164 DR. FRANCIS DARWIN ON THE MECHANISM BY WHICH 
elongated turn in the direction opposed to the hygroscopic torsion of the awn; and this 
elongated “ reverse twist" is seen іп the mature awn when untwisted by wetting. This 
reverse twist is the consequence of the approach of the ribs to each other as they pass 
from the lower part of the awn to the point of bending; and I think that, as in Stipa, 
this alteration in form is connected with the longitudinal partition of the awn into a 
more and a less contractile half. 
HETEROPOGON MELANOCARPUS. 
The general appearance of the awn of this species is shown in fig. 2. In an ordinary 
room in winter it has but one knee, as shown in the figure ; but I have since found that 
by drying it at a higher temperature a lower knee appears; in the wet state a “ reverse 
twist" сап be made out. The seed has a sharp oblique point, and а well-developed 
plume of hairs; the awn, like that of Stipa, Avena, and Anemone, is twisted into a right- 
handed screw. There are no feathery hairs on the non-twisted part of the awn; but it is 
covered throughout its entire length with minute reflexed barbs. I find it capable of 
burying itself on becoming wet; but whether it does so on becoming dry I am not sure. 
HETEROPOGON CONTORTUS 
has a doubly bent twisted awn, of which the vertical part is roughly 13 mm., the part 
between the knees 10 mm., and the non-twisted part 4 cm. in length. It twists in the 
same direction as Stipa &c. ; and, when wetted, I have found it able to thrust itself into 
sand. 
ANDROSCEPIA ARUNDINACEA. 
The strongly bent and twisted awn is shown in fig. 3. I do not possess the complete 
fruit of this species, and could not make any trial of its burying-powers. 
ANTHESTERIA CILIATA 
somewhat resembles Heteropogon melanocarpus, with a bent and twisted awn, and 4 
fine plume of hairs. 
Lagurus ovatus has a bent and twisted hygroscopic awn. And I find the following 
kinds of grasses mentioned as having bent and twisted awns— Aira, Arrhenatherum, 
Holcus*, Streblocheta nutans, Danthonia, Chetobromus Dregeanus, Macr ochloa arenariat. 
I think it is probable that these have the power of burying themselves. 
RANUNCULACES. 
ANEMONE MONTANA. 
Fig. 6 gives the general appearance and proportions of the achene of this plant. The 
slight hairiness of the awn probably aids in the distribution of the seed. It is 
exceedingly remarkable that the general features of the above-mentioned burying-seeds, 
all members of the order of Grasses, should be repeated in a Ranunculaceous plant. 
* ‘British Flora,’ Hooker. 
+ Max Wichura, translated in Taylor's Scientific Memoirs, May and Aug. 1853. 
