116 



MINNESOTA BOTANICAL STUDIES. 



posed of large, thin-walled cells. There is a distinct perime-^ 

 dullar}^ zone. 



The region of cork formation was not determined. 



Co?iipariso7i of Structure of Hypocotyl and E^icotyl. 



In their primary structure the steles of the hypocotyl and epi- 

 cotyl show important differences. That of the former region is 

 quadrangular ; it has four primary vascular bundles and at a 



later time other secondary bundles 

 are intercalated. These latter do not 

 have resin canals. In the epicotyl 

 there are about eight vascular bundles 

 each with a resin canal. 



The pith of the hypocotyl is four- 

 sided, that of the epicotyl circular in 

 outline, when seen in cross section. 



RHAMNACEiE. 

 Berchemia racemosa. 



Structure of Hypocotyl. 



The cells of the epidermis are nearl}'- 

 square in cross section, sometimes 

 radially elongated, but becoming at . 

 length considerably flattened. No 

 hypoderma is developed, although the 

 cells of the outermost layer of cortical tissue are considerably 

 smaller than those below. There are about five layers of cells 

 in the cortex. This tissue is extremely loose, having many in- 

 tercellular spaces. 



The endodermis is quite distinct until nearly the time that 

 cork formation begins. The cells are smJiller than those of the 

 cortex but larger than the pericyclic elements. 



Starch is present in the endodermis from the first, but does 

 not appear in the cortex at all, nor in the pith and phloem till 

 about the close of the season. 



The stele is originally four- angled. There are four xylem 

 and four phloem bundles. These are paired. They soon fuse 

 so that there are two crescent-shaped bundles, and by further 

 growth closed rings of xylem and phloem are produced. 



While the bundles are in the crescent form four small groups 



Scliiiiiis iiiollc 

 Fig. 15. 



