142 PLANT PHYSIOLOGY 



applied in the practice of treating seeds to protect them against 

 fungi. 



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 in comparison to water cultures, while in soil the 

 plant is able to endure several hundred times as large a dose as 

 in water. This is due to the adsorptive capacity of the soil, 

 primarily the clay portion, which binds the poisonous substances 

 on the surface of its particles. 



The action of poisons does not differ essentially from that of 

 the mineral salts, since in strong concentrations these too have an 

 inhibiting effect on growth. But they can also stimulate it, if 

 applied in weaker dilutions. The difference must therefore be 

 considered to be quantitative rather than qualitative, and only 

 substances that inhibit growth in very weak concentrations 

 should be regarded as poisonous. It has been seen already that 

 all cations show a toxic effect in pure solution and that only 

 solutions that are physiologically balanced are suitable for the 

 nutrition of plants. 



Recently, the question of stimulants for growth has been given 

 a new interpretation in the works of Popoff, who is of the opinion 

 that it is not absolutely necessary that the stimulant should act 

 continuously upon the plant during its development. In order 

 to increase growth, it is sufficient to stimulate the germinating 

 seeds. For this purpose, Popoff uses 3 per cent solutions of 

 magnesium chloride and sulphate and of manganese sulphate 

 and nitrate, as well as mixtures of these salts, and a number of 

 organic compounds, such as tannin and various narcotic and 

 antiseptic substances. The seeds are soaked in these solutions 

 for several hours and then sown in the usual way. According 

 to Popoff, as a result of such a treatment, growth has been accel- 

 erated, the size of the plants has increased, and considerably 

 larger crops have been obtained. In some of his experiments, 

 this increase has amounted to as much as 50 to 100 per cent. 



Popoff's experiments have attracted a great deal of interest, 

 since by the stimulation of seeds a considerable increase in yield 

 may be obtained at little expense. But most investigators have 

 not been able to confirm his results, especially under field con- 

 ditions. Stimulation of seeds is a considerably more complicated 



