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. [Illustration: 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.] The driving action of even small reflexed asperities on awns is well illustrated by the fruits of _Hordeum_, which are often made by children to creep up the sleeve. 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. The size varies from lengths of 2 mm. or less (_Poa_, _Aira_) (Figs. 43, 44) to 15-20 mm. or more (_Arrhenatherum_) (Fig. 47) and distinction must be made between the various characters of the caryopsis devoid of its coverings (paleæ, glumes) and such as really belong to the latter.