ZEA MAYS 12.3 



mately the soil level is accomplished by the activity of meristematic 

 cells immediately below the second node. (Fig. 49.) The pri- 

 mordium of the coleoptile is initiated as an open sheath; but, as 

 growth continues, the edges soon unite, forming a closed structure 

 although the line of union remains visible, especially near the top. 



--en 

 --pel 



~ ~P'^ I endarch 

 - -mx\ collateral 

 I bundle 



Fig. 48. Transection of the first internode of the axis showing transitional character 

 with endarch collateral bundles, and radial arrangement of primary phloem and exarch 

 xylem: co, cortex; en, endodermis; mx, metaxylem; pel, pericycle; pf), phloem; pi, pith; 

 px, protoxylem. (After Avery.) 



The structure of the coleoptile is simple, consisting of a relatively 

 compact chlorenchyma and two vascular bundles which are bilater- 

 ally placed. Each bundle is made up of several xylem strands 

 which partially surround the phloem so that it may be regarded as 

 half-amphi vasal. In exceptional cases, the number of vascular 

 bundles in the coleoptile may exceed two; and, in some instances, 

 four are formed by a division of the original bundles. Avery (i) 

 has reported "the presence of as many as five bundles. The extra 

 bundles have ordinarily an origin similar to that of the usual two, 

 although in the case of five, the fifth bundle usually arises de novo 



