THE MIGRATION AND DISPERSAL OF WEEDS. 
Orrsoors. 
Underground stems, creeping stems, or offshoots are the means by 
which a troublesome weed may considerably increase the area of 
ground that it covers. A plant such as St. John’s Wort (Hypericum 
perforatum), which is a perennial with short runners, capable of 
spreading in any direction and sending up new shoots all round the 
original plant, can spread over a large area in a short time. In many 
plants, as the underground stem grows and branches, the original stem 
dies away behind, so that two plants are formed in the place of one. 
Underground stems are seen in Couch Grass (Agropyrum repens), 
Bindweed (Convolvulus arvensis), Stinging Weed or Nettle (Urtica 
dioica), Bracken Fern (Pteris aquilina), and others. 
Prorrction From Exrincrion purring Unravorasin SEasons. 
Many weeds are noted for the persistence with which they reappear 
in a field after they have been eradicated by all of the more simple 
means. ‘This persistence may be due to the possession of underground — 
stems or bulbs which will sprout again every time the shoots are cui off. 
They have to ibe cut a good many times before the vitality of the stem 
is exhausted. ‘The persistence may be due to the longevity of the seeds, 
as many weed seeds can remain dormant during four or five years and 
still be able to germinate. This means that, if the seeds are ploughed 
into the ground to a depth which is unfavorable for germination, instead 
of decaying, they will remain there in a dormant condition, and at the 
next ploughing will stand a good chance of being brought nearer to the 
surface and of being able to germinate. 
The Bathurst Burr (Xanthiwm spinosum) produces two seeds to a 
fruit, but these seeds allow for propagation over two seasons, as one 
ripens one year and one the next, so that it takes over two years to 
eradicate this pest when once it has been allowed to seed. 
Some weeds are able to grow in land in which any other vegetation 
is quickly destroyed. The Onion Grass (Romulea cruciata) seems to 
particularly favour land which is continually being trampled on. ‘This 
accounts for the way in which, when it becomes established along the 
sides. of a road, it appears to enter the fields only through the gates. ; 
The ground just at the gate is usually hardened and bare of vegetation 
because it is continually being trampled on, which makes ideal condi- 
tions for the growth of the onion weed. 
It will be seen from the above that when once a weed has been 
allowed to grow and ripen its seed, it will take time and money to get | 
rid of it. It is, therefore, necessary to take all the precautions possible: 
to prevent the entry of any new weeds, or when they have entered to 
prevent them seeding. 
INDEX TO ILLUSTRATIONS. 
Fig. 1. Head of common dandelion (Taraxacum officinale), slightly reduced. 
‘Fig. 2. Seed of Cat’s Ear (Hypochceris radicata), x 6, 
Fig. 3. Seed of Sow Thistle (Sonchus oleraceus), x 6. 
Fig. 4. Seed of Sheep’s Burnet (Acena sanguisorba), x 2. 
Fig. 5. Seed of Wild Oats (Avena fatua), natural size. 
Fig. 6. Fruit of Bathurst Burr (Xanthium spinosum), natural size. 
Fig. 7. Seed pod of a leguminous plant showing spiral twist, reduced. 
Fig. 8. Fruit of African Box Thorn (Lycium horridum), natural size. 
Fig. 9. Fruit of Blackberry (Rubus fruticosus), reduced. ayies ; 
Fig. 10. Impure lucerne seed—(a) lucerne seed; (b) dodder seed ; (¢) earth particle, enlarged, 
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