102 



MINNESOTA BOTANICAL STUDIES. 



than those of the epicotyl, There is in the former region also 

 a less developed hypoderma. 



The stele of the hypocotyl is 

 originally quadrangular ; it is sur- 

 rounded by a distinct endodermis, 

 and has four xylem bundles and 

 four principal phloem bundles. The 

 stele of the epicotyl is cylindrical, 

 without a distinct endodermis, and 

 even at a very early age, the xylem 

 and phloem form closed rings. 



The hypocotyl has no cortical 

 vascular bundles ; of these the epi- 

 cotyl has four. 



The presence of true collenchy 

 matous hypoderma in the hypocotyl 

 deserves special mention, as this 

 forms an exception to the general 

 rule that collenchyma is not de- 

 veloped in the hypocotyl. 



C^ESALPINACE^E. 



Parkinsonia aculeata. 



Structure of Hypocotyl. 



The epidermis is composed of cells which are rectangular in 

 cross section. They are, at first, radially elongated. Eventu- 

 ally they become elongated in the other direction. 



The cortex is many-layered. There is no hypoderma. The 

 outer cells of the cortex are much smaller than those further 

 down. Very early in the history of the hypocotyl a parenchy- 

 matous sheath of small cells is formed in the cortex about mid- 

 way between epidermis and endodermis. The cells are not 

 arranged in definite rows. The position of this sheath is shown 

 in the last plate accompanying this paper. 



The cells of the endodermis are smaller than those of the cor- 

 tex. They contain starch. The endodermis is quite distinct; 

 it was, however, not definitely distinguished in material col- 

 lected late in the season. The cortex and pith at a later time 

 also have some starch. 



The stele is four-angled. There are originally four phloem 



