DISTRIBUTION OF WEEDS 



21 



This method of seed distribution is highly specialised, but 

 is less effective than others. Comparatively few seeds are 

 formed, and as the risk of life is very great it seldom happens 

 that weeds of this description are very abundant. The 

 geraniums are usually associated with temporary pastures, 

 because when once the seed is sown there is little cultural 

 interference and the weeds are able to ripen seeds in peace. 

 If the ley is succeeded by tillage crops 

 such as cereals or roots, the geraniums 

 usually disappear rapidly, as there are not 

 enough seeds in the soil to withstand 

 the processes of cultivation. 



(c) Fruits and Seeds Distributed by 

 Wind. Adaptations for this purpose are 

 exceedingly common and are usually in 

 the form of wings or hairs arranged to 

 form a kind of parachute. The wings or 

 hairs may be developed either on the seed 

 itself or on the fruit, the latter being more 

 general. Many of the seeds are liable to 

 be lost, but they are produced in large 

 quantities and under favourable circum- 

 stances can be carried for many miles. It 

 is on this account that weeds with these 

 adaptations are so peculiarly dangerous, 

 for if they are allowed to ripen their seeds 

 it is impossible to confine them within 

 bounds, and a single dirty farm may vitiate 

 all attempts at clean farming over a wide FlGt 



area. 



6. FRUIT OF 



N ARROW-LEAVED 



VETCH (Vicia an- 

 gustifolia), showing 

 one pod split open 

 and curled back with 

 the seeds shot out. 



(a) Winged Fruits. The winged 

 part may either be firmly attached to the 

 fruit or may be easily detachable. In 

 several of the Umbellifers, as for instance 

 hogweed (Heradeum sphondyliuni] (Fig. 7 C), a very broad 

 wing is developed by the seed case, which encloses a single 

 flattened seed. In the docks and sorrels the floating organ 

 is formed by the persistent floral envelope or perianth, within 

 which is the hard triangular fruit (Fig. 7 D). 



(ft) Winged Seeds. In toadflax (Linaria vulgaris] and 

 yellow rattle (Rhinanthus crista-galli) (Fig. 7 A) each seed is 

 provided with a broad wing, so that it is easily carried by the 

 wind. In some other cases, as in spurry (Fig. 7 B), a very 



