182 THE ELEMENTS OF VEGETABLE HISTOLOGY 



fragments the ray cells are at right angles to the 

 fibers. 



Pith Parenchyma. — In mature woody stems this 

 element is much reduced in quantity and may be 

 entirely obscured. In other instances its position is 

 indicated by a hollow cavity or space at the center 

 of the stem. The individual cells are irregularly 

 circular with thin and frequently porous walls. Owing 

 to the small amount of pith parenchyma present in 

 a mature woody stem, this element is rarely apparent 

 in powdered materials. 







CELL CONTENTS 



The cell contents of woods are stored in the cavities 

 of the fibers, in the ray cells and in the pith parenchyma 

 cells. The stored materials include crystals, resins 

 and volatile oils. Starch is rarely present in woods. 



FUNCTIONS OF SECONDARY STEM TISSUES 



The functions of the different structures present 

 in secondary stems may be summarized as follows: 



„ C Cork or periderm, 



Covering issues.. . ( phelloderm 



' Bast fibers, 

 Wood fibers, 

 Stone cells. 



Supporting tissues . 



Absorbing tissues Lenticels. 



'* f Sieve tubes, 



Conducting tissues Ducts, 



Medullary rays. 

 Assimilating and 



synthesis tissues Secretion cells. 



Storage tissues Bark parenchyma. 



Meristematic tissues ....{.-, , . ' 

 I Cambium. 



