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 
Fic. 45.—Srcrion turovcH Epipermis or Lear, 
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 igs accompanied by 
characteristic reactions; it stains yellow instead of bluc 
when treated with iodine and sulphuric acid, and becomes 
brown under the action of strong alkalies, such ag 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 
