64 



THIS PLANT CELL. 



(&) Tracheides or vessels with reticulate* thickenings on their 

 walls, the reticulate markings consisting of localised thickened 

 areas, the intermediate parts remaining thin (see Fig. 44, c). 



(c) Elements possessing both the modifications (a) and (b) — 

 viz., reticulo -pitted vessels or tracheides (see Fig. 42, b); in this 

 case the pits occur in the areas enclosed by the reticulations, and 

 are often of the " bordered " variety (see next heading, d). 



(d) Tracheides or vessels possessing "bordered-pits." — These 

 " pits " occur in two main varieties — viz., the simple bordered-pit, 



?5S3l 



Fig. 42.— a, A Pitted Tkaoheide 

 FROM Xylem of Qiierctis. b, 

 A Pitted and Reticulate 

 Tkacheide (Quercus). 



Fig. 43. — Portions of Annular 

 AND Spikal Vessels fkom the 

 Protoxtlk.m of Dicotyledon- 

 ous Stems. 



where an upraised thickened margin occurs round a simple 

 circular or oval pit, and the " bordered-pit," which is met with in 

 the tracheides of such a plant as Pinus — almost to the exclusion 

 of other elements. These pits occur only on the radial walls of 

 tracheides, and have the following structure : — 



i. In surface view each "pit" is circular in contour, the diameter 

 occupying almost the entire breadth of a tracheide. The central part, or 



* The scalariforra vessel met with in the xylem of Ferns is one form of 

 reticulate element (see Fig. 44, c). 



