THE CELL 25 
walled, elongated prosenchyma cells are called SCLEREN- 
CHyMA fibres. If the prosenchyma cell is shorter and 
wider, and not pointed at the end and provided with 
bordered pits it is called a TRACHEIDE (fig. 138). Its 
chief function is to carry water from one part of the 
plant to another. Its walls are always lignified, whereas 
those of the sclerenchyma may, or may not, have under- 
gone this change. The walls of the tracheides may be 
thickened spirally or in rings or reticulately. 
A Cc 
B | sas 
Fic. 14.—Bordered Pits. 
A. Surface view. B, Tangential section. C. Transverse section of a 
tracheide. 
16, Pits are formed in the cell wall by uneven thicken- 
ing of the cellulose wall. They may be circular, ellip- 
tical, or elongated. ‘Tracheides have bordered pits, the 
structure of which can be understood:from figure 14. 
When a number of elongated cells fuse together to 
form a long tube, the latter is called a VussEu. It is 
similar in nature to a tracheide, except that it has been 
formed from several cells instead of from one. 
In the phloem we find another kind of cell called a 
SIEVE TUBE (fig. 15). This is a long, thin-walled, un- 
