96 



MINNESOTA BOTANICAL STUDIES. 



The stele is large. There is a circle of twelve to eighteen 

 conjoint vascular bundles. These soon fuse to form a narrow 

 zone each of xylem and phloem. 



There is a considerable amount of sclerenchyma at the outer 

 edge of the phloem. The cells are, however, mostl}^ isolated or 

 else occur in small groups. 



The pith, which is extensive, is composed of large, parenchy- 

 matous elements with thin, slightly lignified walls. According 

 to Flot [1893], there is a perimedullary zone of five or six lay- 

 ers of crushed, thin-walled cells. The same author states that 

 laticiferous tubes are found in the perimedullary region of 

 young twigs of this species. 



The cork cambium is formed in the outermost hypodermal 

 layer (cf. Moeller [1882], p. 82). 



Comparison of Structure of Hypocotyl and Epicotyl. 



Both hypocotyl and epicotyl have simple epidermal hairs, but 

 the former does not have the pointed or the bulbous hairs found 

 in the latter region. The hypocotyl is also without the some- 

 what collenchymatous hypoderma 

 found in the epicotyl ; its endodermis 

 persists for a greater length of time. 

 The structure of the stele in the 

 hypocotyl is anomalous. A single 

 flat bundle of xylem is flanked by 

 phloem, which eventually surrounds 

 the centrally-lying xylem, the inner 

 cells of which contain starch. There 

 is no pith. The epicotyl, on the other 

 hand, has a large pith, and the vas- 

 cular bundles are originally numer- 

 ous. Starch is absent from the xylem. 

 Cork formation is endodermal or 

 peri cyclic in the hypocotyl, but hypo- 

 FiG. 4. dermal in the epicotyl. 



Broussonetia 

 papyrifera 



MAGNOLIACE^. 



Liriodendron tulipifera. 



Structure of Hypocotyl. 



The epidermis consists of cells which are nearly square in 

 cross section ; at first they are very much bulged. They never 



