124 



MINNESOTA BOTANICAL STUDIES. 



Comparison of Structure of Hypocotyl and Epicotyl. 



The hypocotyl is more nearly cylindrical than the epicotyl ; 

 its epidermis less flat, its stereom is better developed the first 

 year. Stem internodes above the epicotyl are square. 



The primary structure of the hypocotyl is like that of Te- 

 coma. There are four xylem bundles and four phloem bundles. 

 In the youngest epicotyl examined the phloem forms a complete 

 ring and there is a variable number of xylem bundles. 



No internal phloem was recognized 

 in the hypocotyl although it is quite dis- 

 tinct in the epicotyl. The cork of the 

 former region is pericyclic or cortical 

 in origin, that of the latter subepi- 

 dermal. 



BIGNONIACE^E. 



Eucalyptus 



"lobulus 



FIG. 20. 



Tecoma radicans. 

 Structure of Hypocotyl. 



The epidermal ceils are rectangular 

 in outline when seen in cross section. 

 From being originally radially elon- 

 gated they are, at length, nearly 

 square. The cuticle in cross section 

 appears minutely serrate. There are 

 a few short, simple, epidermal hairs. 

 The cortex is of loose parenchyma, generally about six layers 

 in thickness. There is no differentiated hypoderma. 



The endodermal cells are smaller than the cells of the cortex. 

 Originally they are irregularly hexagonal in outline, but toward 

 the end of the year they become elliptical and have slightly 

 thickened walls. Starch is entirely absent, except in the en- 

 dodermal region until nearly the close of the first year, when it 

 appears especially in the pith. 



The stele is originally very small and somewhat four-sided, 

 containing four xylem bundles and four phloem bundles. The 

 latter are next the pericycle. They alternate with the xylem 

 bundles. These are presumably the four "principal bundles" 

 found, according to Hovelacque [1888], in all Bignoniaceous 

 stems. At quite an early stage the phloem and xylem form 

 closed rings. 



