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 
