118 



MINNESOTA BOTANICAL STUDIES. 



The hypocotyl has at first four xylem and four phloem bun- 

 dles, the epicotyl a closed ring of phloem and about eight 



xylem bundles. In the former area 

 there are but four groups of scleren- 

 chyma, while in the latter there is an 

 interrupted circle of thick-walled ele- 

 ments just outside the phloem. 



The pith of the hypocotyl is smaller 

 than that of the epicotyl. 



Bcrchemia 

 racemosa 



FIG. 16. 



Rhamnus purshiana. 



Structure of Hypocotyl. 



The cells of the epidermis are origi- 

 nally square or pentagonal in cross 

 section. They become, at length, much 

 flattened. The cortex is thick and 

 very large-celled. No hypoderma is 

 differentiated. 



The endodermis consists of small, thin-walled cells contain- 

 ing starch. It remains distinct until cork formation takes place. 

 Starch is found, late in the season, in the perimedullary zone 

 and phloem, but not in the cortex. 



The stele is originally somewhat four sided and has four vas- 

 cular bundles which soon fuse and produce closed zones. 



The pith is composed of very large cells. The perimedul- 

 lary zone is rather ill-defined. It is one or two cells in width. 

 The cells are small and contain starch. 



The cork is of endodermal origin. Its formation begins to- 

 ward the close of the growing season. 



Structure of Epicotyl. 



The epidermal cells, at first square or pentagonal in cross 

 section, become at length, considerably flattened. There are 

 numerous short, curved and pointed hairs. A poorly developed 

 hypoderma is present in year-old material. The cells of the 

 cortex are all about the same size. 



The endodermis was distinguished only in young material. 

 The cells are small and contain starch. The perimedullary 

 zone, cortex and phloem have at a later time, small amounts of 

 starch. 



