530 THE STRUCTURE OF ECONOMIC PLANTS 



The histology of the proto- and metaxylem has been described. 

 The secondary xylem is laid down in radial rows, and consists of 

 large vessels, tracheids and parenchyma. The former are heavily 

 pitted with the pits arranged in transverse series, and each vessel 

 segment is two or three times longer than broad. Parenchymatous 

 cells surround the vessels, and are also laid down in radial rows 

 as xylem rays. 



The mature sieve tubes are cylindrical, and a single sieve plate 

 comprises the entire transverse wall. Each plate is perforated 

 by a large number of circular pores, but these do not have sieve 

 fields such as occur in some phloem types. No plates occur on 

 the radial and tangential walls. The inner and outer phloem 

 elements are approximately equal in size, and the sieve tube seg- 

 ments do not vary much in length except that the segments are 

 short and relatively broad where anastomoses of bundles occur. 



In general, the sieve tube arises from an initial which under- 

 goes longitudinal division to form two cells of unequal size, a 

 large sieve tube and a smaller companion cell, but there may be 

 variation in this respect. In some cases, the sieve tube initial 

 does not undergo longitudinal division so that there may be a 

 seHes of sieve tubes without adjacent companion cells. In addition 

 to the sieve tubes and companion cells, phloem parenchyma is 

 differentiated. These cells are elongated and rectangular, with 

 simple pits in their end walls unlike the sieve plates of the sieve 

 tubes. 



The pericyclic fibers have pointed ends and are long and awl- 

 shaped with a small lumen and heavily thickened unpitted walls 

 that later become lignified. The endodermal cells are longer in 

 the tangential dimension than in the radial one and Casparian 

 strips are present but not well differentiated. 



Vascular Anatomy of the Stem. — Each leaf trace consists 

 of five bundles and at the base of the semicircular petiole there 

 are three large centrally located bundles and two smaller ones 

 which lie at its outer edges. The course of the bundles in the 

 stem is illustrated in the accompanying figure. (Fig. x8i.) In 

 this figure, A, B, and C indicate the main bundles, while a and b 

 represent two of the three smaller bundles. The smaller inter- 

 mediate bundles, a and b, become median traces of the leaves. 

 Branches of the three large bundles A, B, and C form the four 

 lateral bundles of the leaf traces. The traces to the wings of the 



