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 awns 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 Brown & Scofield, “Wild Rice: Its 
Uses and Propagation,’’ U.S. Dept. Agric. Bur. Pl. Ind. Bulletin No. 50. 1903 
