THE ANGIOSPERMAE 



1549 



Fruits which become inflated hke bladders are in a special category, for 

 while they may be dispersed by wind, if they are detached without dehis- 

 cence, they also float easily and may often be carried about by rivers. Two 

 familiar garden shrubs, Cohitea and Staphylea, are in this group. Several 

 genera of Leguminosae produce swollen, parchment pods, which normally 

 dehisce on the plant and only occasionally may be dispersed as fruits. 



Wind dispersal of fruits, as of seeds, is usually associated with the 

 production of either wings or plumes. Prominent among the types of wing 

 formation are those in which the wings are outgrowths of the pericarp. 

 They are usually one-seeded, indehiscent fruits, known as samaras, and 

 are either akenes or reduced follicles or legumes. Many examples can be 

 cited without going outside the British flora: Ulnuis, Fraxirius (Fig. 1408) 



Fig. 1408. — Fraximis excelsior. Ash. Bunches of winged fruits 



or " keys ". 



and Acer, all with one wing, but in the latter genus with the winged akenes 

 cohering in pairs, so that the familiar "keys" of the Sycamore have two sym- 

 metrical wings; Betula, Isatis and Heracleum with the edges of the pericarp 

 extended into two wings, as well as several two-winged siliculas of Cruci- 

 ferae, e.g., Thlaspi and Lepidiutn. There are a great number of similar 

 fruits in other parts of the world and the method seems to furnish efiicient 

 dispersal (Figs. 1409 and 141 o). The very light winged akenes of Betula 

 probably fly for hundreds of yards, but heavier fruits may not make more 

 than six to ten yards in falling from the tree, and only travel further along 

 the ground if the wind is strong or gusty. An interesting variant is found in 

 some plants, the wing being twisted so that the fruit rotates in falling, which 

 delays its descent and allows time for considerably greater travel, up to 100 



