HOW SEEDS BURY THEMSELVES. 



47 



^^* In three wettings and 

 three dryings, a little 

 over an inch was buried 



in dry sand. A rise of temperature affects the 

 awns in the same way as increased moisture ; a fall 

 of temperature acts like dryness. Mr. Darwin found 

 that minute strips of the awn, consisting even of two 

 long cells, twisted just as well as the entire awn. He 

 thinks the tortion is produced by the striation or 

 stratification of the cell walls. These are series of 

 parallel lines, alternately light and dark, traversing 

 the surface of the cell. Very frequently the two 

 systems wind spirally round the axis in opposite di- 

 rections. When the tissue expands during the absorp- 

 tion of water, it is due mainly to the swelling of the 

 less dense striae. This is thought to be the 

 cause of tortion in cotton wool. Soon after 

 being buried, where the soil is moist, the awn 

 breaks off at a joint from the apex of the 

 grain. The seeds of some of these species, 

 such as those of Stipa spartea, are very annoy- 

 ing to sheep and other animals, such as are 

 covered with thick hair. They sometimes 

 even cause death. [Dr. M. Stalker in Am. 

 Nat., p. 929, 1884]. Where plenty, they pen- 

 etrate clothing about the ankles of people, 

 and produce considerable trouble. Those, 

 like sweet vernal, which are provided with 

 feeble awns, work their way under leaves, 

 sticks, rubbish, and find every little hole and 

 crack in the dried earth, when the first rain covers them with soil. 

 The fertile flowers of Amphicarpum are not those on top of the 

 culms, but those out of sight and among the roots under ground. 

 Moles, ants, and other small animals move earth and cover seeds. 



Fig. 60. O vary 



and awn of stitxt 

 avenacea 1 x l.-(Sud- 

 worth). 



Fig. 61. Two long 

 cells of an awn iso- 



lated and twisting 

 ch 



rged 

 Lin. Soc). 



when dry. Muc 

 enlar g e d. (Tr; 



