lOO PLANT LIFE. 



the closing does not generally take place fast enough to 

 prevent infection by fungi ; and a tree so wounded is 

 often attacked by these enemies, and even sometimes 

 killed. Leaves are able to protect themselves by a sort 

 of amputation of the wounded parts as has been shown 

 by Dr. Blackman. If short slits are cut in the leaf 

 with scissors, a layer of cork is formed a little way back 

 from the injured cells, and a cork border is produced all 

 round the injury. The scars left by fallen leaves are 

 also always corked over ; the coating is formed before 

 the leaf actually falls, and a special layer of very thin 

 cells is formed in the stalk above the cork layer, so 

 that the leaf comes neatly and easily away. The 

 ordinary cork of commerce is the result of a process of 

 artificially stripping the trunks of the Cork Oak of their 

 natural covering. The tree in response produces a very 

 thick and homogeneous cork layer, which is the com- 

 mercial produce. It should be also noted that cork is 

 used by man to keep in liquids and prevent fungi and 

 insects from entering. The Mediterranean countries 

 produce annually looo metric tons of Cork chiefly from 

 France (Algeria), Portugal and Spain. 



In the younger parts of the tree, it is necessary that 

 the water should sometimes escape. A continual loss 

 of water or transpiration goes on even from leafless 

 branches during winter. Leaves and very young twigs 

 are continually giving off water. Thus both the 

 epidermis and the cork sheath, though covering the 

 entire outside of the plant, require special valves by 

 which this water may be allowed to escape under due 

 restrictions. These openings are the stomata in the 

 epidermis and the lenticels of the cork. Stomata and 

 lenticels are automatic valves which allow the water to 

 escape and the air to enter, but only in a regular and 



