THE ANGIOSPERMAE 1569 



Where the hard part is large, as in Cherry, it may not be swallowed but 

 simply flung away by pecking birds. 



This means of transport is very eflFective over relatively short distances, 

 but in migrant birds the digestive tract may be almost entirely emptied 

 during flight and there is therefore only a small chance of seeds being thus 

 carried to remote places. 



Mechanical Dispersal. Concerning the mechanical dispersal of fruits 

 as distinct from seeds, which we have already dealt with, there is not much 

 to say. The relatively few examples apply to akenes, which are dispersed 

 like seeds mostly by the resilience of elastic tissues. The fruits of Poly- 

 gonum virginianiim are thus shot off with considerable force, when the 

 hardened style is struck, by the elastic elongation of the pith cells in the 

 pedicel, which are held in compression by the abscission layer below the 

 fruit. When this is broken the pith elongates and shoots the fruit off as 

 from a catapult. 



Dorstenia is a genus of Urticaceae, consisting of small herbs with a 

 wide distribution in the tropics generally. The inflorescence axis is a 

 flat expanse covered with minute flowers which form akenes. The pericarp 

 is hard around the top of the akene but fleshy on two sides of the lower 

 part, forming a sort of nut-cracker holding the hard part in its jaws. As 

 the fleshy portions dry and shrink, the hard part breaks away and is shot 

 out by pressure. Two other members of the same family, Pilea and Elato- 

 stema, have also an explosive mechanism, which closely resembles that 

 by which the pollen is dispersed (seep. 1291). There are three staminodes, 

 strongly bent inwards with their ends lodged beneath the carpel. As the 

 latter ripens the staminodes grow bigger and exert a lifting force on the 

 akene, a force which is released when the akene becomes detached. 

 The staminodes spring up and the akene is shot out to a distance of several 

 metres. Circaea alpina is reported to have an explosive projection mechan- 

 ism for its fruits, but its nature still needs investigation. 



The boring awns of some Grasses and of Erodium may be mentioned 

 here, though they are not strictly organs of dispersal, but rather the 

 opposite, organs which help to fix the small fruits in the ground where they 

 have fallen. The principle is the same in all, namely that the awn (or in 

 Erodium the style) is divided into two zones, the lower of which is spirally 

 twisted while the upper is straight and bent at right angles to the first. 

 The fruit comes to rest with the akene pointing obliquely downwards. 

 Changes of humidity cause the spiral to twist and untwist, while the top 

 of the awn is held in place by pressure against the ground. The movement 

 of the spiral portion thus twists the akene round, first one way, then the 

 other, like a bradawl, boring it into the soil. The akenes are always pro- 

 vided with stiff hairs pointing upwards, which act as barbs and prevent 

 the akene from being withdrawn but offer no hindrance to its downward 

 movement. 



These plants bury their fruits after they have been dispersed, but 

 there is a considerable class of plants which bury their fruits themselves, 



