4 THE TRUE GRASSES. 



soon as the culm is placed in a horizontal or oblique 

 position the cells of the node on the side turned towards 

 the earth begin to elongate, and consequently the lower 

 side becomes longer, while the upper side is shortened 

 and often wrinkled from the pressure of the opposite 

 side ; this continues until the upper internodes have re- 

 sumed the vertical position. Several nodes, however, 

 may share in this process at the same time. 



Minute Structure. — In the tissues of the culm a 

 larger or smaller ring of sclerenchyma is especially prom- 

 inent ; it lies close under the epidermis, to which it is often 

 joined by rib-like sclerenchyma bundles. Similar bundles 

 accompany the fibro-vascular bundles both without and 

 within, or even form a ring about them ; the great firm- 

 ness of the culms is due to the mass of strongly developed 

 sclerenchyma. In pithless culms the fibro-vascular 

 bundles are usually arranged in two circles, one of 

 which is on the inside and the other on the outside of 

 the sclerenchyma ring ; in culms containing pith other 

 bundles are present, either scattered through the pith or 

 arranged in indistinct circles. Each bundle contains 

 two or three large, pitted vessels between which and 

 towards the centre is an air-passage resulting from the 

 rupture of an old, ringed vessel, as is shown by the pres- 

 ence, here and there, of isolated rings. For other por- 

 tions of the bundle, see explanation of Fig. 2. 



The fibro-vascular bundles run parallel in the inter- 

 nodes ; the superficial ones join those of the lower inter- 

 nodes directly, the others take the form of a shallow 

 arch bending towards the centre (in culms with pith), 

 where they pass through several internodes, and finally 

 bend outwards to join the superficial ones. In the nodes 

 the bundles cross and interlace by means of small and 

 short cross-bundles, which pass from the axillary shoots 

 or buds towards the centre. In this wa} r arise the dia- 

 phragms or plates of tissue which separate the pith 

 cavities of the internodes. 



The Leaf. — The leaves of grasses usually alternate in 

 two ranks 180° apart, and those near the ground often 

 form tufts or even fan-shaped bunches through the non- 



