REMARKABLE INSTANCES OE PLANT DISPERSION, 



346 



to 6,000 feet. A watercress {Nasturtium amphibium) 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 navigation 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. Darrens observed that the white clover spread over tracts of peaty 

 soil, which until invaded supported a dense and luxuriant growth of the 

 New Zealand flax {Phormium tenax) ; but one of the greatest pests was 

 Bumex Acetosella, the sheep's sorrel ; this, however, was expelled by the 

 white clover. The latter, notwithstanding its extraordinary vigour, was 

 itself unable to hold its own against the cat's-ear {Hypochaeris radicata) 

 or some similar composite, introduced with grass-seeds from England. 

 In Nelson excellent pastures were wholly destroyed in less than three 

 years by this weed, which absolutely displaced every other plant on the 

 ground. Lastly " the dock, the sowthistle and other European composites, 

 the red sorrel, &c, were to be met with all over the country." To this 

 the following testimony has been added. " The most remarkable special 

 instance of all that have been communicated to me," wrote Sir J. D. 

 Hooker, "is, that the little white clover and other herbs are actually 

 strangling and killing outright the New Zealand flax {Phormium tenax), a 

 plant of the coarsest, hardest and toughest description, that forms huge 

 matted patches of woody rhizomes, which send up tufts of sword-like leaves, 

 six to ten feet high, and inconceivably strong in texture and fibre. I know 

 of no English plant to which the New Zealand flax can be likened so as 

 to give an idea of its robust constitution and habit to those who do not 

 know it. (The garden Yucca resembles it somewhat.) It is difficult 

 enough to imagine the possibility of white clover invading our bogs and 

 smothering the trussocks of our Garex paniculata, but it would be 

 child's play in comparison with the resistance the Phormium would seem 

 to offer." 



The preceding facts show an extraordinary amount of vigour in 

 Trifolium repens, the Dutch or white clover. Darwin's experiments 

 showed that if protected it only produced 10 per cent, of the quantity of 

 unprotected and visited by bees ; in another it bore no good seed at all, 

 while twenty unprotected had 2,290 seeds. Hence this plant is extremely 

 self-sterile. Whether it acquired self-fertility in New Zealand and 

 America, where it is equally thriving, is not known ; but it affords a good 

 instance, if it requires insect aid, of " an exception which proves the rule " ; 

 forit rivals the knotgrass, and the ligulate composite, whatever it was, which 

 are both self-fertilizing (if the latter behaves like the dandelion). " The 

 causes of this prepotency of the European weeds are probably many and 

 complicated ; one very powerful one is the nature of the New Zealand 

 climate, which favours the duration of life in individuals, and hence gives 

 both perennials and annuals a lengthened growing season ; and, in the case 

 of some species, more than one seed crop is borne in the year. This is 



* Quoted by Hooker in the Nat. Hist. Rev. 1864, p. 124; and in Pop. Sci. Rev., 

 vi. p. 137. 



