Rat)ialcy : iiypocotyl and epicotyl in woody plants. 117 



of sclerenchyma appear in the pericycle, one opposite each of 

 the original xylem groups. 



The pith is composed of large cells, whose thin walls be- 

 come, at length, somewhat lignified. A perimedullary zone of 

 about two layers may be distinguished but is not always con- 

 tinuous the whole way around the pith. 



The cork has its origin in the inner cortex or in the endoder- 

 mis. Some of the layers of cork carry a brown pigment. 



Structure of Eficotyl. 



The epidermis, composed originally of small cells, square or 

 pentagonal in cross section, eventually becomes strongly cuticu- 

 larized and the separate elements very much flattened. 



There is no hypoderma developed. The cortex is rather 

 large-celled, but very narrow, being only three or four layers of 

 cells in thickness. During the second year the walls of these 

 cells become conspicuously pitted. Many large crystals, chiefly 

 cubical in form, are found in this region. 



The endodermis, composed of flat cells containing starch is 

 distinct till near the close of the first season. Except in the en- 

 dodermis starch is absent until about the end of the first year's 

 growth, when it appears in the pith and medullary rays. 



Even in very young stages the phloem forms a closed zone 

 surrounding a ring of from six to ten, but generally about eight, 

 xylem bundles. These soon become fused. A narrow band 

 of sclerenchyma, for the most part only one cell wide, is found 

 at the outer limit of the phloem ; it does not form a closed ring, 

 but is more or less irregular and broken. Small patches of 

 sclerenchyma are found in the phloem of two-year-old seed- 

 lings. 



The pith is large-celled ; the walls are thin but slightly ligni- 

 fied. No perimedullary zone was distinguished. 



The region of cork formation was not distinguished. Two- 

 year-old material was examined, but the cork cambium had not 

 begun to form. 



Comparison of the Structure of Iiypocotyl and Epicotyl. 



The epidermis of the hypocotj'l remains thinner-walled and 

 exhibits less cuticularization than that of the epicotyl. This is 

 to be expected, since in the former region cork is produced the 

 first year, while in the latter not till a later period. 



