110 THE ELEMENTS OF VEGETABLE HISTOLOGY 



sues or stone cells may be found in all parts of the 

 plant with the exception of the floral organs. The seed 

 coat often contains large amounts of this tissue as 

 it forms a hard protecting envelope for the embryo 

 and other parts of the seed. Stone cells are particularly 

 apt to be present in the stems of leaves, flowers and 

 fruits as well as in barks. Stone cell walls are composed 

 of ligno-cellulose. The deposition of lignin with the 

 consequent formation of stone cells takes place at a 

 later period than the transformation of plerom cells 

 into fibrous tissue. The term schlerenchyma is some- 

 times indiscriminately applied to all hard tissues, and 

 when so used includes wood fibers, bast fibers and 

 stone cells. Stone cells appear as rectangular, polyg- 

 onal and irregular thick-walled cells, the length seldom 

 being more than three times the diameter. The 

 thickness of the wall and, consequently, the size of 

 the cell cavity vary considerably in the different 

 types. Striations and pores may be apparent and the 

 cavity may contain inclusions. 



In describing stone cells the following points should 

 be noted : 



1. Presence or absence of pores; 



porous and non-porous types. 



2. Presence or absence of striations; 



striated and non-striated types. 



3. Branching; 



branched and non-branched types. 



Stone cells, with the possible exception of the 

 branched types, remain intact during grinding and 

 owing to their close contact, usually occur as masses 

 in powdered materials. Various types of stone cells 

 are illustrated in Plate 42. 



