THE PLANT-CELL 53 



largely developed in the outer bark of many trees and shrubs, the 

 cork of commerce being derived from the Cork-oak (Quercus suber) of 

 Southern Europe. Very similar is the cutinization of the exposed 

 epidermal cells of leaves and steins, in which the outer wall of the 

 epidermal cells develops a thick, impervious layer, or cuticle, which 

 in its chemical composition seems to be much like cork. It has been 

 supposed that the character of the suberized membranes was due to 

 an infiltration of the cellulose membrane by a fatty or waxy sub- 

 stance, suberin ; but later researches have made it more likely that 

 the cellulose undergoes a chemical change as well. This is indicated 

 by the destruction of the suberized membranes by reagents which 

 do not attack cellulose. 



The cell-walls of many Algae are mucilaginous in consistence, or 

 they may be imbedded in masses of gelatinous matter, which are 

 probably in part derived from a transformation of an originally cel- 

 lulose membrane, but are with little question also in part a direct 

 secretion of the protoplast. There are, moreover, all intermediate con- 

 ditions between mucilage and cellulose, with which it is chemically 

 closely related. Mucilaginous and gelatinous walls are remarkable 

 for their power of swelling when wet, and it is this property which is 

 of value to the plant. The mucilaginous change in the walls of the 

 cells in the reproductive organs of many plants, e.g. zoosporangia 

 of Algae, sexual organs of Ferns, etc., is the main factor in the open- 

 ing of the organs and the discharge of their contents. The develop- 

 ment of mucilage, either by secretion from the cells or by changes 

 in the cell-walls, is of great importance in protecting delicate parts 

 from excessive loss of water. This is especially well seen in many 

 Seaweeds which are exposed for long periods between tides. 



Inclusions of the Protoplast 



Besides the living cell-contents, there are present various sub- 

 stances which are the products of the activity of the protoplasm, 

 and may be either plastic substances, capable of being used by the 

 protoplasm as food, or they may be excretions or waste products. 



Soluble Substances. Within the cell-sap are dissolved many sub- 

 stances, like sugar and pigments, as well as inorganic bodies. The 

 blue and crimson pigments of leaves and flowers are, with few excep- 

 tions, dissolved in the cell-sap of its superficial cells. Sugar, inulin 

 (found in roots of various Compositse), and similar carbohydrates 

 occur in solution in the cell-sap, and may be crystallized out by 

 proper methods. The shining granules occurring in the cells of the 

 common Alga Zygnema are minute vacuoles filled with a solution 

 containing tannic acid; and other organic acids, e.g. malic, oxalic, 

 occur in solution in many plants. 



