CERTAIN SEEDS BURY THEMSELVES IN THE GROUND. 157 
My brother then suggested that if it were due to the expansion or contraction of the 
water contained in the tissues, the results ought to be exactly reversed when water at 
4° C. was used as the higher temperature, 0° being the lower one, because water expands 
instead of contracting in passing from 4° to freezing-point, and therefore the Stipa-hygro- : 
scope ought to behave as if 0° was a higher temperature than 4°. Preliminary experi- 
ments showed that a difference of 4^ is clearly indicated by the hygroscope; but, to our 
surprise, we found that the hygroscope behaves exactly as if water expanded in passing 
from 0 to 4°. We therefore concluded, like De Luc, that the effects of temperature have 
nothing to do with the expansion of water. He placed his hygrometer in quicklime, 
by which means he obtains what he calls * absolute dryness," and found that changes of 
temperature affected his instrument in ** nearly the ваше” manner as when in water. 
The only explanation left appears to be that the woody tissue itself expands with heat. 
But, from the experiments of Villari *, it appears that the expansion of dry wood with heat 
is extremely small compared with the expansion due to imbibition. I find it impossible 
to believe that the rapid rotation through a considerable angle is due to this cause. Тһе 
following eurious phenomenon negatives such a view, but must also remain quite unex- 
plained. It will be made clear by detailing an experiment. Two vessels of water were 
employed, whose temperatures differed by about 68° C. In the cold water the index of 
the hygroscope stood at 130; on putting it into the hot water the index moved quickly 
to about 240 (not exactly noted), and returned slowly to 115. ‘That is, it first makes а 
rapid wntwisting, or wet movement, and then returns to a point on the dry side of its 
original position—that is, a point representing increased torsion of the awn; so that on 
moving into hot water it untwists rapidly, and then twists slowly up beyond its original 
degree of torsion. Exactly similar but reverse results ensue on removing the awn 
from hot to cold water. These experiments have been frequently repeated by my brother 
and by myself. 
The Mechanism of Torsion.—In Sachs's * Handbook’ t the twisted growth of trees is 
explained by the internal parts not growing as fast as the external tissues, the unequal 
longitudinal tensions satisfying themselves by producing torsion. To apply this to the tor- 
sion of the SZipa-awn, we must suppose that, on drying, the internal parts contract more 
strongly than the external. "This action may be imitated, as Sachs remarks, by slipping 
an elastic tube over a second of smaller calibre; the internal one is then stretched, 
ihe external one being left as it is, and the two are bound together in several places. 
On the release of the internal tube, the external one will not permit any shortening of 
the internal tube except by the whole system turning into an irregular helix. Hil- 
debrand { offers a similar explanation for the torsion of the awn of the * Springhafer ” 
(Avena sterilis). He says that one surface of the awn contracts on drying more strongly 
than the opposite surface, and the awn twists on itself to satisfy the unequal tension 
thus produced.  Hanstein $ explains the torsion of the awns of the Geraniacez ina 
Similar way. For a long time I concluded that some explanation of a kindred nature 
* Quoted in Sachs: English танба, р. 649. + English Translation, 1874, р. 770. 
t Pringsheim's Jahrb. für w. Bot. 1873, Bd. ix. “ Sehleuderfrüchte." $ Loc. cit. 
