Butters: seeds and seedling of caulophyllum. 27 



lively great length of the hypocotyl render the seedling of Caulo- 

 phyUiim a very favorable object for the study of this interesting 

 region. 



From each cotyledon three distinct vascular bundles enter the 

 hypocotyl. These all have the structure and orientation of ordinary 

 stem bundles. The median bundle from each cotyledon is some- 

 what larger than the lateral ones (Text Fig. 3, a). Shortly after 

 entering the hypocotyl these larger bundles show a double structure 

 in the phloem region and the two phloem groups of each bundle 

 gradually diverge (Text Fig. 3, b, c, d), the as yet undifferen- 

 tiated central part of the bundle also divides, and finally the pro- 

 toxylem divides also. Below this there are for some distance eight 

 distinct cotyledonary bundles in the hypocotyl (e, f, g). 



The bundles gradually twist so that the phloem group of each 

 lateral bundle is approximated to the phloem group of the cor- 

 responding median bundle. A little lower, the phloem groups separ- 

 ate from the xylem (g) and at about the same level the xylem 

 groups originating from each of the median bundles approach each 

 other and reunite, forming first a band of xylem with protoxylem 

 in the middle and metaxylem at each end, and along the outside of 

 the band (h). A little lower the protoxylem concentrates into a sin- 

 gle small mass and the metaxylem passes into the centripetal posi- 

 tion which is characteristic of root steles (i). 



At the same time each pair of approximated phloem groups unite 

 into a single group, then the lateral xylem groups twist into the 

 usual root position, and considerably lower each pair of lateral xylem 

 groups unite to complete the regular tetrarch stele of the root (j). It 

 is to be noted that the vascular structures derived from the respective 

 cotyledons remain distinct throughout the entire transition area of 

 the hypocotyl, and unite only at the beginning of the root. 



Throughout the hypocotyl the vascular tissue gradually ap- 

 proaches the center, and by the beginning of the root the pith is re- 

 duced to a small group of cells, which a little lower disappears en- 

 tirely, and gives place to metaxylem (k). 



As may be expected from the function and behavior of the cotyle- 

 dons, their bundles, throughout, have a comparatively large amount 

 of phloem, and correspondingly little xylem. The metaxylem, 

 especially, consists even at a later stage of only a very few cells. 



