82 TBACHEARY ELEMENTS 



xylem will be jiiven later in this Iiitrocluetioii. In secondary 

 xylem, i.e., the wood produced by the vascular cambium, the 

 walls of the tracheid are provided either with transverse, slit-like 

 scalariform bordered pits (as in ferns, and club mosses) or with 

 circular or oval bordered pits (as in most gymnosperms and 

 angiosperms) . The type and arrangement of the pits of tracheids 

 seem to be determined in part by the nature of the cell or cells 

 bordering the tracheid. Thus when two tracheids are in contact, 

 bordered pit-pairs occur while, if a wood-parenchyma or wood- 

 ray cell crosses the tracheid, half-bordered pit-pairs are developed. 

 Frost (1929) in a study of angiosperm xylem, however, has called 

 attention to the fact that the type of pitting in a given tracheary 

 element depends to a large extent upon the degree of phylogenetic 

 specialization of the cell itself rather than upon the type of 

 neighboring cells. Frost finds that the pit-pairs between tracheary 

 elements and parenchyma cells may be either bordered, half- 

 bordered, or simple, according to the species and that this situa- 

 tion constitutes a reliable criterion for distinguishing the xylem 

 of various plants. It seems clear that the whole question of 

 tracheary pitting demands more study both from a comparative 

 as well as an ontogenetic point of view. Indeed, considerable 

 diversity of opinion prevails as to the nature of the most primitive 

 type of pitting in seed-plants (cf. Jeffrey Ch. IV and VII, Brown 

 1918 and Bliss 1921). 



(6 ) The vessel clement. This type of tracheary element is gen- 

 erally interpreted as having evolved, phylogenetically, from some 

 primitive type of tracheid. In the genus Ephedra for example, 

 the sloping end walls of certain of the tracheary elements are 

 provided with circular bordered pits, the membranes of some of 

 which disappear during ontogeny. Cells of this type may typify 

 one of the ways in which the perforations, distinctive of vessel 

 elements, may have orginated (cf. Jeffrey, pp. 94-95, Figs. 72- 

 73). Further evidence of the derived nature of vessel elements 

 is afforded by their distribution in extinct and living vascular 

 plants. According to Jeffrey (p. 93), vessels are absent from 

 the secondary wood of Paleozoic cryptogams. Among existing 

 lower vascular plants, vessels are known to occur in certain species 

 of Sclaginella (Duerden 1934) and in two species of Pteridiitm 



