DESTRUCTIVE METABOLISM 413 



rangemcnt for continuous riddance, such as is found in the excretory 

 organs of animals. It is ab<> 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, C0 2 

 and 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 8 H w O«), 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. 



