56 General Botany 



many flowers, the yellow bark of some trees, and certain yellow 

 fruits. Some of these pigments have a commercial value as dyes. 

 Indigo and htmus are blue dyes of vegetable origin. Madder is 

 one of a group of red dyes used in making artists' colors. 



Cell-wall constituents. Cellulose is the best known of the sub- 

 stances found in cell walls. It belongs to the more complex of 

 the carbohydrates and is a strong, white, insoluble substance. 

 It forms the framework of most plants and is the important con- 

 stituent of all textile fibers, hke cotton, hemp, flax, and jute. It 

 is also the basis of a large number of manufactured products, 

 such as paper, celluloid, acetic acid, artificial rubber, lamp 

 black, charcoal, vegetable silk, and numerous explosives. 



Pectic compounds, which closely resemble cellulose in chemical 

 composition, occur in most cell walls. The middle lamella, which 

 holds together the cells of the higher plants, is made of pectose 

 or of calcium pectate. It is this layer which breaks down in the 

 boiling of fruits and vegetables and allows them to soften and 

 separate. Pectic compounds occur in many fruits, and when 

 these fruits are boiled with sugar, jelly is formed. In the Kving 

 plant pectic compounds aid in holding water in the cell. 



Lignin, suberin, cutin, and wax. Closely associated with cel- 

 lulose is a group of substances which modify the cell walls of 

 certain tissues as they increase in age. These substances form 

 mixtures, or chemical combinations, with the cellulose already 

 present. Lignin increases the hardness and rigidity of cellulose 

 walls and is present in most woody tissues. Suberin is the impor- 

 tant constituent of the walls of cork tissue. Cutin and wax are 

 usually present in the outer walls of the epidermis of land plants. 

 Suberin, cutin, and wax are all related chemically to the fats, 

 and when present in, or on, cellulose walls render them less per- 

 meable to water. 



Resins, gums, and mucilages. Resins and gums are products 

 frequently formed in all parts of plants. Resins are insoluble 

 in water and render walls impervious. They occur usually in 



