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IX. On the Hygroscopic Mechanism by which certain Se $ are enabled to bury 
themselves in the Ground. By Francis DARWIN, M.B., F.L.S. 
(Рһлте XXIII.) п 
\ 
be | 
Read March 16, 1876. 
THE object of this paper is to describe the hygroscopic mechanism by which certain 
seeds are enabled to bury themselves in the ground. This phenomenon as it occurs in 
the Geraniaceze has already been described by Hanstein *, and again by G. Roux t in 
the case of Erodium; lastly, Asa Gray f briefly points out the power of burying them- 
selves possessed by the carpels of some of the Western American species of this genus. 
After describing the process, he adds, * It is the same with the grain and awn of 
Stipa.” I may mention that the latter note only appeared after I had completed my 
researches on this subject. 
The species оп which my observations have been made are Stipa pennata, Avena 
elatior, Heteropogon contortus (Bombay), Heteropogon (Andropogon) melanocarpus, 
(Florida), Androscepia arundinacea, Anthesteria ciliata $ (Khasia), Anemone montana 
(from Switzerland). 4 
It seems to me an extremely remarkable fact that the curious power of burying them- 
selves should be exhibited by plants belonging to several distinct orders, and inhabiting 
various parts of the world; and that the mechanism should be essentially the same in 
all of them is even more remarkable. 
My observations have been chiefly confined to Stipa; a short description of the 
caryopsis of this plant will explain the general method of action common to all the 
other fruits observed by me. 
Fig. 1 represents the seed || of Stipa pennata. It terminates inferiorly in a sharp strong 
oblique point (р) €, armed with a dense plume of barb-like hairs; v, Ё. fis the strong 
woody awn, of which the lower vertical part (v) is strongly twisted on its own axis, the 
appearance of the strands of a rope being given by the ribs r and ғ, fig. 9 (and seen in 
section in figs. 11 & 12), which run spirally up the awn. Above, the awn terminates in a 
long untwisted portion (f), bearing a series of beautiful hairs, giving its well-known 
feathery aspect: when the seed is dry this portion extends nearly horizontally out wards. 
The twisted and vertical part (v) is separated from the feathery part (f) by а curious 
double bend; the two angles thus formed I call the lower and the upper knee (4, and k»). 
* Sitzungsberichte d. Niederrhein. Gesell. Bonn, 1868. See Sachs, * Botany, "Eng. Trans. p. 841, for abstract. 
+ Annales de la Soc. Bot. de Lyon, 1873. 
t Silliman's Journal, Feb. 1876, p. 158. : 
$ Dr. Hooker most kindly supplied me with specimens of the four last-mentioned species. | 
| The use of this expression in place of the strict botanical term шау perhaps be allowed me, on account of its 
Breater convenience. т : 
*| Said to be eapable of injuring the intestines of animals which feed on the grass (Hooker's Trans. of Decaisne 
and Le Maout, p. 892). 
