this country. 
All along the sides of the main lines of road through the plains Polygonum aviculare grows most luxu- 
riantly, the roots sometimes two feet in depth, and the plants spreading over an area from four to 
five feet in diameter. The dock (Rumez obtusifolius or R. crispus) is to be found in every river-bed, 
extending into the valleys of the mountain-rivers until these become mere torrents. The sowthistle 
is spread all over the country, growing luxuriantly nearly up to 6000 feet. The watercress increases in 
our still rivers to such an extent as to threaten to choke them altogether; in fact, in the Avon, a still 
deep stream running through Christchurch, the annual cost of keeping the river free for boat navi- 
gation and for purposes of drainage exceeds £300. I have measured stems twelve feet long and three 
quarters of an inch in diameter. In some of the mountain districts, where the soil is loose, the white 
clover is completely displacing the native grasses, forming a close sward " *, 
Mr. Warens, writing again in the same year, adds further details, and mentions that 
the white clover is spreading over the tracts of peaty soil which, until invaded by other 
plants, supported a dense and luxuriant growth of Phormium tenas (N. Z. flax). One of 
the greatest pests, he says, is Rumex Acetosella; but this, again, like other weeds, is 
expelled by white clover. This latter, however, “ notwithstanding its extraordinary 
vigour, is itself unable to hold its way against Hypochwris radicata (?), which has been 
introduced with grass-seeds from England. In Nelson I have seen excellent pastures 
wholly destroyed in less than three years by this weed, which absolutely replaced every 
other plant on the ground. The radical leaves form an imbricated mass, so closely 
appressed to the soil as to prevent evaporation and ensure the decay of every thing below 
them." Lastly, he notes “the dock, the sowthistle, and other European composites, a 
turnip [Raphanus Raphanistrum?, J. D. H.], the red sorrel, and a variety of other 
introduced plants are to be met with all over the country " f. 
A writer in the * Gardeners’ Chronicle’ for August 2, 1856, inquires for a suggestion as 
to the cause of the unusual quantity of white clover which appeared ** in all the pasture- 
fields, and on roadsides and places where it could not have been sown.” Mr. Rogers, 
replying on August 16, observed :— 
* During a residence of many years in America, I always observed that wherever a clearance was 
effected in the forest, no matter how distant from a settlement, in the course of two years, although no 
seeds of any kind had been sown, and the surface of the ground burnt nearly all over the previous crop, 
a good crop of white clover was sure to appear, considered there the best sheep pasture in America. 
These facts show an extraordinary amount of vigour in Trifolium repens and other 
plants. Mr. Darwin, experimenting on the white clover, found that protected plants 
produced only 10 per cent. of the quantity of seeds produced by unprotected plants 
which were visited by bees, while on another occasion twenty protected plants yielded no 
good seed at all, while twenty unprotected yielded 2290. Hence this plant is sometimes 
* Quoted by Hooker in Nat. Hist. Review, 1864, p. 124, and in Pop. Sc. Rev. vol. vi. p. 137. 
` t Hooker adds in a note :—* Sonchus arvensis is wild in Now Zealand, anà was eaten by tho aborigines but the js 
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