300 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1934 



settle on the silk of the same plants that furnish the pollen. How 

 little naturally falls in this way is shown by the comparative sterility 

 of single isolated stalks. The cob of such plants usually bears only 

 a few seeds or grains. 



POLLINATION OF BLUE GRAMA 



Grasses with perfect flowers may show adaptations that aid in 

 cross-pollination. A common grass of the Great Plains known to 

 stockmen as blue grama {Bouteloua gracilu) illustrates one of these 

 adaptations. The inflorescence consists of two 1-sided spikes at- 

 tached obliquely at the upper part of a slender stem about a foot 

 tall. The spikes or flags are about an inch long and an inch apart 

 on the stem. When the wind blows, the spikes are thrown to the 

 lee side. The spikelets are close together on the axis of the spike. 

 At the time of flowering (at anthesis, the botanist would say) the 

 anthers are exserted to leeward on slender filaments. The two 

 feathery stigmas are also exserted but on the windAvard side. This 

 arrangement practically compels cross-pollination. All the pollen is 

 carried away from the stigmas of the same plant, whereas the 

 stigmas must receive pollen from some other plant. 



SELF-POLLINATION 



It is probable that many grasses are self -pollinated if cross-polli- 

 nation fails. Wheat, for example, opens and exposes the anthers and 

 stigmas for only about 15 minutes in the morning. During this time 

 cross-pollination may occur. Some grasses appear to depend mainly 

 on self-pollination, in which case much less pollen is necessary than 

 in the case of cross-pollinated plants. But continued self-pollination 

 (inbreeding) may result in deterioration. 



Some kinds of grasses produce flowers that are normally self- 

 fertilized (cleistogamous flowers). One such kind, a rather rare 

 species {Am'phicarpu7n purshii) grows in sandy soil of the Coastal 

 Plain from New Jersey to Georgia. It bears a terminal panicle 

 with perfect flowers, but these appear to be normally unfruitful — 

 why, is not at present known. On slender subterranean branches 

 from the base of the plant are borne single large fruitful spikelets 

 that never open, being fertilized by the pollen of their own minute 

 anthers. There are a few other species, curiously enough all Ameri- 

 can, with similar subterranean cleistogamous spikelets. An example 

 of another kind of cleistogamy is furnished by poverty oatgrass 

 {Danthonia spicata), frequent in the eastern United States on 

 sterile soil. The terminal panicle bears a few large spikelets, each 

 with several florets. Hidden in the sheath^ at the base of the stem 



