854 



THE DISPERSION OF SPECIES BY MEANS OF FRUITS AND SEEDS. 



thus), the two wings exhibit a slight spiral twist resembling a propeller; this 

 occasions a peculiar gyratory motion of the fruit as it sails along in the air. 

 Wherever there is only a single wing which projects from one side, the centre of 

 gravity has an eccentric position, and the fruits and seeds of this class spin quickly 



Fig. 468. Dispersion of fruits and seeds by the wind 



as they fall freely through the air. The motion in question is particularly well 

 marked m the half-fruits of the Sycamore and the seeds of Pines 



same object as is attained in the above cases by the development of .late 

 ' brought about in other plants by the transformation of dry bracts or 

 i into light, loose, saccate, or inflated envelopes round the fruits or seeds. 



