162 DR. FRANCIS DARWIN ОМ THE MECHANISM BY WHICH 
due to the other, if any tendency of the cell to twist in one direction more than in the 
other is to result. Nägeli has shown * that imbibition does lead to the torsion of a cell 
on its axis when one system of striation is more pronounced than the opposite. Fig. 10 
is copied from his paper already quoted t: а shows a cotton-wool fibre in extremely 
dilute sulphuric acid; 0 is the same fibre in somewhat more concentrated acid. The 
first treatment merely brings out the stri] more clearly; the stronger acid causes 
the fibre to swell and to twist on its axis. Тһе importance of these figures is this: 
in а 16 may be seen that the direction of the striation changes in the lower half of 
the fibre; and in b it is seen that the direction of the torsion changes precisely where 
the direction of the striation does; not only do cotton-wool fibres twist on the intense 
imbibition caused by acid, but, as I find, on the intense abstraction of water caused 
by strong drying. From these facts we must conclude that the striation of the cells is 
the cause of the torsion of fibres of cotton wool. 
The elongated cells of the 5Нра-алуп present a close analogy with cotton-wool fibres; not 
only do they twist on drying, but also, as a result of the great swelling caused by macera- 
tion in strong Schultz’s solution, they undergo torsion in the opposite direction. 
From all these considerations, I cannot resist the conclusion that the torsion of the 
cells in the Stipa-awn is a direct consequence of unequal contraction of the cell-wall due 
to the striation of the membrane. I have found the investigation of the molecular 
structure of the cells in the Stipa-awn too difficult a task to be included in the pre- 
sent research; I believe, however, I may say that the cells are obliquely and spirally 
striated, and that one system is more strongly developed than the opposite. 
Both МасеН and Hofmeister give explanations of the ultimate mechanism of the tor- 
sion of cells; but in my present need of clear anatomical details it would be useless to 
apply their explanations to Stipa-cells. 
I now pass on to describe the remaining seeds or fruits observed by me. 
AVENA ELATIOR. 
Professor Hildebrand has described the hygroscopic awn of Avena sterilis, both as to 
the mechanism of the twist and the adaptation of the hygroscopic torsion as a means 
of distribution f. 
Fig. 4 shows the awn of Avena elatior (the empty glumes having been removed). It will 
be seen that, supposing the fruit to be held vertically, we have, just as in Stipa, а vertical 
twisted part of the awn, and а more or less horizontal part which is not twisted. In Avena 
sterilis, according to Hildebrand$, the spikelet contains two fertile flowers, and there- 
fore two bent awns. Не describes how, in drying, the pressure of the rotating part of 
the awn against the ground causes the fruit to be projected into the air (“ein Stück 
. fortgeschleudert wird ")|. 
* Münchener Sitzungsb. 1864, July, p. 124. T Tab. i. figs. 8 & 9. 
+ Pringsheim's Jahrb. loc. cit, ; and * Verbreitungsmittel der Gramineen-Früchte," Bot. Zeitung, no. 49, 1872. 
$ Schleuderfrüchte, Separatabd. p. 14. 
| This curious property has gained for it in Germany the popular name of “ Springhafer " (Hildebrand). I have 
met with a eurious mention of the wild oat in the * The Young Man's Companion, or Arithmetic made easy, '1721:— 
