18 ^lEDICINAL HERBS AND POISONOUS PLANTS 



Other plants contain alkaloids whichj though poisonous^ 

 exert in small quantities a stimulating effect of a pleasant 

 nature. Such, for example, are the theobromine of the 

 cocoa plant and the the'ine of the tea plant. 



From the point of view of the plant economy it must 

 be noted that alkaloids are waste -products and of no 

 further use to the plant; their formation was^ as it were, 

 unavoidable, and they are to be regarded as by-products 

 of processes of great importance to the plant. 



Some plants are valuable because of the aromatic 



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siihstancrs which they produce. These, again, are 

 mainly by-products of important processes. Thus when 

 glucosides are split up by ferments the sugar glucose 

 is invariably formed. The production of glucose is an 

 important operation, for the plant utilizes it as food 

 material, but other substances are also formed, each 

 glucoside producing one or two other substances as well. 

 Among the latter are many aromatic substances, the 

 presence of which brings the plants contahiing them 

 into the category of medicinal or poisonous plants. 

 Thus pruHsic acid is formed by the splitting up of the 

 glucoside amygdalin; tannic acid is formed by the split- 

 ting up of the glucoside tannin. As similar by-products 

 we may regard the commoner aromatic acids such as ben- 

 zoic acid, cinnamic acid, gallic acid, and salicylic acid. 



Again, the resins are substances which are always to 

 be found in plants, and in all parts of them. They are 

 semi-solid plant secretions, l)eing found either as emul- 

 sions or oleo-resinous juices. When mixed witli ethereal 

 oils and aromatic acids they are called halsavis] when 

 mixed with gums and ethereal oils they are called gu7n 

 resins. All these are waste-products, although in some 

 plants they, accidentally as it Were, {jlay an important 

 role by warding off from the plant the attacks of micro- 

 organisms. 



