64X THE STRUCTURE OF ECONOMIC PLANTS 



triad, as Compton (7) has termed it, in which the two metaxylem 

 arms are tangentially oriented with respect to the protoxylem. 

 (Fig. 336, G.) At about the level of cotyledonary divergence, the 

 lateral orientation of the metaxylem is more marked; and, instead 

 of a single group of phloem elements lying on the abaxial side of 

 each bundle, there are two strands that are laterally placed and 

 separated from each other by parenchymatous cells. (Fig. 336, 

 E, F.) 



Lower in the hypocotyl, the phloem is differentiated progres- 

 sively in a more lateral position, and may form a varying number 

 of distinct strands that are farther apart from one another than at 

 any point higher in the seedling axis. At this level, the bundles 

 occupy positions nearer the center of the hypocotyl, constituting 

 the primary stele of the axis. Below the cotyledonary node, the 

 opposed lateral bundles of the two cotyledons unite in pairs to 

 form two small bundles which lie at right angles to the larger 

 median bundles coming from the midribs of the cotyledons. 

 (Fig. 336, £.) These laterals end blindly somewhat lower in the 

 hypocotyl, but are later joined to the two large bundles by the 

 development of secondary vascular tissue. 



At about the level at which the lateral bundles terminate, the 

 stelar pattern undergoes further change. The metaxylem arms of 

 each bundle are differentiated in a lateral position so that they 

 form a straight line with their respective protoxylem points. 

 (Fig. 336, D.) The phloem groups lie closer to each other, and 

 each bundle may be divided into two more or less distinct strands. 

 Slightly below this point, the metaxylem is differentiated laterally 

 and centripetally so that two tangentially exarch bundles are 

 formed. (Fig. 336, C.) 



Near the middle of the hypocotyl, the relation of protoxylem 

 and metaxylem is strictly exarch. The diarch character of the 

 stele is evident although there are still parenchymatous cells at 

 the center of the axis separating the two halves of the xylem strand. 

 At this point, the phloem is differentiated laterally in relation 

 to the long axis of the xylem strand and forms two distinct groups. 

 (Fig. 336, B.) In the lower part of the hypocotyl, the stele is 

 similar to the primary root. (Fig. 336, A.') 



The Cotyledons. — The fleshy cotyledons are alike in structure 

 but there may be considerable variation in their size and shape. 

 They have wide bases and the most common form is spatulate 



