142 BRITISH PLANTS 
II. Internal Protective Characters. 
These consist in the presence in the tissues of substances 
noxious or unpalatable to animals and antiseptic to 
diseases. They are either formed as natural by-products 
in the economy of the plant (calcium oxalate), or they 
arose, in the first place, as a result of certain physiological 
disturbances, set up by causes that interfered with 
nutrition or respiration. But, however formed, they have 
subsequently proved useful to their possessors, and have 
developed and specialized in the struggle for existence. 
(a) Latex.—This is a fluid, generally milky in appear- 
ance, which is found in some plants, contained in special 
cells, tissues, or vessels. It is always powerfully anti- 
septic, generally very acrid, and in some cases even 
poisonous. When poured over wounded parts, latex 
congeals and keeps off the germs and spores of disease. 
All parts of the greater celandine (Chelidontwm majus) 
contain an orange juice. In one important section of 
the Compositee—-the Cichoriez—all the plants possess a 
milky latex—e.g., dandelion (Taraxacum), lettuce (Lac- 
tuca), hawkweed (Hieracium), goat’s-beard (T'ragopogon), 
and sow-thistle (Sonchus). The Natural Order to which 
the spurges belong—the Euphorbiacezee—is characterized 
by the presence of latex. The great economic importance 
of latex is realized when we consider that rubber is 
obtained from the latex of certain tropical plants. 
(b) Gums, Resins, and Turpentines.—These bodies are 
either excreted by special glandular cells, or are formed 
as the result of the disintegration of certain tissues. They 
are commonly found in xerophytes—e.g., pines, firs. 
Being antiseptic, they all constitute a means of defence 
against bacteria, while their unpleasant taste and physical 
properties teach animals, by experience, to avoid eating 
the tissues in which they occur. 
(c) Bitter Principles, such as tannin, found in the bark 
and lignified tissues of many trees—e.g., oak—as well as 
in galls. Quinine is a bitter alkaloid obtained from the 
bark of Cinchona (Peruvian bark). 
(d) Fixed Oils are found chiefly in fruits and seeds. 
Some of them are of a fatty nature, others are solid, form- 
ing waxes. In many cases, especially in seeds, they con- 
