354 THE STRUCTURE OF ECONOMIC PLANTS 



with the leaf gap resulting from the outward divergence of the polar 

 bundle leading to the trifid bract. 



The Second Internode. — The epidermal and cortical regions 

 in the second internode are like those of the first, except that, 

 occasionally, one of the cortical fiber bundles may be divided to 

 form two groups which later converge to form a single strand. In 

 the stele, slightly above the second node, only one polar bundle is 

 completely organized, and at the other pole there is a leaf gap of 

 parenchymatous tissue resulting from the divergence of the median 

 trace to the first trifid bract. (Fig. 179, 5.) At a higher level, 

 divergences from the two bands of exarch xylem result in the 

 separation of the central zone of primary xylem into two exarch 

 groups lying adjacent to the leaf gap; and two lateral groups of 

 metaxylem forming the central mass which are separated from each 

 other by a narrow band of parenchyma. Progressive reorientation 

 of the two exarch groups adjacent to the leaf gap results in the 

 formation of a single endarch bundle which becomes the median 

 trace of the third foliage leaf. Near the upper limit of the second 

 internode, which is a relatively short one in most varieties of 

 Pisum, the polar bundle supplying the second bract-like leaf is 

 diverged centrifugally through the cortex. 



The Third Internode. — The third internode is somewhat more 

 quadrangular in transection than the ones below it, but the cortical 

 organization is identical to that described for internodes one and 

 two. In the stele, each lateral group of phloem fibers may consist 

 of three, rather than two, strands and the laterally oriented bands 

 of xylem are more widely separated by the pith. (Fig. 178.) 

 After the divergence of the polar trace to the second trifid bract at 

 the third node, there is a reorientation of the exarch groups of 

 primary xylem to form a new polar bundle, as described for the 

 opposite pole in the second internode, resulting in the formation 

 of the median bundle for the fourth foliage leaf. The remaining 

 lateral bands of primary xylem may be regarded as tangentially 

 mesarch; that is, the protoxylem of each of the bands is centrally 

 located on its inner face and between flanking groups of metaxylem 

 elements. (Fig. 178 and Fig. 179, C) 



One of the quantitative tests for auxin devised by Went (30) 

 utilizes the third internode of etiolated pea seedlings. When the 

 seedlings are about seven days old, and 10 to ix cm. in length, the 

 top is cut off 5 mm. below the terminal bud, and the elongated 



