128 



MINNESOTA BOTANICAL STUDIES. 



Catalpa 

 speciosa 



derma, though in the epicotyl a distinct collenchymatous zone is 



developed. 



The endodermis of the hypocotyl remains distinct for a much 

 longer time than that of the epicotyl ; 

 the stele has about eight vascular 

 bundles, instead of twenty or more ; 

 sclerenchyma is first disposed in four 

 groups instead of a considerable num- 

 ber. 



The pith of the hypocotyl is much 

 smaller in amount than that of the 

 epicotyl. 



RUBIACE^. 



Cephalanthus occidentalis. 

 Structure of Hypocotyl. 



The cells of the epidermis are ob- 

 long or somewhat hexagonal in cross 

 section. About every fifth or sixth 

 FIG. 22. cell is elongated radially and pointed, 



projecting somewhat beyond the gen- 

 eral line of cells. These might be described as extremely 

 short hairs. This characteristic feature continues for a consid- 

 erable length of time. 



No distinct hypoderma is formed. The cells of the two or 

 three outer layers of the cortex are rather thick-walled but not 

 collenchymatous. The cortex is loose with numerous large in- 

 tercellular spaces. 



The endodermis is large-celled. It remains distinct through 

 the first year although the cells become at length very much 

 flattened. They contain starch. Starch is later found in the 

 various parenchymatous tissues. 



The stele is circular in cross section. There are originally 

 four phloem bundles and an equal number of xylem bundles ; 

 they are grouped in pairs. The xylem and phloem soon form 

 closed zones, the xylem encroaching upon the pith which, at 

 the close of the first season is almost entirely obliterated. The 

 hypocotyl thus assumes a root-like structure " rhizelle "' of 

 Van Tieghem [1891]. 



The cork is of epidermal origin. 



