112 



MINNESOTA BOTANICAL STUDIES. 



The endodermis is distinct and can be recognized in year-old 

 material. Starch is present from the first. The various paren- 

 ch3'matous tissues at length have a small amount of starch. 

 Secretion cavities develop in the cortex. 



The stele is small ; much smaller than is usual in most spe- 

 cies. In the youngest material examined the phloem forms a 

 complete ring surrounding a small number of xylem bundles 

 which eventually fuse. 



Numerous groups of elements in the pericycle become scler- 

 otic so that they form an interrupted ring of sclerenchyma sur- 

 rounding the phloem. 



The pith is small, unusually so for an epicotyl. There is a 

 definite perimedullary zone of small-celled parenchyma contain- 

 ing starch. 



The formation of cork takes place in the outermost layer of 

 hypoderma (cf. Moeller [1882], p. 326). 



Comparison of Structure of Hy^ocotyl and Eficotyl. 



« 



The hypocotyl does not have the epidermal hairs and the collen- 



chymatous hypoderma of the epicotyl. 

 The cortex of the former region, though 

 very thick, is but little thicker, in pro- 

 portion, than that of the epicotyl. 



In the stele of the hypocotyl there 

 are four phloem bundles and an equal 

 number of xylem bundles, while in the 

 epicotyl, in the youngest material ex- 

 amined, the phloem forms a closed ring 

 surrounding about six xylem strands. 



The sclerenchyma of the hypocotyl 

 is in four somewhat irregular columns 

 in the pericycle, while in the epicotyl 

 it forms more nearly a closed sheath. 

 In the former region also the peri- 

 medullary zone is poorly developed. 



Ptolea trif'oliata 



Fig. 13. 



SIMARUBACEiE. 

 Ailanthus glandulosa. 



Structure of Hypocotyl. 

 The epidermis consists of small cells, square or nearly so, in 

 cross section, and considerably bulged when young. A few 



