126 



MINNESOTA BOTANICAL STUDIES. 



walls. It is found to be practically in the center of the section, 

 though Pedicino [1876] found that when the plant climbs the pith 

 is eccentric. About the time that the seedling has developed one 

 internode above the epicotyl a medullary cambium has begun 

 to form in the epicotyl. This produces xylem without and 

 phloem next the pith. Considerable masses of phloem may 

 thus be formed. This peculiar cambium in Tecoma was noted 

 by Sanio in 1864 and fully described later by De Bary [1884]. 

 Young branches of the plant were studied by these investigators 

 who did not examine seedlings. 



Cork formation, as previously described by Moeller [1882] 

 for young branches, takes place in the second layer of the cor- 

 tex. The cork cells are nearly square in cross section. 



Comparison of the Structure of Hypocotyl and Epicotyl. 



Although both hypocotyl and epicotyl are originally cylindri- 

 cal, only the former remains so, the latter developing four 

 thickened areas which make it somewhat quadrangular. 



The four small groups of scleren- 

 chyma in the pericycle of the hypo- 

 cotyl are represented in the other region 

 by a considerable number of smaller 

 groups forming an interrupted ring. 

 The endodermis remains distinct 

 in the hypocotyl for a longer time than 



// /^^\ \ in the epicotyl. The medullary cam- 

 bium is formed later and is less active. 



Tc'coma 



radicans 



Catalpa speciosa. 



Structure of Hypocotyl. 



The epidermis consists of small cells, 

 square in cross section. Short, blunt 

 epidermal hairs are rather numerous. 

 No hypoderma is developed ; all the 

 cortical cells are thin-walled. 

 The endodermis remains distinct for a long time. It consists 

 of thin-walled cells which are but slightly smaller than the cells 

 of the cortex. Starch is present, from the first, in the endoder- 

 mis and toward the close of the season appears sparingly dis- 

 tributed in medullary rays and cortex ; it is apparently absent 

 from the pith. 



FIG. 21. 



