92 



MINNESOTA BOTANICAL STUDIES. 



are numerous simple curved and pointed hairs ; there are also 

 some with bulbous ends. 



The cortex is thin. A distinct collenchymatous hypoderma 

 is developed. It usually consists of three or four layers of 

 cells. 



The endodermis, which is originally distinct, soon becomes 

 unrecognizable. The cells are about the same size as those of 

 the cortex ; they contain starch. Eventually all the parenchy- 

 matous elements contain starch. 



The stele is originally somewhat elliptical in cross section. 

 The phloem, in the youngest material examined, forms a com- 

 plete ring. There are generally two large and four small 

 xylem bundles. These soon fuse to form a closed xylem 

 zone. 



An interrupted band of sclerenchyma is developed at the 

 outer limit of the xylem. 



As in Celtis australis (cf. Flot [1893], p. 68) there is a dis- 

 tinct perimedullary zone composed of two or three cell rows. 



Cork formation begins, rather late in the season, in the outer- 

 most hypodermal layer (cf. Moeller [1882], p. 74). 



Comparison of Structure of Hypocotyl and E-picotyL 



A striking difference between hypocotyl and epicotyl is the ab- 

 sence from the former region of the 

 numerous epidermal hairs so abun- 

 dant in the latter. The hypocotyl 

 is without a hypoderma. 



The primary stelar structure of 

 the hypocotyl is peculiar, the phloem 

 forming two crescentric masses and 

 not uniting into a closed ring till 

 after the xylem bundles have fused. 

 The epicotyl possesses a ring of 

 phloem and six xylem bundles. 



At the end of the season the hypo- 

 cotyl has two interrupted bands of 

 sclerenchyma instead of one, and a 

 smaller pith. Aside from these dif- 

 ferences the two regions are the 

 FIG. 2. same in structure. 



Celtis 



occidentalis 



