158 DR. FRANCIS DARWIN ОХ THE MECHANISM BY WHICH 
must hold good for Stipa. I assumed that if there existed unequal longitudinal 
tensions along opposite surfaces of the awn, there would be inequality of contraction 
or expansion in a radial direction, and that the sectional outline would alter in shape 
on drying. I could observe no such alteration, and found, in fact, that on swelling 
in water the awn increased in diameter very nearly equally in all directions. І con- 
cluded, therefore, that the inequality of expansion must be between the internal and 
external tissues, and not between two longitudinal halves. I saw no way of con- 
firming or destroying this hypothesis; but I always felt a difficulty in the fact that 
the Stipa-awn is always twisted іп one direction, whereas the above-mentioned unequal 
distribution of tensions would give a tendency to twist in either direction. Ulti- 
mately all theories of unequal contraction of the awn as a whole were overturned by 
observing that longitudinal sections and mere strips torn from the awn were capable 
of twisting up into precisely the same right-handed screw as the whole awn. ‘This 
proved that torsion-power may reside in а combination of a few cells. The fact that 
a transverse section of the twisted awn of Anemone montana shows only broken- 
down tissue in the centre, convinced me that torsion is possible where the twisting- 
organ has no strongly contracting tissue in the centre. These two observations 
suggested that the power of torsion must reside in the individual cells composing the 
awn. I therefore boiled a piece of Stipa-awn in dilute nitric acid and chlorate of 
potassium; by subsequent teasing in a drop of water, the cells were isolated with great 
ease. The slip of glass was then held over a spirit-lamp, and a light teasing action 
continued, to prevent the cells adhering to the glass on drying. On examining the 
object under the microscope the torsion of the individual cells was beautifully seen; 
besides individual cells, numerous little ropes of two or three cells are seen, all 
twisted in the direction of torsion of the awn, i.e. that of a right-handed screw (see 
fig. 14). 
To this observation it may be objected that the treatment with hot acid may have 
conferred the power of torsion on the cells. То this it may be answered, (1) that small 
portions, separated by mechanical means, exhibit torsion when dried; (2) that pieces of 
awn treated with boiling nitric acid and chlorate of potassium (if they are not teased into 
their constituent cells) undergo torsion on drying, proving that the cause of the torsion, 
whatever it be, is not affected by the treatment with acid. 
I believe that both the internal and the external cells of the awn are capable of inde- 
pendent torsion; but whether this be so or not, it is certain that the small external cells 
exhibit the power in a far higher degree. "This cleared up what had always seemed à 
great difficulty, viz. the instantaneous movements of the Stipa-hygroscope ; for it removes 
the seat of hygroscopic action from the internal tissues to a more accessible part near 
the surface. In accordance with the present theory, it is found that the individual 
cells possess the same delicacy of action as the whole awn. I have actually seen a single 
cell under the microscope untwisting and twisting up again as my hand approached 
and was withdrawn from its neighbourhood. 
Supposmg ы then to be granted that the cells composing the awn are capable of inde- 
pendent torsion, we have yet to show that the twisting of the awn as a whole will be 
