286 



PLANT STRUCTURES 



tissues, of which there are two prominent kinds. The 

 tracheary vessels are for water conduction, and are cells 

 with heavy walls and usually large diameter (Fig. 268). 

 The thickening of the walls is not uniform, giving them a 

 very characteristic appearance, the thickening taking the 

 form of spiral bands, rings, or reticulations (Fig. 268, B). 

 Often the reticulation has such close meshes that the cell 

 wall has the appearance of being covered with thin spots, 

 and such cells are called " pitted vessels." The vessels with 

 spirals and rings are usually much smaller in diameter than 

 the pitted ones. The true tracheary cells are more or less 

 elongated and without tapering ends, fitting end to end 

 and forming a continuous longitudinal series, suggesting a 

 trachea, and hence the name. In the Conifers there are 



no true tracheary cells, as in 

 the Dicotyledons, except a few 

 small spiral vessels which are 

 formed at first in the young 

 stele, but the tracheary tissue 

 is made up of tracheids, mean- 

 ing "trachea -like," differing 

 from tracheae or true tracheary 

 vessels in having tapering ends 

 and in not forming a continu- 

 ous series (Fig. 269). The walls 

 of these tracheids are " pitted" 

 in a way which is characteristic 

 of Gymnosperms, the "pits" 

 appearing as two concentric 

 rings, called "bordered pits." 



The other prominent mes- 

 tome tissue developed in the 

 stele is the sieve vessels, for the 

 conduction of organized food, chiefly proteids (Fig. 268). 

 Sieve cells are so named because in their walls special areas 

 are organized which are perforated like the lid of a pepper- 



Fio. 269. Tracheids from wood of 

 pine, showing tapering ends and 

 bordered pit?. CHAMBERLAIN. 



