82 OAK. [CH. VI 



lignified tissue in giving a yellow instead of a blue colour 

 with Schulze's reagent. It is however especially re- 

 markable for its resisting power. If a section is placed 

 in strong sulphuric acid, ordinary cellulose walls are 

 destroyed, but the cuticle is not destroyed. Plants in a 

 state of nature are not subject to baths of sulphuric acid, 

 but this test shows at any rate a resisting power which 

 gives the cuticle value as an external armour-plating to 

 the epidermis. 



In older branches the epidermis disappears, and its 

 place is taken by several layers of cork-cells, whose walls 

 have a similar but not identical resisting quality: the 

 walls of cork-cells are not said to be cuticularised, but 

 to be suberised. 



The young oak twig is green, because the cortical cells 

 contain chlorophyll, but it begins to turn brown in its 

 first year, the brown colour being due to the growth of a 

 layer of cork covering up the green cortex like a veil. 

 This film-like appearance would suggest that the cork 

 arises on the surface of the cortex. It does not however 

 arise in the most superficial cells, i.e. in the epidermis, 

 but in the cells immediately under the epidermis. In 

 this layer a remarkable change takes place precisely like 

 that rejuvenescence which gives origin to the inter- 

 fascicular cambium. The sub- epidermal cells begin to 

 divide by tangential walls, and thus a cambium-like ring is 

 formed immediately inside the epidermis (ph in fig. 38). 



This meristematic layer has, like the vascular cambium, 

 a double activity : it adds to the cortex on its central side 

 and manufactures cork on its external epidermal side. It 



