PLANT TISSUES 
103 
chyma in being usually more elongated and in conducting soluble 
food materials with greater celerity. 
Reserve Parenchyma. —This resembles ordinary parenchyma in 
many particulars of structure but differs from it mainly by its cells 
being filled with^ starch, protein crystals, or oil globules. It is 
usually found in seeds, fleshy roots, or underground stems such as 
tubers, corms, and bulbs. 
Collenchyma.—' This form of tissue is characterized by its cells 
being prismatic, more elongated than ordinary parenchyma, and 
thickened in their angles with a colloidal substance. The cells, like 
those of parenchyma tissue contain protoplasm and a nucleus, and 
frequently chloroplasts (Fig. 40). Collenchyma is generally found 
underneath the epidermis, and gives strength to that tissue. It is 
frequently observed forming the “ribs” of stems and fruits of the 
Umbelliferce and “ribs” of stems of the Labiates. In many leaves 
it has been found as the supporting and strengthening tissue between 
the stronger veins and the epidermis. 
Sclerenchyma or stony tissue comprises a variety of supporting 
elements having thickened cell walls composed of lignocellulose. 
When first formed these cells resemble those of ordinary parenchyma 
in having walls of pure cellulose, but later lignin becomes deposited 
on the inner surface of the walls in one or more layers. (Occasion¬ 
ally as in the rhizomes of Ginger no lignin is deposited on the walls 
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- 
