Page:Journal of botany, British and foreign, Volume 34 (1896).djvu/374

 846 THE DISPLACEMENT OF SPECIES IN NEW ZEALAND. Europe. Twigs of these trees are easily detached, and are floated by the river to new situations, where they quickly take root and develop with rapidity, so that in certain situations navigation is impeded. Introduced Plants on Broken Soil. Introduced plants compete with indigenous species for the pos- session of any newly-loosened surface, and especially for waste land. The margins of newly-formed roads are speedily clothed with a dense growth of sheep's-cress, docks, thistles, Yorkshire-fog, and many others, mixed with the native piripiri [Acana sanc/idsorbcB), toad-grass (Juncus bufonius), Danthonia semi- annularis, and when neglected form splendid nurseries for injurious insects and fungi. Crumbling places on hillsides in many localities are quickly covered with a strong and permanent growth of the blessed- thistle {Silybum Marianum), which distributes vast quantities of seeds, and over- comes indigenous and introduced plants alike, forming continuous masses of variegated foliage in the early spring, but presenting a ragged and untidy appearance during the autumn and winter months. The common spear-thistle {Cnicus lanceolatus) furnishes a striking example of the ability of a plant to seize upon situations suitable for its growth ; in many districts immediately after the bush is burnt off the entire area is overrun by this rapacious invader, which exhibits a dense luxuriant growth often 4 ft. to 5 ft. high, preventing the growth of grass, and forming an almost impenetrable mass. The growth becomes less luxuriant during the second season, so that the grass is able to make headway, and by the end of the fourth season only a few old thistles have retained sufficient vigour to reassert themselves. The so-called Californian thistle (0. arvensis) is the only naturalized species capable of injuring pasturage to any serious extent, and, unhappily, it is often the cause of serious loss to the pastoralist and agriculturist. The Gundagai thistle, as it is called in New Zealand (Carduus pycnocephalus), flourishes on newly- disturbed soil in many localities, but is comparatively rare on grass-land. Whenever the finely-comminuted basaltic scoria of the Auckland isthmus is disturbed, a luxuriant crop, chiefly of naturalized plants, speedily makes its appearance, but amongst them one of the most abundant is the indigenous Chenopodiuni carinatiim, although not a specimen may have been seen in the vicinity until the surface was disturbed. After the second year the number of plants is greatly diminished, and during the fourth year only solitary specimens are to be found. A similar instance has been observed at Cape Whan- brow, near Oamaru. Whenever the fine silt which covers the surface is disturbed, Lepidium tenuicaule and the indigenous form of Atriplex patula make their appearance in abundance, although usually both plants are only to be found in small quantity. Naturalized Aquatic Plants. The increase of the watercress {Nasturtium amphibium [officinale] ) in streams and watery places is phenomenal, and attracts the attention of new arrivals on account of the excessive luxuriance