THE SKELETON OF THE PLANT 



55 



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. These are made up of corky 

 cells very loosely arranged, and 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, 

 a fact which affords evidence that the primary function of 

 both is the same. 



FIG. 46. OUTER PORTION OF CORTEX 



OF YOUNG TWIG OF LIME. 

 per, cork layer ; ph, meristem layer. 



FIG. 47. SECTION OF A LENTICEL. 

 /, lenticel ; per, cork layer. 



Like the substance of the middle lamella, both lignin 

 and cutin are soluble in warm nitric acid containing potassic 

 chlorate. 



In some cases the cell-walls of the epidermal protoplasts 

 are impregnated with various matters that do not proceed 

 from their own disintegration. Among these are various 

 fatty bodies, while wax is sometimes very conspicuous. 

 The Uoom of such fruits as the grape and the plum is 

 composed of very fine waxy particles ; the impregnation 

 in their case having been so great that certain particles 

 have passed beyond the walls and formed a layer on the 

 outer surface. The leaves of the wax-palm show an even 

 denser deposit. 



Mineral matters are also of frequent occurrence in the 



