196 BRITISH PLANTS 



2. Explosive Fruits and Seeds. — In explosive fruits, the 

 walls, when ripe, suddenly explode, and ejeot the seeds 

 with some force into the air. In the balsam, touch-me- 

 not {Impatiens Noli-me-tangere), and wood-sorrel, the walls 

 are somewhat succulent, and explode suddenly through 

 the tension set up in warm, dry weather by the rapid 

 loss of water from them. The geraniums afford an 

 excellent example of explosive schizocarps (see p. 190). 

 In the squirting cucumber, a berry-liise fruit becomes so 

 turgid that it bursts, shooting out a watery pulpy mass 

 in which the seeds are embedded. In the wood-sorrel 

 '^xalis Acetosella) the seeds are not only expelled from the 

 capsule, but out of their own arils ; in this case the seeds 

 themselves may be regarded as explosive. The pods of 

 the gorse and the siliquas of the cuckoo-flower {Carda- 

 mine pratensis) pop in dry weather. In the Viola the 

 seeds are shot away one after another from the open 

 capsule, by the drying and contraction of the margins of 

 the dehisced sections (Fig. 105). 



3. Fruits and Seeds distributed by Birds. — These are 

 generally succulent, in which case some provision is 

 always made to prevent the seeds from being eaten. In 

 berries the seeds are hard and indigestible ; in drupes the 

 seeds are enclosed in a stone, and this is always dropped. 

 Some seeds ejected from capsules are covered with a 

 sucQulent coat — the aril ; this is eaten by birds, and the 

 liernel within rejected — e.g., spindle - tree, foetid iris, 

 wood-sorrel. Among the conifers the seeds of the yew 

 and juniper have succulent integuments ; in the yew it 

 is an aril ; in the juniper fused bracts. 



Seeds of all kinds are eaten by birds, and many are 

 destroyed in this way, especially the grains of cereals. 

 Some of them, however, may, by some accident, escape 

 complete destruction and get distributed. 



Darwin points out that birds may be the means of 

 dispersing seeds in another way. If the ground is damp, 

 they pick up a good many seeds on their dirty feet. In 

 one case he found that eighty seeds germinated from a 

 small pad of soil which he took from the foot of a bird. 



4. Animal-Dispersed Fruits and Seeds.— (a) Seeds eaten 

 by animals— e.gr., nuts and oily seeds— are enclosed in 

 tough skins or hard shells which have to be torn or broken 

 open before the food can be obtained. Many are de- 



