H H H H | | | | C C C C // \ / \\ // \ / \\ HC C CH HC C CH | ║ | | ║ | HC C CH HC C N \\ / \ // \\ / \ // C N C C | | | H H H Quinoline, C_{9}H_{8}N Isoquinoline, C_{9}H_{8}N The common alkaloids are distributed in the several groups as follows: Pyrridine--piperidine group; piperine, coniine, nicotine. Pyrrolidine group; hygrine and stachydrine. Tropane group; atropine, hyoscine, cocaine, lupinine. Quinoline group; quinine, cinchonine, strychnine, brucine. Isoquinoline group; papaverine, hydrastine, morphine, codeine, berberine. The composition and properties of the individual alkaloids have been extensively studied, because of their medicinal uses. As they have no known metabolic use to the plants which elaborate them, it will not be worth while to consider all of these investigations in detail here. The following facts with reference to certain typical members of each group will serve to illustrate the general constitution and properties of the alkaloids. =Piperine=, C_{17}H_{19}O_{3}, is found in black peppers. Its constitution is represented by the following formula, the group which is united to the piperidine ring, in this case, being piperic acid: H_{2} ║ C / \ H_{2}C CH_{2} | | /\ --O H_{2}C CH_{2} | | \ \ / | | CH_{2} N----OC·CH=CH·CH=CH--| | / \/ --O =Coniine=, C_{8}H_{17}N, is found in the umbelliferous plant, _Conium maculatum_. Structurally, it is a propyl-piperidine, represented by the following formula: H_{2} ║ C / \ H_{2}C CH_{2} | | H_{2}C CH-C_{3}H_{7} \ / N | H =Nicotine=, C_{10}H_{14}N_{2}, is the alkaloid of tobacco leaves. It is an extremely poisonous, oily liquid, with a strong odor and a burning taste. Its structural formula shows it to contain both a pyrridine ring and a pyrrolidine ring, linked together thus