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EE A an eee eee Le TRS PT nT ae oN eR ee te TEN rm 1 
BRIEFER ARTIGLES: 
NOTE ON THE MECHANICS OF THE SEED-BURYING 
AWNS OF STIPA AVENACEA. 
(WITH FIVE FIGURES) 
In the parts of many plants where hygroscopic movements take 
place in dead tissues, the cause is found in thick-walled mechanical 
cells of peculiar structure or varying chemical composition. 
The ripe and dry awn of Stipa avenacea, holding the seed at the 
lower end, is strongly twisted over half its length next to the seed ina 
direction Opposite to the movement of watch hands. The remainder 
of the awn, not having the spiral structure of the former, is not 
twisted, but is bent at an angle to the body of the awn, thus fur- 
nishing a brace or support when the seed begins its boring motion, 
driven by the alternate twisting and untwisting of the dry or wet awn. 
Little barbules on the upper part of the awn directed away from the 
seed, assist its progress forward while preventing any backward move- 
ment. 
The seed is tipped with a short, sharp point, slightly curved, the > 
ter to lead the way into the ground. Stiff hair-like barbs on the 
Wer portion of the seed hold it in the ground when once started. 
The onward motion is still farther assisted by the increased length of 
the awn when wet, which amounts, by actual measurement, to 20 per 
cent. of the whole length. On drying a corresponding withdrawal of 
the seed is prevented by the barbs. So the alternate wetting and dry- 
ing of the awn serves the twofold purpose of moving Se er 
(although this is, no doubt, more commonly accomplished by being 
fastened with its appendages to some moving object) and placing it in 
a favorable Position in t 
Th 
the lat 
Aveng barbata 
bet 
lo 
The moist awn straightens out completely. , 
potash or-other macerating fluid it twists with watch 
113 
Placed in caustic 
1900] 
