INTRODUCTION 81 



elements in certain angiosperms. (2) The secondary wall of ma- 

 ture traclieary elements consists of lignified cellulose and is 

 deposited as rings, spiral bands, bars, a reticulum or as pitted 

 layers upon the thin primary wall; (3) at maturity, traeheary 

 elements lack a protoplast and the lumen is occupied by gas 

 or fluids. The principal distinction between the two types of 

 traeheary elements consists in the fact that the tracheid is an 

 imperforate cell with a continuous primary wall, while a vessel 

 element is provided with distinct openings or perforations which 

 are usually located in the end-walls of the cell. When longi- 

 sections of the xylem are examined, it is evident that inter- 

 communication between adjacent tracheids is possible by means 

 of the bordered pit-pairs on their lateral walls. In contrast, 

 vessel elements occur in more or less distinct vertical series in 

 which the perforations of adjacent elements exactly coincide. 

 Thus, collectively regarded, a series of vessel elements constitutes 

 an open "pipe-like" structure which is termed a vessel. The 

 literature devoted to the structure and pitting of traeheary ele- 

 ments is so extensive that it will only be possible to outline below 

 some of the most salient features of tracheids and vessel elements. 

 (a) The tracheid. From a phylogenetic standpoint, the 

 tracheid is usually regarded as the "fundamental" cell type in 

 the xylem of vascular plants. According to Eames and Mac- 

 Daniels (p. 62), "tracheids alone probably made up the xylem 

 of very ancient plants." Among living plants, tracheids con- 

 stitute the only type of traeheary element in the xylem of most 

 lower vascular plants and, except for the Gnetales, are the 

 dominant cell type in the wood of gymnosperms. Tracheids are 

 also characteristic of angiospermous xylem where together with 

 vessel elements, fibers and parenchyma they contribute to pro- 

 duce the great histological complexity typical of the wood in this 

 group. Structurally, the tracheid is an elongate cell, the second- 

 ary wall of which is laid down in a variety of patterns. In 

 primary xylem, i.e., the xylem which develops first in the ontogeny 

 of the root, stem and leaf, the secondary wall has the form of 

 rings, spiral bands, bars, a network or else is provided with dis- 

 tinct pits. A more detailed discussion of the "fibrous" types of 

 secondary wall-thickening in the traeheary elements of primary 



