138 A TEXT-BOOK OF GRASSES 



of Stipa (Fig. 36) have already been described (Par. 156). 

 By the alternate drying and wetting they twist and un- 

 twist, bend and straighten. The fruits, being provided 

 with a sharp callus, covered with retrorse hairs, gradually 

 insinuate themselves into the porous covering of the soil 

 and finally into the soil itself. As the fruit is heavier at 

 the base, it tends to fall point down. Awns of this kind 

 are found upon the fruits of a number of genera, the burial 

 being brought about by the rotation of the twisted portion 

 or by the bending and straightening of a geniculate portion 

 or by a combination of these. Straight a^vTis or bristles 

 that are antrorsely scabrous undoubtedly act in the 

 same manner. 



Examples of tortion: Stipa, Aristida, Heretopogon, Chryso- 

 pogon, Sorghastrum, Arundinella, Avena, Danthonia. Examples 

 of antrorsely scabrous awns: Hordeum, Sitanion. 



174. Water grasses. — The seeds of water grasses fall 

 into the water and remain moist until germination. It 

 has been shown that the seeds of Zizania palustris are 

 injured by exposure for any considerable length of time* 

 to the drying influence of the air. 



If the caryopsis at the time of germination is normally' inclosed 

 within outer envelopes, as lemmas or glumes, the embryo must be 

 able to push its root and plumule through or around these parts. 

 The usual method is for the root to break through the obstruction 

 and for the plumule to push up between the parts. 



Some of the grains (wheat, rye, corn and kafir) are naked 



caryopsides and the growth of the embryo is unhampered. The grain 



or caryopsis of the oat is permanently invested by the lemma and 



palea. The root breaks through the back of the lemma near the 



base and the shoot pushes up between the grain and the lemma, 



emerging at the apex. The fruit of barley also consists of the grain 



inclosed in the lemma and palea and more or less adherent to the 



*For a full discussion of this subject, see BroTv-n & Scofield, "Wild Rice: Its 

 Uses and Propagation." U. S. Dept. Agric. Bur. PI. Ind. Bulletin No. 50. 1903 



