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tivated areas alone, since carried about in the hair of horses, in 
the wool of sheep, or exuded in the dung of cattle, many of them 
are now to be found in exclusive possession of the best soils. 
With some, as the thistles, the invasion is transitory, as after 
exhaustion of the soils of the special constituent required by 
them, the growth becomes weaker, and the plants finally dis- 
appear. 
The extension of railways throughout Australia has also 
greatly assisted in a wide and rapid distribution of imported 
plants, not only fromthe seeds present in the hay on which the 
horses used in the construction of the various lines, but also in 
the facilities offered for the transportation of fodder grown in 
coastal regions, into the most distant pastoral districts, and as 
the soil throughout the interior regions of Australia is commonly 
of very excellent quality, climatic conditions seem the only bar 
to their indefinite increase. : 
It is, however, in the regions of cultivated lands and their 
immediate vicinity that the most striking examples of the effects 
of these introductions are to be noted. Two or three plants in 
particular may be instanced which under certain conditions take 
complete possession of the soil, to the utter exclusion of any 
other ; the most universally spread and the best known is the 
corkscrew- or clover-burr (Medicago denticulata). Coming up 
annually with the first fall of rain, it spreads in dense, close 
masses with prostrate stems, and entirely chokes the growth of 
any other annual; when the heat of summer is felt it quickly 
dries up and its abundant seed-vessels then freely cover the 
ground. No introduced plant has produced greater loss to the 
woolgrowers than this, for although a fairly good fodder, the 
seed-pods, coiled up into two or three whorls, readily become 
affixed by their serrated edges to the wool on the bellies and 
legs of sheep. To the flock-master the presence of these trouble- 
some seeds means a loss of one penny per lb. on all those parts of 
the fleece which contain them, so that the total yearly loss 
amounts to hundreds of thousands of pounds. The best soils 
with the best climate producing it in the greatest abundance; so 
that our native clovers, especially the delightfully-scented and 
invaluable Trigonella suavissima, have no chance to survive 
against so vigorous an enemy. 
Another plant, the Bathurst Burr, Xanthiwm spinosum, a 
South American introduction, is also yearly the cause of great 
loss to the wool-grower, and though it is only a summer-growing 
plant, and ripening its troublesome burrs in autumn, yet is useless 
as a stock food, and to some extent also interferes in certain 
situations with the reproduction of native plants. 
A third example is the South African Cryptostemma calen- 
O 
