462 



A TEXTBOOK OF THEORETICAL BOTANY 



Stoma 



i^-l — J. Sc/ere/d 



Fig. 449. — Hakea pectinata. Transverse section of 

 leaf, showing sclereids. 



The pointed ends of fibres in the sclerenchyma overlap considerably, 

 and as this is not true of the cells from which they develop, it has been 

 suggested that during elongation the points must push in between each 

 other. This movement is called sliding growth, as the cell surfaces are 

 assumed to slip on one another during the process. Its existence has not, 

 however, been completely established. If it occurs it can only be of limited 

 duration, particularly during the early amyloid stage of the cell wall, at the 

 beginning of cell expansion. The maturation of the wall and of the middle 

 lamella would probably soon put an end to such possibilities. 



Tracheids 



Fibre cells are too narrow to be of any use for conducting liquids, and 

 this is the principal function of the tracheids (Fig. 450), which make up a 

 large part of what is generally called " wood." Tracheids are shorter and 

 broader than fibre cells and their ends are less elongated, the taper being 

 usually in two planes only, so that they are chisel-pointed. It is difficult to 

 draw a sharp line between them and true fibres, and intermediate types or 

 fibre tracheids are common. Tracheids are normally lignified, but the 

 lignified material may be disposed on the wall in several difi^erent patterns. 

 The thickness of the wall is much less than in fibres so that there is a large 

 central lumen. The protoplasm disappears at maturity and only the wall 



remains. 



The existence of a practically continuous series of forms connecting true 

 tracheids with true fibres has led to much confusion in the anatomical 



