130 MINNESOTA BOTANICAL STUDIES. 



stage examined, forms a closed ring surrounding six xylem 

 bundles. 



At the end of the first year the structure of the two regions is 

 essentially the same except that the hypocotyl is without pith or 

 differentiated hypoderma. 



GENERAL CONCLUSIONS. 



The following summary and conclusions are based on the 

 facts shown in the foregoing pages. It is not intended to re- 

 peat here all the points which are there given, but merely to 

 bring together under appropriate headings the most important 

 facts of structure of the hypocotyl and epicotyl in the plants 

 studied. 



General Shape of Hypocotyl and Epicotyl. In cross sec- 

 tion the hypocotyl is usually circular in outline, the epicotyl is, 

 however, not infrequently hexagonal in outline and somewhat 

 flattened. The hypocotyl has usually in early stages much the 

 greater diameter. 



Comparison of the Epidermis of Hypocotyl and Epicotyl. 

 The epidermal cells of both regions when seen in cross section 

 appear at first square or radially elongated. After a time, 

 however, they became elongated tangentially, being stretched 

 by the growth of the stelar tissues and not continuing to divide. 

 In quite young stages some plants have in the epicotyledonary 

 region, epidermal cells which, in cross section, appear tangen- 

 tially elongated, viz. : Ulmus americana, Liriodendron tulip- 

 ifera, Butneria florida, Cercis canadensis. Trichome struc- 

 tures are usually fewer and less complex in the hypocotyledon- 

 ary region, e. g"., Ulmus americana, Broussonetia papyrifera, 

 Butneria florida, Robinia pscudacacia, Ailanthus glandulosa, 

 Catalpa speciosa, Cephalanthus occidentalis. In the following 

 species the epicotyl has trichome structures but they are absent 

 from the hypocotyl : Celtis occidentalis, Toxylon pomiferum, 

 Gleditsia triacanthos, Ptelea trifoliata, Rhamnus purshiana, 

 El&agnus umbellata, Tecoma radicans. 



Hypoderma tn the Hypocotyl and Epicotyl. Only one of the 

 species examined has a definite collenchymatous hypoderma in 

 both regions. This is Butneria Jlorida. The following plants 

 have a hypoderma in the epicotyl, but not in the hypocotyl : Celtis 



