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 alkalies, such as caustic potash. 



More efficient and prolonged protection is afforded by 

 the formation of sheaths of cork, certain layers being 

 differentiated as meristem tissue, or actively dividing cells, 

 for the continued production of this material. The walls 

 of true cork cells are thin, but the presence of eutin is a 



^f^ffe'S^ws 



% &_;<~>(^> <=> c? S? g=>^ 



FIG. 46. OUTER PORTION OF CORTEX 

 OF YOUNG TWIG OF LIME. 



per, cork layer ; ph, meristem layer. 



FIG. 47. SECTION OF A LENTICEL. 

 I, lenticel ; per, cork layer. 



conspicuous feature in them. They are very regular in 

 form, and are closely arranged together without any inter- 

 cellular spaces (fig. 46). Coming as they do between the 

 exterior and the metabolic tissue of the cortex of stems, 

 thus cutting off the intercellular 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 communication needed (fig. 47). During the winter a 

 layer of cork is formed below the lenticel. 



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, 



4 



