I04 PLANT LIFE. 



bark in spring contains from 4-20 per cent, whilst 

 Australian Wattle has 20-36 per cent of tannin. 

 Gall nuts are often rich in such secretions. This is 

 specially interesting ; for it is clear that, after the insect 

 has punctured the bark, the cells all round the place 

 must be stimulated to pour out an excessive amount of 

 the tannin, with the view of resisting its attack. A 

 regular commerce is carried on in the wooden Oak- 

 apple-like galls of Queracs infectoria, simply because 

 of their rich tannin contents. 



Acids and Alkaloids. — A very large series of Acids 

 and Alkaloids (which will undoubtedly keep off danger- 

 ous enemies of some kind) are produced by plants. 

 Amongst the best known of these are Nicotine, Quinine, 

 and Theine. They occur dissolved in the cell-sap, or 

 as amorphous bodies inside the cells (often in connec- 

 tion with the tannin), and appear to be generally manu- 

 factured in the leaves ; a single leaf may form about 

 .00005 gramme of quinine alkaloid every day. The 

 quinine of the Cinchona is a poison to the malaria-germ, 

 which lives in human blood corpuscles, but we have no 

 knowledge of the particular enemy which attacks the 

 tree in a wild state. A great many of these alkaloids 

 are very deadly poisons, and, in the early stages of 

 human history, the desire of men to avail themselves of 

 this property in their struggles with their enemies, must 

 have been a great incentive to the study of plants. The 

 use of arrow-poison must especially have been a very 

 great advantage. It seems to have been very widely 

 used by the more helpless races of mankind, and it is 

 still largely employed by savage and semi-savage races 

 in all parts of the world. These poisons are found in 

 every part of the plant ; in the seeds, as in Strychnos 

 nux- vomica, Strophanthus, Laburnum, etc. ; in fruits, 



