172 



MONOCOTYLEDONS 



4. Leaf. — Each leaf consists of two quite different parts, the , 

 sheath and the blade. Where both join, tliere is a small mem- 

 braneous appendage, called the Ugiile. The 

 sheaths arise from the nodes and form tubes 

 protecting the stem. If a young Padd}* plant 

 is cut lengthwise, as in the illustration tig. 

 158, the sheaths will be seen to form a case, 

 in which the stem {St.)^ the younger leaves, 

 and also the Hower-buds {Fl.) are enclosed. 

 These parts are so extremely tender that even 

 a feeble breeze could break or the heat of the 

 sun scorch them. The sheaths of the older 

 leaves that rise above them thus form a 

 protection to these tender xmrts. Only when 

 they have attained sufficient strength, they 

 grow out of their protecting cover. Besides, 

 they afford the culm more support, which it 

 requires very much, as those parts of it 

 which lie immediately over the nodes continue 

 to grow for a longer period. This is a pecu- 

 liarity of tlie grass-culm. We know that the stems and stalks 

 of other plants grow only at their ends; the stems of the Grasses, 

 however, rjroiv aljove every node and thus lengthen out telescopically, 

 a fact whicli explains why Grass plants generally have such a 

 rapid growth. A bamboo stem, for instance, has been known to 

 grow three feet in twenty-four hours. 



The Jjlade is linear and waves like a flag in the wind. Conse- 

 quently the wind meets with no resistance from it and cannot 

 easily overturn the plant. The nerves or ribs of tlie leaf run 

 i)i }mrallel lines from the base to the tip of it, iu accordance 

 with its linear structure (tig. 159). 



The ligule between the sheath and the blade prevents the 

 rain-water running down the leaves from entering under the 

 sheath and thus rotting the tender parts of the i-ulm. 



If the leaves are drawn swiftly through the hand, they cut. 

 This is due to the presence of silica, a sul)stance, which consti- 

 tutes some of the hardest stones like (juart/ and Hint, and is 



Fig. 159.— The tip of 

 a Grass blade. 



