THE LEAVES 



47 



The foliage leaves are of the ordinary linear gramineous type, possess- 

 ing sheath, blade, and ligule, with a pair of auricles at the base of each 

 blade. 



The leaf-sheath, which is split in the upper parts but entire near the 

 base, encircles the culm or straw, protecting the latter from damage by 

 frost, drought, and insect attacks ; it possesses considerable strength and 

 serves as a support for the young growing internode within it, and especially 

 for the basal intercalary growth-zone, which remains soft and weak after 

 the rest of the internode has become rigid. 



The leaf-blade develops much more rapidly than the sheath, and in 

 young plants reaches a length of two or three inches, while the sheath is 

 not more than a few millimetres long. The 

 sheath grows very little before the blade 

 has attained its maximum size, but grows 

 rapidly when the stem internodes begin to 

 lengthen. 



The greater portion of the sheath is 

 somewhat thicker and more transparent 

 than the blade, with thin transparent 

 margins. For a distance of 3-5 mm. 

 above the point of insertion on the stem 

 it is considerably thickened ; this short 

 swollen part is about i mm. thick, and 

 is often erroneously assumed to belong to 

 the node of the culm (Fig. 39). 



In many varieties the surface of the 

 sheath is glabrous, while in others it is 

 clothed more or less uniformly with short 

 deflexed hairs. Sometimes there is a line 

 of cilia near the outer margin of the sheath, 

 and in T. aegilopoides, T. monococcum, and 

 T. dicoccoides the hairs on the swollen base are longer than those of other 

 wheats, forming a conspicuous white muff-like band round it. 



The leaf-blade is linear- and parallel-veined ; its midrib projects on the 

 back and is continued some way along the sheath as a raised line. It is 

 twisted into the form of a loose right-handed screw, the twist being 

 increased by holding the tip of a young leaf and turning clockwise (Fig. 

 40). The direction of the screw is normally the same for all leaf-blades, 

 although exceptions are occasionally found, and it is independent of the 

 alternately reversed convolute rolling of the young leaves previously 

 noticed. 



The halves right and left of the midrib are of different widths, the 

 wider and narrower portions alternating regularly in successive leaves. 



FIG. 39. i, Node; 2, longitudinal 

 section through the node ( x 2). 

 b, Diaphragm ; a, thickened leaf- 

 base. 



