DEFENCE. 103 



cut down. Tar, pitch, resin, and turpentine are of 

 course extremely valuable products, and appear to have 

 been used by man since a very early period. A whole 

 series of substances secreted by plants, and akin in 

 character to resin might be mentioned here. Probably 

 all are secreted or excreted to defend the plant either from 

 animal enemies or from the danger of losing too much 

 water when accidentally wounded. Thus there are the 

 true Gums, such as Gum Arabic, which is produced by 

 various Acacias found in Upper Egypt, and was an 

 article of Egyptian commerce in very remote times. 

 Another kind of gum can be easily found on Cherry 

 branches which have been broken or injured. 

 Amongst other similar substances are the true resins 

 such as Copal and Grass-tree-gum ; Inodorous Gum- 

 resins such as Gamboge ; Odorous Gum-resins such 

 as Myrrh, Bdellium, and Opopanax ; the Oleoresins 

 such as Lacquer and Turpentine, etc. ; True Balsams 

 such as Balsam of Peru ; and Volatile Oils such as 

 Camphor, Menthol, Spirits of Turpentine, etc. (see below, 

 p. 105). These are used by man, sometimes merely as 

 strong perfumes, but more often as insecticides, pre- 

 servatives, or for varnish, i.e. for protecting substances 

 from the attacks of insects or bacteria. Some of them 

 are as characteristic of dry climates, as the Pine is of 

 northern latitudes, and the latex of tropical jungles. 



Tannins. — Another group, the Tannins, is very 

 widely distributed in the bark of trees. Tannin gives 

 the peculiar astringent taste to most barks, and is, 

 almost certainly, used for defence against bacteria and 

 small insects. Man employs this tannin for rendering 

 the leather of boots and shoes impregnable to bacteria. 

 A little more than one per cent, of tannin is sufficient 

 to destroy the growth of bacteria and fungi. Oak 



