54 VEGETABLE PHYSIOLOGY 



through its thickness. This change in the outer walls 

 of numbers of contiguous cells renders it possible to 

 strip off from such a tissue a piece of apparently structure- 

 less membrane, which is technically called the cuticle, and 

 which consists of nothing more than these altered layers of 

 the outermost walls of the contiguous cells. The alteration 

 of the chemical character of this membrane in forming the 

 cuticle of the epidermis is due to the transformation of its 

 cellulose or pectose constituents into a substance known as 

 cutin. Its properties are very different from those of the 



FIG. 45. SECTION THROUGH EPIDERMIS OF LEAF, 

 SHOWING THE OUTER WALLS MATERIALLY 

 THICKENED AND CUTICULARISED. 



a, epidermis ; &, cells of mesophyll. 



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

 and it is not easy for gases to pass into or through it. 

 This difference 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 cutin is a 

 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 



