pe ELEMENTS OF PLANT ANATOMY. 
carbon. Of their manner of origin and connection with the 
rest of the wall, still less is known than in the case of lgnin. 
The outer walls of epidermal cells are generally cutinized, some- 
times only the outer portion, and sometimes the entire wall, and 
in some instances the cutin extends throughout all the epidermal 
walls. Suberin is found in the walls of the periderm cells which 
are known as cork, or cork cells. Of the three layers of these 
walls, the primary seldom contains any suberin but is often 
lignified, the second is suberized, the third contains but little 
suberin, and in many instances is found to consist of pure 
cellulose. 
5. Formation of Mucilage and Gums. 
The third change, or breaking down of cellulose into muci- 
lage, is caused by the swelling of the membrane on the entrance 
of water. In higher plants, this occurs frequently in the 
epidermal cells, as in flax-seed; here the cellulose part of the 
epidermal wall swells till the outer cutinized portion bursts 
off and the entire layer of cells is reduced to a mucilage. In 
lower plants one or more of the outside layers of the walls 
swell, so that a mucilaginous coating is formed about the plant. 
The formation of various gums is a particular phase of this 
change of cellulose and requires a special chapter for treatment. 
The most prominent physical characteristics of cell mem- 
brane have been mentioned in the preceding description. Its 
power of double refraction has been taken as evidence of the 
crystalline nature of the individual micella or unit of structure. 
It possesses the quality of hardness in varying degrees, depend- 
ing upon the amount of mineral matter in its construction. It 
is elastic as well as flexible, being able to assume its normal 
size after it has been stretched beyond this by turgescence. 
It is capable of imbibition, and is in nearly all cases permeable 
by liquids and gases, though the process is a slow one. 
