xvi INTRODUCTION 



Sclerenchyma, like collenchyma, is a supporting tissue. Unlike 

 collenchyma cells, sclerenchyma cells have thick, hard, lignified 

 walls and are low in water. During maturation their protoplasts 

 degenerate, leaving a non-living sclerenchyma tissue. Sclereids and 

 fibers are sometimes conveniently distinguished in sclerenchyma, 

 but these forms are part of a morphological continuum. Fibers are 

 found in many tissues, but most profusely in cortex, pericycle, 

 xylem, and phloem. 



The sculpture of fiber walls varies from the complex bordered 

 pits of xylem to simple pits in the other complex tissues. Fiber 

 cells may occur singly or in scattered clusters, but most 

 commonly, they are present as strands or sheets of considerable 

 length. 



The anatomically defined fiber is not to be mistaken for seed 

 hairs (cotton), foliar vascular bundles (hemp), wood cells (paper), 

 etc., which are commonly termed fibers on the basis of their 

 properties. 



Sclereids tend to be isodiametric (but not exclusively so), al- 

 though irregular and quite variable. They are hard, even gritty, and 

 are sometimes known as stone cells. The pits in sclereid walls 

 are simple and form branching cavity systems. 



The anatomical array in the complex tissues —xylem and 

 phloem— is far more varied than it is in the simple tissues. In xylem, 

 tracheitis, fibers (or fiber-tracheids), vessels, and wood parenchyma 

 may be distinguished. Of these types, the tracheid occupies the 

 most fundamental position. The tracheid wall is extensively pitted 

 and, although the individual cells are closed off with tapered ends, 

 their lumens are in communication through the delicate, sometimes 

 perforated, pit membranes. This continuous, or anastomosing 

 system can sometimes be demonstrated by the forced passage of 

 carbon particles from cell to cell. 



The primitive, rather "all-purpose"tracheid is present in some 

 species, but it is often displaced by the apportionment of functions 

 to the other cell types— support to fibers, conduction to vessels, and 

 storage to parenchyma. Vessels possess both pits and end-wall 

 perforations. The pits occur in those regions of the wall where 

 vessels are contiguous. The end walls may contain simple perfora- 



