110 



MINNESOTA BOTANICAL STUDIES. 



The pith is composed of parenchymatous cells which acquire 

 thick lignified walls. There is a well-differentiated perimedul- 

 lary zone three or four cells in width. The cells are small and 

 have thick lignified walls. 



Cork formation, according to Flot [1890], takes place rather 

 deep in the cortex. 



Structure of Epicotyl. 



The cells of the epidermis are square or oblong in cross sec- 

 tion, and become in time greatly flattened. There are numerous 

 epidermal hairs. A narrow collenchymatous hypoderma is 

 present. The cells of the cortex are about the same size as the 

 epidermal cells. 



The endodermis was distinguished only in very young stages. 

 The cells are rather small and closely packed with starch. 

 Starch is found at a later time in the various parenchymatous 

 tissues. Troschel [1879] states that in year-old twigs starch is 

 present in some of the elements of the wood but disappears the 

 next year. 



In young material the epicotyl is elliptical in cross section. 

 The stele follows this closely in shape. The phloem forms a 

 closed ring surrounding a variable number of xylem bundles. 

 There are usually more than eight of these bundles. The 

 xylem also soon forms a complete zone in which medullary rays 



are prominent. 



The pith cells become, at length, 

 thick-walled. There is a well-de- 

 fined perimedullary zone. 



Cork arises in the fourth, fifth or 

 sixth layer of the cortex (cf. Moel- 

 ler [1882], p. 384). 



Comparison of Structure of Hypo- 



cotyl and Epicotyl. 

 The hypocotyl is without the col- 

 lenchymatous hypoderma of the epi- 

 cotyl ; it has fewer epidermal hairs ; 

 the cortex is thicker ; there are four 

 large groups of stereom with some 

 very small ones instead of a broken 

 FIG. 12. r i n g f medium-sized bundles. 



Robin ia 



pseudacacia 



