ANATOMY OF THE CELL. 37 
classes. Those containing oxygen and hydrogen in the same 
proportion as in water are called carbohydrates. There are 
various other products which contain the same elements but 
in different proportions. The principal group is that known as 
oils, but there is one class of these which lacks oxygen. 
10. Fixed Oils. 
These are combinations of fatty acids with glycerine. <Ac- 
cording to the kind of acid they are either liquid or solid. They 
are probably always present in protoplasm in very minute par- 
ticles ; and often as drops of considerable size, which are in some 
cases so numerous as to nearly fill the entire cell cavity. This 
is especially true of the fungi, in whose cells fat takes the place 
of starch. Wax is a form of solid fat which is not a reserve 
material, but a secretion in the outer wall of the epidermis, 
thrown to the surface in the form of small grains, rods, or scales. 
It also occurs in very fine particles on the surface of certain 
fruits, forming the so-called bloom. 
11. Volatile Oils. 
These differ from the fixed in possessing a volatile or ethereal 
quality by which they give out a pungent odor. They probably 
never occur as reserve materials, but as secretions which are 
either held suspended in the cell sap, or thrown out into inter- 
cellular spaces, or into peculiar cells designed to receive such 
secretions. Several of the volatile oils lack oxygen, — for ex- 
ample, turpentine, whose formula is C,,H,,. A few others con- 
tain not only the three elements carbon, oxygen, and hydrogen 
but also a little sulphur. 
12. Resins. 
These are closely related to the ethereal oils, some being re- 
garded as their oxidized products. They are widely distributed 
and are divided into three classes, true, balsam, and gum resins, 
