THE SKELETON OF THE PLANT 49 



constituents into a substance known as cutin. Its 

 properties are very different from those of the original 

 cell-wall ; it is but slightly permeable by water, and it is 

 not easy for gases to pass into or through it. This dif- 

 ference of physical property is accompanied by characteristic 

 reactions ; it stains yellow instead of blue when treated with 

 iodine and sulphuric acid, and becomes brown under the 

 action of strong alkalis, such as caustic potash. 



More efficient and prolonged protection is afforded by 

 the formation of sheaths of cork, certain layers of cells 

 being differentiated as meristem tissue for the continued 

 production of this material. The walls of true cork cells 

 are thin, but the presence of cutin is a conspicuous feature 



FIG. 46. OUTER PORTION OF CORTEX 

 OF YOUNG TWIG OF LIME. 



per, cork layer. 



FIG. 47. SECTION OF A LENTICEL. 

 I, lenticel ; per, cork layer. 



in them. They are very regular in form, and are closely 

 arranged together without any intercellular spaces (fig. 46) . 

 Coming as they do between the exterior and the metabolic 

 tissue of the cortex of stems, thus cutting off the inter- 

 cellular space system of the latter from access to the air, 

 they are usually penetrated by special structures known as 

 lenticels, which are made up of corky cells very loosely 

 arranged, and which consequently set up the communica- 

 tion needed (fig. 47). 



In the corky cell-wall the cutin is frequently associated 

 with a certain amount of lignin. 



The thin corky walls possess almost exactly the same 

 physical properties as the thickened cuticle of the epidermis, 

 a fact which affords evidence that the primary meaning of 

 both is the same. 



E 



