GKASSES OF IOWA. 75 



the fruit to bore deeply into the ground." This boring into 

 the soil takes place in a short time. Beal says, in speaking of 

 the Feather grass {Stipa pennata): "Francis Darwin found 

 that the rate increases up to the fifth revolution, and then 

 diminished quickly. In three wettings and three dryings, a 

 little over an inch was buried in dry sand. A rise of tempera- 

 ture affects the awns in the same way as increased moisture; a 

 fall of temperature acts like dryness. Mr. Darwin found that 

 minute strips of the awn, consisting even of two long cells, 

 twisted just as well as the entire awn. He thinks that the 

 torsion is produced by the striation or stratification of the cell 

 walls. These are a series of parallel lines, alternately light 

 and dark, traversing the surface of the cell. Very frequently 

 the two systems wind spirally round the axis in opposite direc- 

 tions. When the tissue expands during the absorption of 

 water, it is due mainly to the swelling of the less dense striae. 

 This is thought to be the cause of torsion in cotton wool. Scon 

 after being buried, where the soil is moist, the awn breaks off 

 at a joint from the apex of the grain. " 



Mr. R. M. Christy states that a large number of the seeds 

 are often found beneath the skin of sheep, especially about the 

 shoulders. Dr. M. Stalker states that in many of the north- 

 western counties of Iowa they occasion much annoyance to 

 sheep, and, in some instances, cause the death of animals. 

 They penetrate and bury themselves in the flesh. Dogs, and 

 even persons, are also affected. F. H. King says that he was 

 much annoyed by the fruit of this grass. Stipa capillata of 

 southern Russia, Aristida hygrometrica of Queensland and Heter- 

 opogon contortus of New Caledonia inflict similar injuries. 



The beautiful south European grass {Stipa pennata) also 

 buries itself in the soil. The long, feathery awn enables the 

 wind to carry it away. The "seed" is small, the flowering 

 glume has a pointed callus and above it obliquely pointed stiff 

 hairs. As in the other species it is provided with strong, 

 twisted awns that end in a long, beautiful feather. As the 

 seeds are carried by the wind they eventually fall, with the 

 "seed" end to the ground, as it is heavier at the lower end. 

 The "seeds " remain in this position as long as dry, but when 

 moist the spirally twisted awn unwinds the plume and helps to 

 hold it in the soil; thus, finally, the "seed "is buried in the 

 ground. 



