ECOLOGY 137 



varieties of maize are unable to force the mesocotyl to a 

 length greater than 10 cm. (Collins, Journ. Agr. Res. 

 1:293. 1914). 



172. Impervious seed-conveyers. — ^At maturity all 

 seeds are moderately dry within, that is, for the preserva- 

 tion of the endosperm during the dormant stage the moist- 

 ure has been reduced to a minimum. To protect the 

 contents against further loss of moisture which would 

 injure or kill the embryo, the seed is enveloped by an 

 impervious coating, which serves the double purpose of 

 preventing the loss of moisture from within and the 

 absorption of moisture from without. The protecting 

 coating may be in immediate contact with the seed or it 

 may be developed from some outer coating or organ. If 

 an outer coating such as the glumes become hardened for 

 this purpose, then the inner organs, lemma and pericarp 

 are comparatively thin. 



The protective coating is developed from the seed-coat (Sporo- 

 bolus), pericarp (wheat), lemma and palea (Panicum), glumes 

 (Andropogon), rachis and glume (Tripsacum), sheathing bract 

 (Coix), involucre (Cenchrus), or various combinations of these. 

 In some cases, as in Cenchrus, several seeds are protected by the 

 same outer coating. 



173. Self -burial. — The dormant stage continues 

 through the season unfavorable for germination, that is, 

 winter or a dry season. When the season for germination 

 arrives, the s,eed, under the influence of moisture and 

 higher temperature, gradually absorbs water, growth is 

 started, the embryo swells and bursts through its sur- 

 roundings, and germination has begun. Ordinarily the 

 seeds are more or less covered with earth or debris by the 

 action of the wind. But some seeds are aided in self- 

 burial by the torsion of the awns they possess. The awns 



