276 LEAVES 



those of grape sugar and only slight changes in their pro- 

 portion. 



Cellulose is a suitable material for cell walls; for its elasticity 

 permits cells to enlarge, and its permeability allows water 

 and solutions of food to reach the protoplasm. In actively 

 growing cells thin cellulose walls are essential, so that the 

 cells can enlarge; but when the cells are to afford strength, 

 as in case of bast fibers, then the cellulose is so deposited as to 

 form thick walls. Commonly when an important function of 

 the cell walls is to afford strength, another substance called 

 lignin is formed from the sugar and combined with the cellulose, 

 thus forming the wood characteristic of the trunks of trees and 

 shrubs but also common in herbaceous plants. Also in the shells 

 of some nuts and the coats of many seeds, lignified walls are 

 common. Woody walls, like cellulose walls, are permeable to 

 water and solutions, and for this reason are not adapted for 

 protective coverings, where the prevention of loss of water is an 

 important function. In forming waterproof walls, a portion 

 of the sugar or cellulose is converted into fatty or wax-like 

 substances known as cutin and suberin. Cutin is common in the 

 outer walls of epidermal cells, while suberin occurs throughout 

 the walls of cork. 



Occasionally in the formation of cell walls, some of the sugar 

 is converted into substances which swell and become mucilaginous 

 when wet, as the seed coats of Flax and some Mustards illustrate. 



Some other substances which are formed from sugar and asso- 

 ciated with cellulose, lignin, cutin, and suberin in cell walls are the 

 pectic compounds. The pectic substances (pectin, pectose, and 

 pectic acids), although much like cellulose, are more easily decom- 

 posed by certain acids and alkalies. They are often combined with 

 minerals, and one of the mineral compounds, known as calcium 

 pectate, is the chief substance of the middle portion (middle 

 lamella) or oldest portion of walls separating cells. 



Besides the various substances formed from sugar, cell walls 

 are often infiltrated with mineral matters, notably silica, and 

 these minerals often add much strength to the frame work. 



Cellulose and its closely allied compounds serve man in many 

 ways. From cellulose paper is made, and long cellulose fibers, as 

 those of Cotton and Flax, are woven into clothing. The wood 

 of plants is the source of lumber. Being oxidizable, cellulose 



