132 MINNESOTA BOTANICAL STUDIES. 



species the stele of the hypocotyl does not have th-e typical 

 structure just mentioned, the number and arrangement of vascu- 

 lar bundles being different. Ulinii.s amcricana has two xylem 

 crescents and numerous small phloem bundles ; Broiissonetia 

 ^apyrtfera has a root-like structure ; in Catalpa speciosa there 

 are about eight vascular bundles. 



Primai'y Structure of the Stele of the Epicotyl. — In shape the 

 stele of the epicotyl is often originally somewhat hexagonal, 

 though, as in the case of the hypocotyl, becoming at length 

 cylindrical. As is well known there are usually from six to 

 very many vascular bundles. Sometimes the phloem is com- 

 pletely fused into a closed zone even in very young stages. 



Arrangement of Sclercnchyma in the Hypocotyl. The scler- 

 enchyma of the hypocotyl first appears as four masses in the 

 pericycle in Toxlyon poniferimi^ JLiriodendron tulip fera^ Cer- 

 cis canadensis, Gleditsia triacanthos, Robinia pseudacacia, Ail- 

 anthus glandulosa, Vitis cord folia, Eucalyptus globulus, 7e- 

 coma radicans and Catalpa speciosa. In the plants just named 

 this original disposition of the sclerenchyma becomes altered 

 either by the intercalation of parenchymatous elements in the 

 areas of sclerenchyma or by the development of sclerenchyma 

 at other points. In the following plants, however, there is 

 practically no change in the sclerenchyma during the first year 

 and the four original masses remain to the end of the season : 

 Parkinsonia aculeata, A^norpha fruticosa, Ptelea trifoliata, 

 Bercheniia racemosa. 



Comparison of the Hypocotyl and Epicotyl tvith Reference 

 to the Distribution of Sclerenchyma. Commonly the scleren- 

 chyma in the two regions becomes, at the close of the first grow- 

 ing season, equally well developed and similarly arranged. Ex- 

 ceptions to this rule will now be noted. Sclerenchyma is absent 

 from the hypocotyl of Menispermum canadense, Butneria fo- 

 rida and Rhamnus purshiana although present in the epicotyl. 

 In Ptelea trifoliata and Berchemia racemosa at the close of the 

 first year there is a greater development of sclerenchyma in the 

 epicotyl than in the hypocotyl. The reverse of this condition 

 obtains in Celtis occidentalis and Eucalyptus globulus. Only 

 scattered sclerenchyma in small amount was recognized in 

 either region in year-old material of Broussonetia papyrijera ; 

 in Cephalanthus occidentalis a few only of the pericyclic cells 

 of the epicotyl become sclerotic. 



