VIIl] DISSEMINATION 125 



fruits of Agrodis easily foil and are scattered by the wind, 

 but in many cases the glumes (Holcus) or paleae (Briza) 

 are expanded and serve as "wings" offering extensive 

 surfaces to the wind. In Arwido, Galamagrostis, Aira, 

 &c., fine silky haira attached to the rachilla serve a 

 similar function, reminding us of the coma of true seeds 

 and the imppus of Composites. In Hordeum juhatum 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 

 aw^n 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 w^ool 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. Stipci capillafa in Russia, S. spartea in 



