Ramalcy : HYPOCOTYL AND EPICOTYL IN WOODY PLANTS. 133 



Cork Formation in Hypocotyl and Epicotyl. The cork cam- 

 bium is developed in the layer of cells next below the epi- 

 dermis in both hypocotyl and epicotyl of Celtis occidentalis^ 

 Liriodcndron tulipifera, Butncria florida, Ptelca trifoliata^ 

 Ailanthtis g'landulosa, Catalpa spcciosa. In Cephalanthus oc- 

 cidentalis it is of epidermal origin in both regions. In the 

 following species cork formation is sub-epidermal in the epi- 

 cotyl but the cork is produced in deeper cell layers of the hypo- 

 cotyl : Ulnms americana, Toxylon -pomiferum, Broussonetia 

 papyrifera, Rhanmus purshiana, El&agnus umbellata* Euca- 

 lyptus globulus. In Cercis canadensis, Gleditsia triacanthos^ 

 Amorphafruticosa, Robinia pseudacacia, Vitis cordifolia and 

 Tecoma radicans cork formation in the epicotyl is cortical, while 

 in the hypocotyl it takes place in some cases in the same cell 

 layer, in other cases in deeper layers. Details are given in the 

 previous descriptions for the separate species. 



Pith and Perimedullary Zone of Hypocotyl and Epicotyl. 

 The pith of the hypocotyl is smaller than that of the epicotyl, 

 sometimes it becomes nearly obliterated, e. g., Cephalanthus oc- 

 cidcntalis. The perimedullary zone is sometimes not distin- 

 guished in the hypocotyl though present in the epicotyl, e. g., 

 Parkinsonia aculeata, Gleditsia triacanthos, Eucalyptus globu- 

 lus. The opposite condition is found in Cercts canadensis and 

 Berchemia racemosa. More often where a perimedullary zone 

 is recognized it is equally developed in both hypocotyl and epi- 

 cotyl. 



Structure of Hypocotyl and Epicotyl at the close of the first 

 years growth. Owing to secondary changes the two regions, 

 though at first quite dissimilar in structure, may come to be very- 

 much alike. The xylem and phloem always form closed rings ; 

 the endodermis often becomes indistinguishable ; the cells of 

 the cortex become flattened. The differences of pith, perime- 

 dullary zone and sclerenchyma have already been given. 



Condensed Summary. 



Although secondary changes may cause a great resemblance 

 in the structure of hypocotyl and epicotyl, the two regions are, 

 in their primary structure, essentially dissimilar. 



The epidermis of the hypocotyl is more often without trichome 

 structures, the cortex is thicker and composed of larger cells, 



