140 DERMAL SYSTEM 



beyond 4 per cent. ; it is probable that the true extensibility of the 

 tangential walls is no greater. 



In considering the functions of corky tissues it must be borne in 

 mind that cork is a very poor conductor of heat, mainly because its 

 cells are filled with air. On account of this property cork would seem 

 to be peculiarly well-fitted to act as a protective covering in the case 

 of perennial aerial organs. Perennial branches contain, in addition to 

 their various permanent tissues, two peripherally situated meristematic 

 layers which require to be protected against sudden fluctuations of 

 temperature, namely the phellogen and the cambium. It may, of 

 course, in many cases be a matter of indifference whether the thawing 

 of a frozen organ takes place rapidly or slowly ; on the other hand 

 there can be no doubt that rapid thawing is in general dangerous, 

 especially where the same organ freezes and thaws several times in 

 rapid succession. Plants therefore find it advantageous to guard 

 against sudden variations of their internal temperature, and thus to 

 minimise the deleterious effect of such violent changes. The gardener 

 is, therefore, merely copying nature when he protects the more tender 

 of his woody plants against frost by means of wrappings of straw or 

 tow ; only the materials employed by the plants themselves namely 

 cork and bark are much better suited to their purpose. 



The barest reference must be made to the fact that cork also 

 constitutes an excellent means of protection against the attacks of 

 parasitic Fungi and against the assaults of animals of every kind, a 

 function in which it is very frequently assisted by various tannins, 

 alkaloids and other poisonous or bitter substances which are so often 

 deposited in bark. 



Cork is also particularly well fitted to serve as a tissue of cicatrisation. 

 As a matter of fact, wounds in the parenchymatous tissues of steins, 

 roots or leaves are generally occluded by so-called wound-cork. The 

 uninjured cells adjoining the wounded surface give rise by tangential 

 division to a phellogen, which in its turn produces corky layers. It is 

 in this way, for example, that the numerous wounds produced on woody 

 twigs by the autumnal leaf-fall are occluded. Dead and diseased 

 tissues also usually become cut off from the healthy parts of the organ 

 in which they occur by layers of cork. 



B. THE PHELLOUEN. 



The increase in thickness and the growth in surface of the outer 

 epidermal wall are dependent upon the activity of the epidermal 

 protoplasts ; similarly the initiation and the continuous regeneration of 

 cork are due to the activity of the meristematic tissue known as the 

 phellogen. Typically the phellogen consists of a single layer of thin- 



