20 STRONG DRINK AND TOBACCO SMOKE. 



The structure of tlie mature leaf will have to be 

 considered presently. I draw attention now to the 

 leaf in its youngest state, in order to illustrate a pecu- 

 liarity in its structure that has a most important relation 

 to the mechanical support of the entire plant. Examine 

 one of the leaves of barley in as early a stage of its 

 growth as possible (Plate 2, fig. 11), and here will be 

 found, at the base of it, an elevated ridge of cellular 

 tissue. This never lengthens to any great extent during 

 the growth of the leaf, but it thickens considerably, and 

 becomes, in a mature leaf, very tough and elastic. It is 

 called the strap (figs. 11, 12 st), and performs the most 

 useful office of preserving all that portion of the leaf 

 which grows after it, for a time, in a cylindrical form. 

 It thus encloses and protects the inner younger leaves 

 (and, as we shall presently see, the flowers and fruit) 

 until these, becoming too bulky, snap the strap asunder. 

 The economy of Nature, vast as it is, can scarcely show 

 a single contrivance more simple, or more admirably 

 adapted for the purpose which it serves. 



Fig. 13 represents a portion of a transverse-vertical 

 section of a mature leaf of barley. In the centre of the 

 section is the bundle of woody tissue (fibrous cells, spiral 

 and other vessels) forming the middle vein of the leaf, 

 and on either side of it three other such bundles runninsf 

 parallel to it. They are surrounded by loose cellular 

 tissue, and enclosed or covered on both surfaces by 

 abular-shaped cells, forming the upper and under skins 

 of the leaf. 



