DEFENCE. 99 



air ; it is also antiseptic and elastic ; so it is very well 

 adapted to keep in water, to keep out air, and to 

 prevent insects or fungi from entering into the 

 stem, where they might do great harm. The elasticity 

 allows it to yield when the branches are swayed or 

 distorted by wind, and within certain limits to adapt 

 itself to the increase in thickness of the stem ; but, 

 generally, the cork- cambium goes on forming new cork, 

 and the older parts of the sheath crack or scale off. 

 Sometimes the old dead cork scales off in strings or 

 irregular flakes, but in most trees it remains in the 

 shape of irregular ridges, which differ in pattern 

 according to the age and species of the tree. The 

 importance of the sheath can be judged from the way 

 in which a potato, when peeled or stripped of cork, loses 

 in weight ; about 64 per cent, of its weight will be lost 

 through the evaporation of water in 24 hours. 



A hailstorm may do an infinite amount of damage to 

 plantations by breaking through the cork-sheath, and so 

 enabling fungi and insects to enter; yet the plant has 

 arranged for accidents of this nature. For instance, if 

 a small twig is torn off a tree, or, if any accident pro- 

 duces a wound deep enough to reach the woody part, 

 the plant at once begins to cover the injured place. 

 The walls of the broken wood vessels become changed 

 into gummy matter of which a drop seals up the 

 aperture of every open vessel. At the same time the 

 living bark cells set about making a new cork-cambium. 

 This begins all round the wound, and is formed by cells 

 which are quite uninjured. Thus the injury is soon 

 surrounded by a basin of cork, which, as growth goes 

 on, swells inwards forming a sort of wave of corky 

 matter which gradually closes in over the wound, and 

 eventually covers it. But if the wound is very large, 



