74 



GRASSES OF IOWA. 



A most interesting case of animal dissemination is afforded 

 by a native species of Stipa spartea. The cylindrical grain is 

 provided with a sharp- pointed cuUus which makes it easy to 

 penetrate clothing, the skins of animals and soil. The grain 

 is enclosed by the hard, persistent, flowering glume which sur- 

 rounds the palet. The lower portion of the coriaceous flower- 

 ing glum9, as well as the pedicel, are covered with stiff hairs 

 that point upwardly in an oblique direction. The flowering 

 glume bears a strong awn twisted like a rope. This awn is 

 very sensitive. The coils unroll or unwind when damp, and 



Fig. 51. Dissemination of Aristlda. Aristida tuheradosa. awned flower, lower detail; 

 Arwtida purpurea, awned flower, detail at the right; Aristida oUgantha, awned flower, 

 detail at top. 



when dry they return to their former condition. Whenever 

 this drying-out takes place the points of the flowering glume 

 are rotated with lateral variations. The upper half of the awn 

 remains straight and soon becomes bent at nearly a right angle 

 to the twisted part. This portion of the awn is of very mate- 

 rial assistance when it comes in contact with surrounding 

 objects. The bristles at the lower end prevent the seed from 

 being pulled out. " This rotation and nutation, together with 

 the action of the bristles, soon causes the bract surrounding 



