DESTRUCTIVE METABOLISM 413 
rangcment for continuous riddance, such as is found in the excretory 
organs of animals. It is also particularly noteworthy that among the 
wastes there are few or none except the alkaloids that contain nitrogen. 
Even these are not necessary products of metabolism, for the very plants 
that produce alkaloids most abundantly may be so grown, and healthily, 
as not to contain any. 
Gaseous wastes. Among gaseous wastes, the most important, CO 2 
and O 2 , have already been mentioned; and the water resulting from 
respiration, while not produced as a gas, leaves the body mostly in this 
form. In a few plants, notably in the stinking goosefoot and flowers of 
hawthorns, a very disagreeable odor makes known the escape of a gas, 
trimethylamin ; but this is formed only in trifling amounts. 
Essential oils. Most of the odors of plants, fragrant or not, are due 
to the essential (volatile) oils, which are distinguishable from true oils, 
to which they are not at all allied chemically, by leaving only a transient 
spot on paper. They are especially abundant in the foliage and flowers, 
though there is no part but may be the seat of their production or storage. 
They are the more volatile constituents of complex mixtures, secreted 
by glands of various forms (see p. 337), whose solid residues, after the 
" oils " have been driven off, are resins (see below). These secretions 
may escape at once upon the surface, or they may be stored in inter- 
cellular receptacles and released only by crushing. In the flower leaves 
they are curiously distributed, being formed in the epidermis of both 
petals and sepals, or only in one, or only in the cells of one face, or only 
in lines or patches of cells. From such parts, even when in very small 
amounts, they may be distilled, and when more abundant they may 
be expressed and purified. Some are medicinal, and some are commer- 
cially valuable as perfumes for soaps, ointments, and other toilet articles. 
Chemically they are quite diverse; many of their constituents belong 
to the class of compounds known as terpenes. 
Gums and resins. Gums and resins occur in great variety, and 
often in mixtures called gum-resins and balsams. These terms are 
rather loosely used, and do not designate definite chemical groups. 
The true gums are in large part carbohydrates, arabinose being especially 
abundant (C 5 H 10 O 5 ), and arise from the transformation of the cell wall 
and growing tissues in woody plants. They swell readily in water. Gum 
arabic and gum tragacanth are well known commercially, and the gum 
of cherry and peach trees is familiar. Resins are yellowish solids, 
usually derivatives of essential oils, that occur dissolved in essential oils. 
