Interesting biological adaptations are met with in the distribution of grass “seeds." The very small and light fruits of Agrostis easily fall and are scattered by the wind, but in many cases the glumes (Holcus) or paleæ (Briza) are expanded and serve as “wings" offering extensive surfaces to the wind. In Arundo, Calamagrostis, Aira, &c., fine silky hairs attached to the rachilla serve a similar function, reminding us of the coma of true seeds and the pappus of Composites. In Hordeum jubatum of the prairies, the axis breaks up and the disarticulated portions with their attached tufts of fruits are blown away by the wind, and something similar occurs in our own H. murinum to a less extent. In the exotic Spinifex whole heads of fruits are thus detached and blown over the sands as “tumble weeds.”
In Stipa pennata we have an example of perhaps the most complex of all such adaptations: the exceedingly long awn terminating the palea is plumose at the upper end and twisted below, and the hard sharp rachilla at the base of the fruit is furnished with short, stiff hairs directed upwards. The plumed awn serves as a wind surface, the whole fruit flying like an arrow through the air. The stiff hairs below serve to fix the lower end between particles of soil, and by their alternate drying and wetting, the warping of these and of the twisting and untwisting awn drives the sharp base into the soil. (Fig. [42].) Similar mechanisms exist in Avena and others.
These bristles and awns also subserve dissemination in other ways, especially by clinging to the wool and fur of sheep and other animals, and cases occur where the twisting awns and reflexed hairs on the hard pointed fruit-base drive the latter into the bodies of sheep with fatal effects—e.g. Stipa capillata in Russia, S. spartea in America, Aristida hygrometrica in New Zealand, Heteropogon contortus in New Caledonia.
Fig. 42. Awned fruit of Stipa. The reflexed stiff hairs and hard point favour penetration into the soil. The long twisted awn performs hygroscopic movements, and its terminal plume offers surface to the wind. Lubbock.
America, Aristida hygrometrica in New Zealand, Heteropogon contortus in New Caledonia.
When we come to examine the external features of the “seeds" of grasses—usually the caryopsis enclosed in one or more paleæ, but sometimes in glumes as well—the following diagnostic characters are of importance.