GROWTH OF PLANTS 



141 



water, which approaches the proportional composition of salts 

 in the blood and lymph. This caused Loeb to say that we all 

 carry in our bodies a part of the ocean, where once animal life 

 originated. A balanced medium for plants is represented by 

 the solutions that are used for water cultures; a number of 

 optimal mixtures have been described by Shive, Tottingham, 

 and others. 



In studjdng the effect of different neutral salts upon growth, 

 it is very difficult to separate their participation in the structure 

 of protoplasm from their 

 influence as specific stimulants. 

 The phenomenon of stimula- 

 tion may be observed best in 

 its pure effect by applying 

 poisonous substances. 



By poisonous substances are 

 understood chemicals that even 

 in small doses inhibit growth 

 and in considerable quantities 

 will kill a plant. Such, for 

 instance, are the salts of heavy ^ig. 36.— The comparative growth of 



, 1 1 M roots of wheat: (1) in a balanced solution 



metals, copper, lead, silver, and of NaCl + KCl + CaCh; (2) in a partly 



mercury, as well as many balanced solution of NaCl + CaCh; 



(3) in a pure solution of CaCh; (4) in a 

 organic substances hke ether, pure solution of NaCl {redrawn after 



chloroform, toluol. Certain Osterhout). 



metabolic products of an organism also may be poisonous, such 



as the organic acids, notably oxalic and carbonic acid. 



A detailed study of the influence of poisonous substances on 

 growth usually shows that not only the severity but the character 

 of their action depend on their concentration. When applied 

 in very weak doses, the majority of even the most poisonous 

 substances will not depress, but stimulate, growth. If for pur- 

 poses of calculation, a solution of 1 gram molecule per 100,000 1. 

 of water is assumed as unity, then phenol, for example, in a 

 concentration of 100 to 200 units will stop the growth of plants 

 in water cultures, but it will have a stimulating effect in con- 

 centrations of 4 to 8 units. Ethyl alcohol, a weaker poison, 

 stimulates in a concentration of 25 to 75 units and checks growth 

 in concentrations of 2,700 to 7,500 units. A similar stimulating 

 effect is produced by mercury preparations, which are widely 



