511 THE STRUCTURE OF ECONOMIC PLANTS 



In the region of the midrib and the principal lateral veins, the 

 palisade is replaced by mechanical tissue above the vein which 

 forms a protecting ridge on the adaxial surface; and a zone of 

 collenchyma develops below the vein. The midvein is bicollateral 

 at the base of the leaf, but there is a reduction in the amount of 

 adaxial (inner) phloem toward its tip until finally the collateral 

 arrangement is attained. The lateral veins are collateral; but, 

 like the midvein, there is a reduction in the amount of phloem, in 

 this case the abaxial (outer) phloem, and the ultimate veinlets 

 terminate as single spiral tracheids. 



At the base of the leaf, there are five principal veins which extend 

 for some distance down the petiole without anastomosing. (Fig. 

 2.6^, B.) The three median ones are bicollateral, while the two 

 laterals are half-amphicribral or collateral. At the base of the 

 petiole, the two lateral bundles anastomose with the right and left 

 median bundles, so that at this point there are two large lateral 

 bundles and a smaller median one. The leaf traces of the first, 

 second, and third foliage leaves constitute the primary vascular 

 tissue of the first internode, which consists of eight or nine endarch, 

 bicollateral bundles. Near the cotyledonary plate, these converge 

 to form two lateral zones of the stele which become confluent with 

 the lateral bundles of the cotyledonary traces in the intercotyle- 

 donary plane. 



LITERATURE CITED 



I. Artschwager, E., "On the anatomy of the sweet potato root, with notes 

 on the internal breakdown." Jour. Agr. Res. zj: 157-166, 1914. 



1. Groth, B. H. A., "The sweet potato." Contr. Bot. Lab. Univ. Venn. 4: 1-104, 

 191 1. 



3. Hand, T. E., and Cockerham, K. L., The Sweet Potato, The Macmillan Co., 



1911. 



4. Herail, J., "Recherches sur I'anatomie comparee de la tige des Dicotyle- 



dones." Ann. Set. Nat., Bot. Ser. VII, 2: 103-314, 1885. 



5. IsBELL, C. L., "Regeneration in leaf cuttings of Ipomoea batatus." Bot. Gaz- 



91: 411-415, 1 93 1. 



6. Kamerling, Z., "Sind die Knollen von Batatas edulis Choisy Wurzeln oder 



Stengel?" Ber. Deut. Bot. Gesell. jz: 351-360, 1914. 



7. Lamounette, B., "Recherches sur I'origine morphologique du liber interne." 



Ann. Set. Nat., Bot. Ser. VII, 11: 193-181, 1890. 



8. Lee, E., "Observations on the seedling anatomy of certain Sympetalae. 



I. Tubiflorae." Ann. Bot. z6: 717-746, 1911. 



9. Lubbock, J., Seedlings, Vol. II: 167-190, D. Appleton and Co., 1891. 



