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 cutin is a 
Fic. 46.—OvuTER PorTION oF CoRTEX 
or Youne Twie or Lime, 
per, cork layer; ph, meristem layer. Fic, 47.—Srction or A LENTICEL. 
1, lenticel ; yer, 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 
