]0 Nov., 191 o.] The Spread of Weeds and of Plant Diseases. 691 



hold their own without spreading further. On Doughljoy Lshmd, wliere 

 formerly a small residence stood, the ground is largely covered with alien 

 weeds, whereas on other of the islands round the Park, which have not 

 I ten occupied or are little visited, the native flora holds its own, although 

 many alien weeds with air-horne seeds grow on the mainland within the 

 distance the seeds may be carried by the wind. In addition, in many 

 parts of Victoria, it has often been noticed that on cleared ground 

 subsequently left to run wild, an originally luxuriant crop of the .so-called 

 Scotch Thistle may gradually thin out and ultimately almost disappear 

 as the land reverts to its natural condition. Onion Grass again, actually 

 follows in man's footsteps, since it flourishes in the hard soil at the borders 

 cf paths and in uncultivated overgrazed pastures in which the continual 

 trampling of the stock has made the .soil hard and given the Onion Grass 

 tiie conditions it needs to enable it to suppress useful grasses and clovers. 

 Travelling stock are common agents in spreading weeds to new districts 

 Jind in providing the conditions which will enable them to gain and main- 

 tain a foothold, but even irrigation channels may aid in the spread of 

 certain weeds. For instance. Stink wort has in .some cases been spread 

 from one district to another by means of irrigation channels, the seeds 

 floating on the surface of the water, and many other .seeds could be spread 

 m the same way. In this case, fixing a board across the channel so as 

 X) hold back the surface scum, would prevent this source of spread. 

 The reticulation channels themselves, f)eing moist and not under crop and 

 usually composed of worked and fertile soil, are localities specially suited 

 for the growth of weeds, and the running water is admirably adapted 

 to provide for the dispersal of floating weed seeds. 



The influence of animals, particularly of stock animals, upon the 

 spread of weeds and also upon the spread of disease, is very pronounced, 

 and this fact is recogni.sed in the regulations for the prevention of the 

 spread of certain contagious diseases. Such regulations are not un- 

 natural interferences with the order of nature, but are rather attempts 

 to bring our artificial conditions into harmony with nature again. In 

 nature, sick or diseased animals are immediately killed by carnivorous 

 animals or are thrust out by the herd and left to die in solitude. The 

 only improvement man has made upon the natural conditions is by destroy- 

 ing or segregating animals which have been sul)jected to the chance of 

 infection, as well as those which are actually diseased, whereas the natural 

 conditions for extirpation only come into play when the disease has 

 advanced sufficiently far to aff^ect the strength and vigour of the animal. 



The way in which stock, and particularly sheep, carry the seeds of 

 Barley Grass and Spear Grass is sufficiently well known to need no com- 

 ment, but even here it is possible in the simplest of ways to lessen the 

 spread by such means. For instance, suppose that a run contains ^ 

 tract covered with any of the troublesome species of Aristida, " The Three- 

 awned Spear Grass." Left to indiscriminate grazing, this will soon be 

 carried all over the run as the better grasses are eaten out, but if the in 

 fested tract is fenced off and the sheep crowded in while the grass is 

 young or while it is shooting after being burnt off, and if they are taken 

 out before it flowers, its spread may l)e prevented or the total amount 

 of it even steadily decreased. 



To take another instance, the hard seeds of the Onion Grass lie on 

 the surface of the ground in abundance from October to December, and 

 are commonlv carried on the hoofs of stock from one place to another. 



