4o8 THE BIOLOGY OF FLOWERING PLANTS 



As an example of a seed which secures wide distribution 

 we may take the willow, with the tuft of hairs by means 

 of which it can float on the wind for long distances. As an 

 example of a seed which travels only a very short distance 

 we may take the poppy. The capsule, opening by apical 

 pores, is borne on a stiff, elastic stalk, which jerks back and 

 forwards in the wind or when brushed against by an animal. 

 The seeds do not fall out, they are thrown to a little distance. 



The unit of dispersal may be either the seed or the fruit. 

 The latter is the case in dry one-seeded fruits which do not 

 open but become detached from the plant as a whole, like 

 the achene of the buttercups, the nuts of the beech and 

 hazel, and the acorn of the oak. In dehiscent fruits which 

 open and liberate the seed, the seed is the dispersal unit ; and 

 this is also the case in fleshy indehiscent fruits, the walls of 

 which are digested by animals or rot away. 



Dispersal by the Plant. — The apt term " censer 

 mechanism " has been apphed to the means of dispersal 

 of the poppy. It is capable of greater refinement. In the 

 genus Campanula the capsules, which are again borne on 

 elastic stalks, open by three pores. In some species, e.g. C. 

 pyrenaica, the capsules are erect ; in others, e.g. C. lattfoUay 

 they nod. In the former the pores are apical, in the latter 

 basal, and thus in both cases the capsule opens at the upper 

 end, so that the seed cannot fall out but must be thrown out 

 (Fig. 64). In Lychnis dioica and many other Caryophyllaceae 

 the capsule opens by teeth which carry out hygroscopic 

 movements. They curve back and open the capsule in dry 

 air, while in moist or rainy weather they quickly bend up 

 and close the entrance. The seeds are thus saved from the 

 effects of wetting which would tend to mat them together ; 

 they escape only in conditions favourable to their being 

 thrown to some little distance (Fig. 65). In some plants of 

 dry regions the opening movement takes place when the 

 fruit is wet, e.g. in Mesembryanthemum. 



In pods of the broom and whin the opening, by splitting 

 into two valves, is again connected with a hygroscopic 

 mechanism. DiflFerent tissue lavers contract to different 



