1918] Kendall: Ahscission of Flowers and Fruits in Sokmaceae 363 



HISTOLOGY AND CYTOLOGY OF THE PEDICEL 



1. Histological and Cytological Conditions of the 



Mature Pedicel 



a. NICOTIANA 



The vascular system in Nicotians, as in all the other genera 

 examined, is characterized by intraxylary phloem. Nicotiana differs 

 slightly from all others in that the xylem seems in cross-section 

 to be composed of a continuous ring of radial strands of trachea 

 rather than composed of a broken ring of distinct bundles. When a 

 branch of the vascular system (fig. 1, a) containing twenty to thirty 

 xylem strands is given oft' to the pedicel, it assumes the shape of a 

 crescent in cross-section, with the opening of the crescent on the ven- 

 tral side. A short distance distal to the groove which marks the sep- 

 aration zone (tig. 1, &), the crescent closes and throughout the 

 remainder of the pedicel the vascular system forms a complete cylin- 



-■-a 



Fig. 1. 



a — vascular system. 

 b — separation zone. 

 c — pedicel cortex. 

 sc — stem cortex. 

 e — epidermis. 



Diagram of pedicel of Nicotiana 



f — chlorophyllous tissue. 

 ff — groove. 

 h — separation layer. 

 p — pedicel pith. 



