94 



MINNESOTA BOTANICAL STUDIES. 



An interrupted sclerenchymatous ring is formed at the outer 

 edge of the phloem ; the cells are thick-walled but do not be- 

 come lignified the first year. 



The pith is rather large. There is a small-celled perime- 

 dullary zone which is quite definite. 



Cork formation in the epicotyl takes place in the outermost 

 cortical layer (cf. Moeller [1882] ). 



Comparison of Structure of Hy^ocotyl and Epicotyl. 



In the hypocotyl the cortex and pericycle are much better 



developed than in the epicotyl. The 

 former region is without epidermal 

 hairs. This point of difference was 

 previously noted by Klebs [1885]. 



The stele of the hypocotyl has orig- 

 inally four vascular bundles, instead of 

 from eight to sixteen ; the pith is small 

 in extent. 



The sclerenchyma is first formed in 

 four patches but afterwards forms al- 

 most a complete ring. Cork formation 

 in the hypocotyl begins in a deeper 

 layer of the cortex. 



At the close of the year the two 

 regions have nearly the same structure, 

 about the only difference being the size 

 Fig. 3. of the pith. 



Toxylon 



poniiferum 



Broussonetia papyrifera. 



Structure of Hyfocotyl. 



There is an epidermis of small cells nearly square in outline 

 when seen in cross section. These cells become greatly elon- 

 gated tangentially as the tissues within increase in thickness. 

 Short, blunt, unicellular epidermal hairs are numerous. 



The cortex is composed of about six layers of large, thin- 

 walled parenchymatous elements which, like the epidermal 

 cells, become stretched toward the close of the season. 



The endodermis is small-celled. It sometimes remains dis- 

 tinct till nearly the close of the first season. Starch is present 

 in the endodermis, but absent from all other tissues for a long 



