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 {Chelidonium majus) 

 contain an orange juice. In one important section of 

 the Compositse — the Cichoriese — all the plants possess a 

 milky latex — e.g., dandelion (Taraxacum), lettuce (Lac- 

 tuca), hawkweed (Hieracium), goat's-beard (Tragopogon), 

 and sow-thistle (Sonchus). The Natural Order to which 

 the spurges belong — the Euphorbiacese — 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. 



(6) 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, whUe 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 seeds. They con- 

 stitute reserves of food which in many cases make them 

 palatable for man and beast — e.g., walnuts, brazils. 



