90 



MINNESOTA BOTANICAL STUDIES. 



of the stem developed, is partially sclerenchymatous. At a later 

 time numerous groups of sclerenchyma are found in the phloem 

 and cortex. 



The pith becomes quite small. There is a small-celled peri- 

 medullary zone. 



The formation of cork cambium, as noted by Plot ( [1890], p. 

 29 ), takes place in the inner cortex. 



Structure of EpicotyL 



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

 somewhat rectangular in outline. The tangential diameter is 

 the longer. Numerous hairs are present. No collenchymatous 

 hypoderma is produced. The cortex is rather narrow. The 

 cells are all about the same size. 



The endodermis is distinct only in young material. The cells 

 are small. They contain starch. At the end of the season 

 starch is found in the pith and inner xylem and is sparingly dis- 

 tributed in the cortex and phloem. 



In the youngest material examined the phloem forms a closed 

 ring surrounding a number of xylem groups. There are usually 

 eight of these. They soon fuse to form a complete ring. 



Numerous small groups of thick-walled cells finally make 

 their appearance in cortex, pericycle and phloem. 



The pith becomes quite small. The cells have thin unligni- 



fied walls. The perimedullary zone 

 is easily distinguished ; it consists of 

 from one to three layers of small cells 

 which are often somewhat flattened. 



Cork formation, as is well known 

 in this species, begins in the outer- 

 most cell layer of the cortex. 



Comparison of Structure of Hypo- 



cotyl and EpicotyL 

 The epidermal cells of the hypo- 

 cotyl in young material appear radi- 

 ally elongated, those of the epicotyl 

 tangentially elongated. The former 

 region has a thicker cortex, fewer epi- 

 dermal hairs, sclerenchyma developed 

 FIG. i. earlier in the pericycle. 



Ulmus 



americana 



