m " 



70 , PROTECTION FROM INJURIES AND LOSS OF WATER 



well as in the wood. This is seen in bottle cork as alternating 

 light and dark bands. 



Cork as a Protective Tissue.— The fact that the cork tissue 

 is several cell-layers in thickness makes it better than the epi- 

 dermis as a buffer against stresses from without; and since in 

 perennial parts the phellogen may keep up its activity from 

 year to year any injuries and losses to the cork are quickly re- 

 paired, and therefore the cork is, on the whole, better than the 

 epidermis where chances of injury are multiplited by years. 



The many suberi-zed walls of the cork tissue interpose a series 

 of barriers against the ingress of fungal parasites. The sweet 

 and Irish potatoes afford examples of the effectiveness of cork 

 in this respect. Here the cork is quite thin, averaging hardly 

 more than six layers of cells in thickness, and yet the potatoes 

 remain sound until the cork covering is broken through, when 

 decay is apt to set in very quickly, because the omnipresent 

 microscopic bacteria and spores of fungi can now get at the deeper 

 and less resistant tissues. 



Cork also affords protection from danger of another kind; 

 air is a very poor conductor of heat, and each^cork cell embraces 

 a dead air space which prevents sudden interchanges of tem- 

 perature. If the outside temperature suddenly falls its effect 

 cannot at once be felt by the deeper tissues, and only little by 

 little can the heat of the plant be dissipated through its cork 

 jacket. Again, after the plant has once frozen, if the exterior 

 temperature suddenly rises, the cork may prevent death that is 

 so apt to result to the tissues from sudden thawing. 



The Cork as a Waterproof Covering. — The walls of the 

 cork cells are permeable with great difficulty to water and gases. 

 Experiments with the Irish potato have shown that in 48 hours 

 a potato with the cork covering removed lost sixty times as much 

 water as an unpeeled one of equal weight. It was found by 

 Wiesner that a film of cork two or three cells in thickness allowed 

 no air to pass through it, even under a pressure of more than a 

 third of an atmosphere continued for several weeks. This gives 

 us an exact statement of a fact that everyone apprehends in 



