Ra?>ialcv : HVl'OCOTYI. AND El'ICOTVI. IN WOODY PLANTS. Ill 



In its primary structure the stele of the hypocotyl differs con- 

 siderably from that of the epicotyl. There are four xylem 

 bundles instead of eight or more and eight phloem bundles in- 

 stead of a closed ring of phloem. 



The cork, although of cortical origin in both regions, arises 

 in the hypocotyl in deeper layers. 



RUTACEiE. 



Ptelea trifoliata. 



St7'ncturc of Hypocotyl. 



The cells of the epidermis, when seen in cross section, appear 

 nearly square. They, at length, are flattened. There are a 

 few short, blunt, unicellular hairs. The cortex is large-celled. 

 There is no distinct hypoderma differentiated. 



The endodermis is large-celled and contains, at first, very 

 little starch. It later becomes closely packed with starch. The 

 various parenchymatous tissues at length also contain starch 

 in the cell cavities. Numerous lysigenous reservoirs are pres- 

 ent in the outer part of the primar}^ cortex. 



The stele is originally four-angled. There is one phloem 

 bundle and one xylem bundle in each angle. The phloem soon 

 forms a closed ring surrounding the now greatly enlarged xylem 

 bundles which enclose, at this stage, a somewhat cruciform pith. 

 The xylem bundles also finally fuse. 



Four very small groups of sclerench^^ma appear, toward the 

 end of the season, in the pericycle. They are equidistant. 

 There are about six cells in each group. Some sections do not 

 show all these groups, as the sclerenchymatous elements do not 

 form continuous strands in the hypocotyl. Some sections show 

 no sclerenchyma at all. 



The pith is eventually quite small. The perimedullary zone 

 is not well developed. 



The formation of cork begins earlv in the outermost cortical 

 layer of cells. 



Sh- net lire of Epicotyl. 



The epidermis is composed of cells which appear slightly rec- 

 tangular in cross section. They are elongated in the tangen- 

 tial direction. Numerous epidermal hairs are present. There 

 is a narrow collenchymatous hypoderma. The cells of the in- 

 ner cortex are very large. 



