388 THE STRUCTURE OF ECONOMIC PLANTS 



Coincident with the reorientation and branching of the primary 

 phloem, the metaxylem elements are differentiated in a more and 

 more tangential position in relation to their respective protoxylem 

 points. There are four metaxylem groups, each of which is 

 adjacent to a phloem strand and centrad to it. At about the mid- 

 point in the hypocotyl, the four metaxylem groups and their 

 collateral phloem strands are each bifurcated so that eight 

 vascular bundles are formed which constitute the cotyledonary 

 traces. At higher levels, two groups, each of four metaxylem 

 strands, are formed; and these become oriented in a more nearly 

 lateral position with respect to the protoxylem poles. As this 

 reorientation of the two groups of cotyledonary bundles proceeds, 

 they become widely separated, each group of four bundles con- 

 stituting the vascular supply to a cotyledon. (Fig. 197, C.) 



At the cotyledonary node, the two lateral bundles of metaxylem 

 and the accompanying phloem in each group of four, become more 

 widely separated from the two centrally located ones, and continue 

 without branching or anastomosing into the cotyledons forming 

 the lateral veins. At higher levels, the two centrally located 

 metaxylem groups are oriented in a more centrad position with 

 reference to the protoxylem; and, at the point of divergence of the 

 cotyledons, the primary xylem is endarch. The two metaxylem 

 groups anastomose, forming the median double bundle of each 

 cotyledon; but the two phloem groups of each double bundle 

 are distinct at this level, anastomosing x or 3 mm. farther up in the 

 cotyledon. (Fig. 197, D.) 



The vascular system of the epicotyl is differentiated somewhat 

 later than that of the cotyledons and hypocotyl. The bundles 

 which constitute the traces of the first foliage leaves above the 

 cotyledonary plate are differentiated as endarch, collateral bundles 

 and extend downward into the hypocotyl where they may anasto- 

 mose with groups of metaxylem and phloem; but, in some in- 

 stances, end blindly in the parenchymatous tissue. 



There is disintegration and a partial or complete resorption of the 

 primary xylem in the seedling axis, similar to that described for 

 Allium by Chauveaud (5) and Hoffman (12.)' ^^^ because of this 

 ephemeral character of the primary xylem. Crooks used plants 

 about six days old. In older axes, the protoxylem is first crushed 

 and stretched; later, in the upper and middle hypocotyl, the first- 

 formed metaxylem disintegrates; and, ultimately, nearly all of the 



