IOWA ACADEMY OF SCIENCES. 205 



The sheath in cross section shows (Plate IX), beginning at the 

 inner or upper surface, the epidermis of Jarge, thin- walled 

 cells, immediately inside of which is stereome in patches, which 

 are located opposite the large bundles on the outer side. Then 

 comes the inner area of the sheath, made up of large, polygonal, 

 colorless, thin- walled parenchyma cells. 



The outer or lower surface of the sheath presents an entirely 

 different aspect and varies greatly with the variety of corn. 

 Generally speaking, it is more or less ribbed, caused by the 

 large fibrovascular bundles. The creases have colorless uni- 

 cellular hairs which are usually not developed on the epider- 

 mis over the bundles. The epidermal cells are small and 

 thick-walled, serving together with the hairs to protect the 

 plant against drouth and other injuries. Beneath the outer 

 epidermis occur the bundles referred to above, usually with 

 intervening smaller ones, but this varies with different corns. 

 For example, a Mexican corn (No. 1) shows two sizes of bundles 

 not connected with each other, forming no external ridges, and 

 the epidermis shows only a few very short spur-like hairs, 

 while a form from South America shows heavy ridges, many 

 hairs, and only one kind of regular sized bundles. 



In all cases there exists an area of stereome between the 

 bundles and the outer epidermis. 



The only chlorophyll in the sheath is located in the chloro- 

 phyll parenchyma sheath which surrounds the bundles, except 

 a portion on the outer side which is occupied by stereome. 



The leaf -blade is made up of the midrib or keel and the blade 

 proper. On each side occur the veins with the fibrovascular 

 bundles. A cross section of the leaf shows the keel (Plate 

 X), on the upper or inner surface the epidermal cells are small, 

 rather thin- walled, and immediately underlying which are sev- 

 eral layers of stereome or sclerotic cells, which gradually 

 increase in number, and toward the margin of the keel only 

 occur in patches over the large bundles on the lower or outer 

 side, and as the keel merges into the blade proper the stere- 

 ome areas unite with the sheath of the large bundles. On the 

 lower or outer surface the epidermal cells are thick-walled. 



Three different kinds of bundles occur within the keel, 

 regularly arranged as follows: The large, perfect bundles con- 

 nected with the lower epidermis by a broad area of stereome, 

 and a line of chlorophyll parenchyma down each side, but not 



15 [la. Acad. Scl., Vol. v.l LJuly 19, 1898 ] 



