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 



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, 



FIG. 113. A small portion 

 of a section through an Irish 

 Potato, r, rind composed of a 

 number of layers of cork cells. 

 a, tissue filled with food. Highly 

 magnified. 



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 

 thoroughness of the healing depends much upon how the wound 

 is made. 



FIG. 114. Some collen- 

 chyma cells from the stem 

 of a Dock (Rumex) showing 

 the cells thickened mainly 

 at the angles. After Cham- 

 berlain. 



