Exercise X 



TRACHEARY ELEMENTS 



I. Introduction. — One of the most characteristic features of 

 the sporophyte of the "Tracheophyta" is the presence of a well- 

 defined conductive or vascular system. This system is his- 

 tologically complex in that its two component "tissues," i.e., 

 phloem and .rylem, are formed of a variety of cell types differing- 

 in their arrangement, form, protoplasts and wall structure. While 

 the functions of storage and mechanical support are performed 

 to varying degrees by both the phloem and the xylem, these 

 "complex tissues" are concerned first of all in the trans-location 

 of water and solutes between root and shoot. This conductive 

 function is possible not only because of the structural character- 

 istics of the cells themselves but also because the vascular tissues 

 in the root, stem and leaves are interconnected and form a con- 

 tinuous system. From this standpoint, the vascular system may 

 be visualized as an internal "skeletal" framework to which new 

 increments are added during growth by the activity of the apical 

 meristems and, in plants with secondary growth, the vascular 

 cambium. This exercise is devoted to an introductory study of 

 the xylem, with particular emphasis upon the structure and 

 development of its definitive tracheary elements, viz. : the tracheid 

 and the vessel. In the exercise dealing with the anatomy of the 

 root, stem and leaf, additional information regarding xylem aiul 

 directions for studying it will be given. 



1. Structure and morphology of tracheary elements. The ex- 

 pression "tracheary elements" is used in this book to designate 

 collectively the tracheid and the vessel element which represent 

 the two chief types of water-conducting cells present in the 

 xylem of vascular plants. The following characteristics are com- 

 mon to both tracheids and vessel elements, viz.: (1) they are 

 typically prosenchymatous in form with oblique or pointed ends. 

 An exception to this is furnished by tiie cylindi-ical form of vessel 



80 



