20 WEEDS OF FARM LAND 



difficult to get rid of them. They spread over the surface of 

 the ground with their long trailing branches, form flowers and 

 fruits in rapid succession along the stems, and drop their seeds, 

 which are usually several in a fruit, directly into the soil when 

 they are ripe. 



Charlock (Brassica spp.) produces an abundance of heavy 

 seeds which to all appearance have no ready means of dis- 

 tribution other than that of falling into the soil in the neigh- 

 bourhood of the mother plant. The true method of charlock 

 distribution, however, is still a mystery, as it has a habit of 

 turning up in profusion in all sorts of places at a distance 

 from the nearest source of the seed. 



Dodder (Cuscuta spp.} is little other than a seedbox, as its 

 clusters of small flowers, ranged at frequent intervals along the 

 twining stems, give rise to multitudes of seeds which either 

 fall into the soil in the immediate vicinity or are carried away 

 when the crop is cut, only to start a fresh infestation of the 

 parasite elsewhere. 



A certain number of taller and more upright weeds also 

 form quantities of seeds, but these are usually so small and 

 light that they are easily carried about by the wind ; conse- 

 quently their range of distribution is less limited, and the 

 danger of loss when they are carried away is compensated for by 

 their abundance. Shepherd's purse, toadrush (Juncus bufonius}, 

 broomrape (Orobanche spp.}, and poppies are good examples, 

 and in some cases, as poppy, the seed, capsules have adapta- 

 tions which prevent the escape of the seed in unsuitable 

 weather when there is danger of it becoming damp and clogged. 



(b) Seeds Shot Out. A limited number of plants take a very 

 active share in their own distribution by shooting the ripe 

 seeds out to various distances. Crane's-bill (Geranium spp.} and 

 stork's-bill (Erodium spp.\ which are often present in temporary 

 pastures, have a fruit consisting of five pieces or carpels joined 

 to a central column, one seed being present in each carpel at 

 the base. When the seeds are ripe, some amount of de- 

 siccation occurs, the carpels break away suddenly at the base 

 and roll up backwards, jerking the seeds out as they go. 

 The fruit of vetch ( Vicia spp.} (Fig. 6) is a pod containing one 

 row of seeds. When ripe the pod splits up both sides and 

 curls suddenly into a spiral, shooting the seeds for some 

 considerable distance, often several feet. In the spurges 

 {Euphorbia spp.) the seeds are jerked away when ripe with 

 considerable force. 



