104 A TEXT-BOOK OF GRASSES 



130. Leaf-base and blade. — As regards development, 

 the leaves of flowering plants consist of two portions — ■ 

 the leaf-base and the blade. The leaf-base is prominent 

 in the early development of the organ, but generally 

 includes only a small portion of the mature leaf. In the 

 leaves of common deciduous forest trees, the swollen 

 portion at the base of the petiole represents the leaf-base. 

 In reduced leaves, the leaf-base often includes a relatively 

 greater portion or may even be the only portion developed. 

 In grasses, the sheath represents the leaf-base. In scales 

 and bracts, the blades have failed to develop or consist 

 of a mere tip or point. 



131. The prophyllum. — At the point where a branch 

 shoot originates from a main shoot, there is produced on 

 the side next to the parent shoot a 2-keeled organ called 

 the prophyllum. Through pressure the back of the 

 prophyllum between the keels is concave against the 

 parent shoot, while the two lateral portions, outside the 

 keels, bend forward clasping the new shoot. At first the 

 prophyllum completely covers the young shoot or bud, 

 but later is spread or opened as the shoot develops. 



The various subdivisions of the leaf will now be dis- 

 cussed more in detail. 



132. The sheath is the lower portion of the leaf that 

 encircles the culm above the node from which it arises. 

 From the developmental standpoint it represents the 

 leaf-base. The sheath is usually open from the base on the 

 side opposite the blade, the right and left margins of suc- 

 cessive sheaths overlapping alternately. 



The sheath usually fits close to the stem but may be 

 loose or inflated, especially the uppermost. The old sheaths 

 at the base of the shoots may persist in perennial grasses 

 and assume a characteristic appearance. 



