266 HOUSE, GARDEN, AND FIELD 



that in a day or two the lines become a good deal wider 

 apart on the side from which the haulm is bending. The 

 organ of movement changes its form, swelling on one side, 

 and either not swelling at all, or swelling to a less extent, on 

 the other side. This power of swelling unequally accord- 

 ing to circumstances is due to absorption of water. The 

 knot, or some structure in communication with it, evidently 

 possesses sensibility ; it can feel, so to speak, when it is 

 displaced, and absorb so much water as to bring the haulm 

 back to the upright position. 



Why do the bases of grass-leaves ensheathe the haulm ? 

 While the grass-haulm is still growing, the outer leaf- 

 sheaths protect the inner ones, and the inner ones protect 

 the haulm. As the haulm attains its full height, the inner 

 parts are gradually withdrawn from the outer ones like the 

 joints of a telescope, and the sheaths become free from 

 one another. A young and soft shoot is stiffened, being 

 made up of a nearly solid mass of sheaths, one within 

 another, but an older and firmer shoot is hollow, light and 

 springy, and needs no support from the leaf -sheaths. 

 No better plan could be devised for the rapid lengthening 

 of the flowering stalks. Something too is gained by 

 carrying higher the base of the free leaf -blade, for to 

 overtop its rivals is a leading feature in the policy of most 

 grasses. 



Why are the sheaths of grass-leaves split along one side ? 

 To permit of expansion without tearing as the parts 

 within enlarge. The haulm within the sheath rapidly 

 expands in diameter as it becomes older. Sometimes a 

 growing ear or mass of flowers is lodged within a leaf- 

 sheath, and needs room for its expansion. 



Why are most grass-leaves ridged on the upper surface ? 

 The ridges when cut across are seen to be more or less 

 triangular, and fit neatly together when the leaf is rolled 

 up. Make a model of a grass-leaf by glueing triangular 

 bars of wood to a strip of canvas, and see how neatly such 

 a model can be rolled up or expanded, as circumstances 



