PLANT TISSUES 
71 
of the sclerenchyma fibers). When sclerenchyma is composed of 
cells which are more or less isodiametric or moderately elongated, 
with thickened lignified walls and conspicuous pores, its elements 
are called Stone Cells. Stone cells are distributed in fruits, seeds 
and barks of many plants, rarely in woods. They have been found 
forming the gritty particles in the “flesh” of certain fruits as the 
Pear, the endocarp or stone region of drupaceous fruits as the Olive, 
Peach, Cubeb, Pepper, etc., the hard portions of seed coats as in 
Physostigma, Walnuts, etc. Each stone cell presents for examina- 
tion a cell wall of cellulose with one or several layers of lignin on its 
inner surface which surround a central lumen. The latter is in 
communication with radial pore canals leading outward to the 
middle lamella. Longitudinal pore canals are also evident. 
When sclerenchyma is composed of cells which are greatly elon- 
gated and more or less obtusely or taper ended, its component ele- 
ments are termed Sclerenchyma fibers. These fibers are frequently 
spindle-shaped, contain air and exhibit oblique slits in their walls. 
They are either polygonal, rectangular or somewhat rounded in trans- 
verse section. They occur in various parts of roots, stems, leaves, 
fruits and seeds as supporting elements. When sclerenchyma 
fibers occur in the xylem region of fibro-vascular bundles they are 
termed Wood Fibers; when they appear in the phloem region, Bast 
Fibers. 
EPIDERMIS 
Epidermis is the outer covering tissue of a plant and is protective 
in function. Its cells may be brick-shaped, polygonal, equilateral or 
wavy in outline. Their outer walls are frequently cutinized (infil- 
trated with a waxy-like substance called cutin). Among the epider- 
mal cells of leaves and young green stems may be found numerous 
pores or stomata (sing, stoma) surrounded by pairs of crescent- 
shaped cells, called guard cells. • The stomata are in direct commun- 
ication with air chambers beneath them which in turn are in com- 
munication with intercellular spaces of the tissue beneath. The 
function of the stomata is to give off watery vapor and take in or 
give off carbon dioxide, water and oxygen. In addition to stomata 
some leaves possess groups of water stomata which differ from trans- 
