Grass 



the earth affects these basal parts. The side next the 

 ground grows rapidly, whilst the upper part shrinks and 

 contracts. The result is to twist up the whole grass 

 stem, and it becomes, if not quite upright, yet lifted up 

 again, so as to be almost in its natural position. 



When fully ripe, the grains of grasses are scattered 

 by some modification of the protecting bracts. 



The steppe grasses often possess a long slender awn 

 which is very hygroscopic. It twists up in dry weather 

 and untwists again when the atmosphere becomes moist. 

 The tip of the spikelet is hard and sharp, and there are 

 also short stiff hairs on its base. When such a Stipa 

 fruit is lying on the ground, its long awn gets entangled 

 in the grasses near it. Every twist or untwisting forces 

 the point into the ground, and as the stiff hairs prevent 

 its being pulled out, it is driven right below the surface. 



If it should have fallen on the back of a. sheep, the result 

 is often that the seed is driven right through the animal's 

 skin, and it may cause a serious wound or even death. 



It is a fact that sheep are killed in this way, indeed 

 no less than four kinds of grass are guilty of sheep- 

 killing. Stipa capillata commits these murders in 

 Russia, and S. spartea also in North America ; Aristida 

 hygrometrica in Queensland, and Heteropogon contortus 

 in New Caledonia have also been convicted of this crime. 



But we must turn to those grass-problems which are 

 of real importance to mankind. Natural grass occurs 

 under the most different conditions as regards rainfall 

 and temperature, and then again there is the very 

 difficult question, what is natural grass-land ? There 

 are, of course, many sorts of grass-land. The Phrag- 

 mites thickets grow actually in water ; bank-foot colo- 

 nisers like Phalaris arundinacea or Glyceria aquatica 

 are also more or less under water. 



Such forms as Aira casspitosa grow just outside the 



266 



