170 TEXTBOOK OF BOTANY 



than one-third of it is converted into cellulose alone. The youngest cell 

 walls of nearly all plants are composed primarily of pectic compounds 

 and cellulose. If thin sections of a plant are treated with a strong solution 

 of chlorozinc iodide, or with a solution of iodine followed by 70 per cent 

 sulfuric acid, and then observed through a microscope the location 

 of cellulose in the cell walls is often indicated by a blue color. If pectic 

 compounds are abundant in the wall, a light brown color is produced. 

 In some cell walls it is necessary first to remove lignin, cutin, or other 

 substances that interfere with the test for cellulose. 



Cellulose is formed in every cell of all plants, except in certain groups 

 of algae (diatoms) and perhaps in certain bacteria. It is also found in 

 the cell walls of a few species of animals. When formed it becomes a 

 part of the permanent framework of the cell, for there are very few 

 organisms that possess the enzyme celhdase, that digests cellulose. The 

 digestion of cellulose in the alimentary tracts of animals is due to the 

 presence of certain non-green plants and sometimes certain protozoa. 

 In contrast to cellulose, hemicelluloses may be formed as secondary 

 thickenings on the cell walls of a plant and later digested to sugar. They 

 are most conspicuous in certain kinds of hard seeds, such as the seeds 

 of dates, palms, coflFee, and iris. The manufacture of cloth, paper, cellu- 

 loid, collodion, acetic acid, artificial rubber, charcoal, rayon, cellophane, 

 and explosives is but a few of the hundreds of industrial uses made of 

 cellulose. 



Pectic compounds. Apparently pectic compounds are present in the 

 walls of all plant cells. They consist of large molecules formed mainly 

 by the condensation of certain sugars and an acid derived from sugar. 

 Various amounts of pectic compounds are usually intemiingled with 

 cellulose in the walls of plants. The middle lamella, which holds the cells 

 of plants together, is composed of pectose or of calcium pectate. The 

 softening of fruits and vegetables when boiled in water or as they ripen 

 under natural conditions or in storage, is due to the breaking down of 

 this layer and the partial separation of the individual cells. A similar 

 process occurs in the absciss layer at the bases of the petioles of leaves, 

 pedicels of fruits, and at the bases of petals and stamens of flowers. 

 The pistils of flowers often abscise in the same way, and no fruits are 

 formed. In simple structures, such as algae and root hairs, the pectic 

 compounds are mainly in the outermost exposed layer of the cell walls. 

 Many plant mucilages and gums are also composed largely of pectic 



