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THE MIGRATION AND DISPERSAL OF WEEDS. 
Some seeds are surrounded ‘by a gummy, gelatinous mass, which 
enables them to cling to each other and to the beaks and claws of 
birds. The small seeds of Hoary Cress (Lepidium draba) and ‘Narrow 
Leaved Cress (Lepidium ruderale) as long as they are wet are enclosed 
in a sticky, mucilaginous mass which will cling to anything. 
The active part played by some animals, but’ more especially by 
birds, is due to the development of edible fruits. These, when they 
are ripe, assume a conspicuous colour, and the fruit stalk twists or 
elongates, so that they will be in a more prominent. position and be 
more noticeable. Although the fruit itself is edible, the seeds enclosed 
are indigestible, and pass through the alimentary canal unharmed, to | 
be deposited at some distant spot. Attractive edible fruit are seen 
in the Blackberry (Rubus fruticosus) (Fig. 9), African Box Thorn 
(Lycium horridum) (Fig. 8), Sweet Briar (Rosa rubiginosa), and 
Prickly Pear (Opuntia monacantha). The edible fruit may not 
always be conspicuous; if it is small and green it will probably be 
devoured by stock with the rest of the plant, the seeds as before passing 
through unharmed. In some cases, especially with grasses, the seed is 
not only edible, but particularly palatable, with the consequence that 
the greedy bird swallows more than it can digest, flies a little distance, 
and then disgorges the excess. 
It will be seen from the above that roads along which stock 
habitually travel will be especially liable to receive the seeds of 
numerous aliens, and if these are allowed to germinate and the plants 
to flourish, the roads will form the starting point for all sorts of 
troublesome weeds. 
Srive Fruits. 
The spread of some weeds over short distances may be accomplished 
by the plant itself without outside aid. The pods of many plants of 
the pea family, e.g., the Common Gorze (Ulex ewropeus), as they ripen 
are burst open along one edge. The two valves undergo. a spiral torsion 
and the seeds are flung to a little distance by the violence of the action 
(Fig. 7). The whole movement is due to the drying and shrinking of 
a particular layer of cells when exposed to a hot sun. The action is 
accompanied by quite a loud report, and the popping of these seed pods 
can be heard on a sunny day all along a gorze hedge: The little 
Wood Sorrel (Ovwalis acetosella) has its seeds enclosed in a four-valved 
capsule. When the fruit is ripe these valves split apart, and curl 
outwards with rather a violent motion, which jerks ‘the seeds away. 
Any of these actions are sufficient to carry the seed well clear of .the 
parent plant, and gives the weed an opportunity to spread. 
OrEEPING Frurrs. 
The stiff bristles and awns of many grasses may function in two 
ways. ‘They may become attached to some animal as described above, 
or they may alter the position of the seed by a movement of their own. 
They are hygroscopic, and change their position according to the per- 
centage of moisture in the atmosphere. When once the seeds have 
fallen to the ground, these movements propel them in a definite direction. 
The distance covered would never be very great, but it would be quite 
sufficient to remove the seed from the direct vicinity of the parent 
plant. . 
C.19077—4. 673 
