16 MEDICINAL HEEBS AND POISONOUS PLANTS 



from the soil; and tliab when forniecl there iy stored 

 in the protoplasmic molecule a large quantity of energy. 

 Under appfopriate conditions this energy is liberated 

 and wo?k accomplished. The work here takes the 

 form of cTfowth, movement, and other manifestations 

 which necessitate the expenditure of energy for their 

 accom]Jishment. The whole process whereby this energy 

 is liberated is called respiration, and in essentials is the 

 same in the plant as in the animal. With the exception 

 of one or two special cases, oxygen is necessary for the 

 proper fulfilment of the process, and the liberation of 

 energy is attended by a splitting up of the protoplasmic 

 molecule, or of substances, whether protein or otherwise, 

 which have been built up by the plant, into simpler 

 components. It seems as though oxygen played the 

 role of tlie match to the gunpowder. Now no two 

 plants are alike in the nature of the substances which 

 are formed as a result of these breakiag-down processes; 

 although it is true that in all of them carbonic acid is 

 jaltimately formed. The routes by ^Yhieh they reach tlie 

 ground are very varied. It is interesting, however, to 

 note that members which have been put into the same 

 fanuly on structural grounds often exhibit the same kind ' 

 of end-products, showing that there is a relationship in 

 function as well as in structure. Thus bitterness is the 

 prevailing characteristic of the plants which compose 

 the Composita^; irritant poisons are common among the 

 members of the Ranunculacese; astringent plants prevail 

 in the Gentianaceee. 



Of course, in the present state of our knoAvledge it is 

 not possible to explain all the chemical and physical 

 changes which intervene before all comes to a stop and 

 end -product are formed; we can only indicate- the 

 general trend, and study more carefully the stable pro- 

 ducts which are found at the end, and which underoo 



