ii8 THE STRUCTURE OF ECONOMIC PLANTS 



in mature stems, since the internodes are very long and the outward 

 curvature of the bundles occurs in the internodes prior to inter- 

 nodal elongation by intercalary growth. 



The Leaf. — The leaf consists of a sheath, a collar-like ligule, 

 and a blade. All the main veins are parallel and there are cross- 

 connecting veinlets. The mesophyll is comprised of relatively 

 compact chlorenchyma with few intercellular spaces, and there is 

 no differentiation into palisade and spongy tissue. The collateral 

 vascular bundles are of two sizes, the larger type resembling the 

 stem bundle except in two respects. The mechanical tissue, instead 

 of completely surrounding the bundle, consists of a zone of thick- 

 walled elements, above and below it. This tissue abuts the upper 

 and lower epidermis and, with the mechanical elements of the mid- 

 rib, serves as the chief support of the leaf. Surrounding the bundle 

 is a sheath of chlorophyllose cells in which the chloroplasts are 

 definitely larger than those found in the mesophyll. The smaller 

 type of leaf bundle has no adjacent mechanical tissue and consists 

 of a few xylem and phloem elements surrounded by a sheath of 

 parenchymatous cells containing large chloroplasts. The trans- 

 verse bundles which cross-connect the small parallel veins with one 

 another or with a larger bundle consist of one or a few xylem 

 elements with or without accompanying phloem elements. (Fig. 



Randolph (14) investigated the cytology of the several types of 

 chloroplasts in corn and found them to be the same in their initial 

 structure. They originate as very minute proplastids and gradu- 

 ally enlarge, developing chlorophyll until they reach maturity. 

 He also observed that both partially developed and fully matured 

 plastids may divide, but was unable to determine the origin of the 

 proplastid because of its small size. The question as to whether 

 the plastids are permanent cell organs with an unbroken genetic 

 continuity or arise de novo is still undetermined. 



Both epidermal surfaces are cutinized, the lower being more so 

 than the upper; and both contain stomata arranged in parallel 

 longitudinal rows. Varying stomatal counts have been made on 

 maize, but all are in agreement that the number in the lower sur- 

 face exceeds that in the upper. Moore (ii) has made a study of the 

 histology of the upper epidermis and has recognized three zones 

 that run lengthwise of the leaf. These are the zone of bulliform or 

 motor cells, the zone of narrow epidermal cells, and the stomatal 



