STEMS 37 



Kinds of Tracheal Tubes (Fig. i6). — There are the fol- 

 lowing sorts of tracheal tubes: 



1. Annular Tracheal Tubes.— Here and there in the tube 

 are thickened rings of lignin, which have the appearance of 

 barrel hoops. 



2. Spiral Tracheal Tubes. — The thickening material is in 

 the form of a loose spiral. 



3. Reticulated Tracheal Tubes. — In these, the 

 strengthening material Is laid down in such a 

 fashion as to form a network on the wall. 



4. Dotted or Pitted Tracheal Tubes. — In these, 

 lignin has been deposited over the inner wall in 

 such a manner as to leave numerous circular thin 

 places, which give the tube a dotted or pitted 

 appearance. 



Tracheids (Fig. 17) are single cells, elongated 

 and modified. They have thick, lignified walls 

 with numerous bordered pits. In shape tracheids 

 are like a spindle, and they fit closely together 

 making up a strong supporting tissue. xf^heids, 



Wood parenchyma cells are usually thin-walled with bor- 

 and with unbordered pits. ^^^ ^' °' 



Wood fibers are long, taper-pointed at the ends and thick- 

 walled. The pits are unbordered. 



Functions of Wood Elements. — The functions of the dif- 

 ferent wood (xylem) elements are as follows: 



1 . Tracheal Tubes. — (a) Carry water and Solutes from the 

 soil to and throughout the leaves; (6) give strength to the 

 stem. 



2. Tracheids. — (a) Carry water and solutes; (6) give 

 strength to the stem. 



3. Wood Parenchyma. — (o) Store water and foods; (6) and 

 also conduct them short distances. 



