they act in similar ways when taken into the system. Poisoning may be of 

 various kinds. Thus certain substances combine with proteins in ways that 

 interfere with normal metabolism. Some substances retard, others accel- 

 erate, metabolism. Some substances seem to attach themselves to special 

 tissues. 



Among inorganic poisons the most dangerous for human beings are 

 compounds of lead, mercury and phosphorus, which are used in certain in- 

 dustries. Since about 1910 legislation has stopped the use of white phosphorus 

 in the manufacture of matches because the fumes caused serious injuries in 

 the workers. More recently, radium compounds, used for luminous watch 

 and instrument dials, were found to disturb seriously the metabolism of 

 those who work with such materials; and strict regulations have been 

 adopted to prevent further injury. 



Some of the most useful drugs obtained from plants are poisons of the 

 alkaloid group, such as morphin, atropin and quinin. These act in specific 

 ways, but often produce undesirable results along with their useful results; 

 and they are dangerous in large doses. For these reasons scientists have 

 been trying to find substitutes that are more readily controlled, in the form 

 of artificial synthetic compounds. The specific substances are not all useful, 

 nor do they act equally on all living things. Hens, for example, appear to 



REGENERATION IN LEAF OF BRYOPHYLLUM 



The leaves of bryophyllum, of begonia, and of a few other genera will form complete 

 plants if removed from the stem. In some experiments with bryophyllum leaves, a 

 plant was regenerated at each notch when the leaf had been cut into strips from 

 the edge to the midrib 



231 



