308 TEXTBOOK OF PLANT PHYSIOLOGY 



a chemical character, will now be considered. The foremost place 

 must be assigned to the cations and anions. It has been noted 

 (Arts. 25 and 26) that the presence of a definite combination of 

 ions in the solution entering into the cells is a necessary pre- 

 requisite of plant growth. In a study of the indispensable indi- 

 vidual ions, it is, however, very difficult to distinguish their 

 growth effect in the building up of new particles of living matter 

 from their action as specific stimuli. Hence, there is more infor- 

 mation on the influence exerted on growth by the less necessary and 

 even poisonous substances. By poisonous substances are under- 

 stood chemicals which even in small doses inhibit growth and in 

 considerable quantities will kill a plant. Such, for instance, are 

 the salts of heavy metals, copper, lead, silver, etc. ; as well as many 

 organic substances like ether, chloroform, toluol, etc. Certain 

 metabolic products of an organism also may be poisonous, such as 

 the organic acids, notably oxalic acid. 



A detailed study of the influence of poisonous substances on 

 growth usually shows that not only the severity of their action, 

 but its character depend on their concentration. When applied in 

 very weak doses, the majority of even the most poisonous sub- 

 stances will not depress, but stimulate, growth. For purposes of 

 calculation, assume a solution of 1 gram-molecule per 100,000 liters 

 of water as unity, then phenol, carbolic acid, for example, in a con- 

 centration of from 103 to 200 will stop the growth of plants in water 

 cultures, but it will have a stimulating effect in concentrations of 

 4 to 8. Ethyl alcohol, a weaker poison, stimulates in a concen- 

 tration of 25 to 75 and checks in concentrations of 2700 to 7500. 

 Some poisons, as copper salts, do not produce a stimulating effect. 1 



It is interesting to note that the effect of a poison depends also 

 on the properties of the medium in which a plant is growing. 

 The poisonous quality of many substances markedly decreases in 

 sand cultures, while in soil the plant is able to endure several 

 hundred times as large a dose as in water culture. This is due to 

 the adsorption capacity of the soil, which binds the poisonous 

 substances on the surface of its particles. 



The action of poisons does not differ essentially from that of 

 substances such as the mineral salts, since in strong concentra- 

 tions these too have an inhibiting effect on growth. But they 



1 In some soils of the United States small additions of copper salts have 

 markedly stimulated plant growth. 



