THE MIGRATION AND DISPERSAL OF WEEDS. 
in every direction and enable the plant to increase rapidly, and at the 
same time make it much more difficult to exterminate. Merely cutting 
the part above ground will not have any permanent effect on the weed, 
as it sprouts again from the horizontal stem, and this stem has to be 
ploughed up and destroyed before the plant will disappear. 
Weeds first appear along the sides of some main thoroughfare, along 
a railway line, or a main stock route. They very often become -estab- 
lished on the waste ground in or around a town, and from there are 
liable to be dispersed by various means throughout the whole district. 
Winp. 
The seeds of many plants are especially adapted for transport by 
wind. Many members of the Composite (thistles, dandelions, prickly 
lettuce, groundsel, &c.) are fitted with an apparatus resembling a tiny 
parachute, and known as a pappus (Figs. 1, 2, and 8). This is caught 
up by the wind and lifts the seed with it, carrying it a little way before 
the heavy seed drops off. In this way, a windy day could pretty well - 
infect a wide area with the seeds of a plant such as Prickly Lettuce 
(Lactuca scariola). Other seeds are fitted with wing-like outgrowths 
‘to the seed coat; the seed itself is very light, and the wings expose a 
comparatively large surface to the force of the wind, which picks the 
whole thing up and carries it for some distance. The best example of 
this, although it can hardly be classed as a weed, is the seed of the 
common Elm Tree (Ulmus campestris). Many seeds are covered with 
thick woolly or hairy coverings, which serve to assist in their dispersal 
by wind, e.g., the seeds of Cape Weed (Cryptostemma calendulacee ) 
are buried in a mass resembling cotton wool. a 
In some cases, the seeds have no special adaptation for wind dis- 
persal beyond their size. They are so tiny that they are blown about 
like dust, in fact, dust is usually composed, to a certain extent, of tiny 
seeds. ‘The obnoxious Bracken Fern (Pteris aguilina) has such small 
seeds or spores that they are hardly visible to the naked eye, but they 
are practically always present in the air, and this enables bracken to 
get a hold in land in which the former vegetation has been destroyed 
before any other plant. 
It is not always the seed only which is carried along by the wind, 
but the plant itself, or a large part of the plant, can be rolled along the 
ground for considerable distances. In Windmill Grass | (Chloris 
virgata), the whole flowering head, which consists of a number of 
spikes radiating from one point like:a windmill, breaks off and rolls 
along the ground, dropping the seeds as it goes. In some of the 
Medics, Lucerne (Medicago sativa), Snail Medic (Medicago scutellata), 
and Burr Medie (Medicago denticulata), the seed pod is rolled into a 
tight ball, which can be blown along the ground. 
WATER. 
Many seeds are carried for long distances down streams and water- 
courses, and become established at points along the banks far from their 
original home. hese will chiefly be wind-blown seeds which have 
reached the river from neighbouring fields with the help of the wind. 
As a general rule, new weeds do not have a very good chance of getting 
a hold along the banks of a normal water-course—the vegetation is 
usually too thick along the banks to allow much room for strangers. 
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