46 



HOW SEEDS BURY THEMSELVES, 



In countries subject to drought, where the soil is sandy and 

 light, the awns ot Siipa, Danthonia, Avena, Heteropogo7i, Anthis- 

 tiria, Air a, and some others, assist the seeds in thrusting them- 

 selves beneath the dry surface to a place of moisture, where they 

 may germinate. Some of these literally bore their way into the 

 soil. 



When dry, the lower part of the awns of these grasses twists 

 about, the upper portion bending off at an angle ; when wet, the 

 awn untwists and finally becomes straight. The lower part of 

 the chaff which envelopes some grass seeds is furnished with 

 sharj) or oblique beaks, provided with stiff hairs, which act as 

 beards. By dropping such seeds on the surface, and alternately 

 drying and wetting them, an experimenter will find that they 

 penetrate the sand, even in some instances extending down six 

 inches below the surface. It seems to make no difference whether 

 the grains are dropped among sticks or stubble, or on smooth sand, 

 they alike penetrate the soil. Even in clay soil the seeds work 

 themselves into the cracks where the sun has dried it ; on the re- 

 turn of rains the cracks close, or soil covers the seeds. 



If the stubble, straw or any other objects prevent the awn 

 from turning around, the seed will revolve on its axis. Besides, 

 if the awn is wet and held down by any object, as it tries to 

 straighten itself it will help push the seed, like a brad-awl, into the 

 ground. On wetting the grain and awn of Stipa pennata, Fran- 

 cis Darwin (Transactions of the Linnean Society, p. 149, 1876), 

 found that the rate increases up to the fifth revolution, and then 

 diminished quickly. This is shown in the table : 



