30 MINNESOTA BOTANICAL STUDIES. 



twist into the root position (e, f). The exception is the strand 

 which corresponds to the middle of the first epicotylary leaf. This 

 stands opposite to the sinus between the cotyledons on one side of 

 the seedling. This bundle extends considerably farther down the 

 hypocotyl than any of the other epicotylary strands (f, g.). It is 

 also exceptional in the early differentiation of its primary tissues. 

 Protoxylem develops early in the lower part of its course, and it 

 forms for some distance a small but conspicuous bundle of the or- 

 dinary stem type standing between the two lateral xylem strands on 

 one side of the hypocotyl. Like all the other epicotylary bundles it 

 ends blindly at the bottom. 



Secondary Thickening of the Hypocotyl. 



There is no secondary thickening in the cotyledonary bundles 

 down to the level of the insertion of the epicotylary structures 

 (Text Fig. 4, a, d, g). In the epicotyl, on the other hand, the 

 zone of meristem serves as a cambium which develops a ring of 

 bundles, the elements of which are almost entirely secondary. 

 These epicotylary bundles pass down between the cotyledonary 

 bundles in such a way that one stands on each side of the hypo- 

 cotyl between the two lateral xylem strands and two or three in each 

 cf the spaces between a lateral xylem strand and the adjacent 

 median bundle (g). 



Continuous with this epicotylary cambium, a cambium layer de- 

 velops in the lower part of the hypocotyl between the separating 

 groups of primary xylem and phloem ( h, i), and thence downward 

 into the root where it occupies the ordinary position. This cam- 

 bium in the lower hypocotyl and root produces continuous second- 

 ary xylem so that the epicotylary bundles as they pass downward 

 widen out, fuse with one another and with the primary xylem of 

 the hypocotyl into a. zone, at first interrupted over the primary 

 xylem of the median bundles, a little lower closing over these re- 

 gions, and finally as the root is reached, forming a solid central 

 cylinder. 



It will thus be seen that the connection between the vascular tis- 

 sue of the epicotyl and that of the root is accomplished almost en- 

 tirely by the secondary thickening of the hypocotyl, and that in this 

 region a solid zone of secondary xylem is formed, points the more 

 notable in view of the facts that secondary thickening is not strong- 



