108 MINNESOTA BOTANICAL STUDIES. 



About the time that the epicotyl has reached its full length 

 four small areas of sclerenchyma appear in the pericycle, one 

 adjoining the phloem of each vascular bundle. These increase 

 somewhat in size, and are still visible in two-year-old material. 

 Numerous isolated sclerenchymatous elements are found scat- 

 tered through the phloem. 



The pith is composed of rather large cells with thin walls, 

 which soon become lignified. As the plant grows older the pith 

 becomes almost obliterated. No perimedullary zone was dis- 

 tinguished. 



Cork formation takes place in the outer pericycle, at length 

 cutting off all tissues outside, leaving the bundles of scleren- 

 chyma which are at the inner limit of the pericycle. 



Structure of Epicotyl. 



The epidermis is composed of cells nearly square in cross 

 section. These abut directly upon a large-celled, few-layered 

 cortex. There is no hypoderma. 



A definite endodermis was not distinguished at any time al- 

 though in a very young stage certain starch containing cells 

 were recognized as having the appearance of endodermis ; a 

 continuous ring of them was not traced. With the exception of 

 the endodermal and medullary region, starch does not occur 

 until the plant has developed a number of internodes above the 

 epicotyl. The cells of pericycle and phloem are at length filled 

 with starch. 



The stele is cylindrical from the first. Owing to fusions the 

 number of vascular bundles is variable. There are, however, 

 generally about five or six bundles. The phloem and xylem 

 eventually form closed rings. 



There is a narrow interrupted ring of stereom at the outer 

 edge of the pericycle. Toward the end of the first season 

 numerous small patches of thick-walled fibers appear in the 

 phloem and the pericycle. 



The pith is large-celled ; it does not decrease appreciably in 

 size as the stem grows older. 



In the lower part of the epicotyl cork formation takes place 

 in the pericycle below the ring of stereom mentioned above, 

 thus cutting off the cortex and epidermis which soon die and 

 disappear. In the upper part it takes place in the cortex 

 (cf. Moeller [1882], p. 383). This plant shows a distinct " re- 

 gion tigellaire " in two-year-old material. 



