128 CELLS AND TISSUES 
much thicker and more protective than an epidermis. (Fig. 113.) 
The cork covering may be more or less flexible, as the rind of an 
Fic. 113. — A small portion 
of a section through an Irish 
Potato. 7, rind composed of a 
number of layers of cork cells. 
s, tissue filled with food. Highly 
magnified. 
Irish Potato or Sweet Potato, or 
harder and more brittle, as in the 
bark of trees, where it reaches its 
extreme thickness. Cork tissue con- 
sists of dead cells in the walls of 
which there is deposited a waxy 
substance much like cutin but called 
suberin to which much of the pro- 
tective character of cork is due. 
Cork coverings afford more protec- 
tion than an epidermis, but on ac- 
count of their opaqueness, they are 
not suitable except where it is not 
necessary for light to penetrate to 
the inner tissues. 
The protection afforded by an 
epidermis and cork is often brought 
to our notice in case of fruits, tubers, 
and fleshy roots. Thus Apples, 
Oranges, and most fruits which may be kept a long time, if 
uninjured, soon decay when their rinds 
are broken. The efficiency of a corky 
rind to protect against the loss of water 
is shown by the experiment in which a 
peeled Irish Potato lost sixty times as 
much water in 48 hours as an unpeeled 
one of equal weight. 
Furthermore, cork tissue has an ad- 
ditional function in the healing of 
wounds where, by the development of 
a callus-like mass of cork, the open- 
ing of the wound is closed and the 
break in the protective covering of 
the plant thereby repaired. It is im- 
portant to recognize this fact in prun- 
ing where the promptness as well as the 
Fic. 114. — Some collen- 
chyma cells from the stem 
of a Dock (Rumex) showing 
the cells thickened mainly 
at the angles. After Cham- 
berlain. 
thoroughness of the healing depends much upon how the wound 
is made. 
