64 PLANT GEOGRAPHY 



split apart as in many Caryophyllacecs, Primulacece, 

 Delphinium, Viola, etc., is, perhaps, the least specialised 

 type of sling-fruits. Somewhat more elaborate are the 

 cases of the pods of many Leguminosce and the valves 

 of the short fruits of such Euphorbiacece as Hura, Heiea, 

 Mercurialis, Ricinus, etc., in which diagonal contraction 

 produces spiral torsion, often throwing the seeds to a 

 considerable distance. Resilience, or elastic spring, in 

 dry fruit-stalks, and hygroscopic action, like that whxh 

 ruptures the sporangia of Ferns, aid in dispersal in such 

 cases as many of the Composite, the Geraniums, and 

 the " hopping " awned grasses. In Centaur ea, fc * 

 example, there is an erect, rigid, resilient peduncle, 

 common receptacle which by drying may detach its 

 numerous fruitlets at their bases; dry, hygroscopic 

 bract-scales highly polished on their inner surfaces, and 

 a shuttlecock-like pappus of bristles to each fruitlet. In 

 dry air censer-action jerks the loose fruitlets over the 

 polished curve of the expanded involucre, the pappus - 

 bristles serving, as do the feathers of a shuttlecock, to 

 determine the direction in which each fruitlet shall fall. 

 The curling upwards or away from the central axis or 

 carpophore of the awn or stylar appendage to each of 

 the five carpels in Geranium seems also largely hygro- 

 scopic; as certainly are the curious jer kings, crossings 

 and uncrossings of the dry awned glumes of such grasses 

 as the Barren Oat (A vena sterilis L.). Equally strikin 

 are such cases as the Squirting Cucumber (Ecballiu 

 Elaterium A. Rich.) and the Balsams (Impatiens), oJ 

 which some tissue of the fruit becomes so turgesceko 

 with watery sap as to burst the fruit from its stalk, . 

 to separate the carpels, hurling the contained seeds : > 

 a distance. The rapid spread of exotic species ^of 

 Impatiens and of Oxalis (in which one of the seed coats 

 becomes turgescent, splits and turns inside out) demon- 

 strates the value to the plant of such dispersive 

 mechanisms. 



HYDROCHORES. The carpels of the Stonecrop (Sedum 

 acre L.) remain closed in dry weather; but when a drop 

 of rain falls into the basin-shaped hollow at the top of 

 the fruit they open, and the small seeds are carried over 

 steep rock-surfaces by trickling runnels. The two most 

 important adaptations to water transport are, however, 

 (i.) the imprisonment of air in structures surrounding 



