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 piotoplasm 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 

 Umbellifera and "ribs" of stems of the Labiata. 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- 



