4 Introduction 



while there is no evidence that growth is proportional to supply. The 

 substances fall into two groups : 



(1) Those that apparently become indifferent in. high dilutions and 

 never produce any increase in plant growth. 



(2) Those that cause a small, but quite distinct, increased growth 

 when applied in quantities sufficiently small. 



The former group may be legitimately regarded as toxins; the 

 latter present more difficulty and even now their function is not settled. 

 It is not clear whether they stimulate the protoplasm or in some way 

 hasten the metabolic processes in the plant, whether they help the 

 roots in their absorbent work, or whether they are simple nutrients 

 needed only in infinitesimal quantities. The two groups, however, cannot 

 be sharply separated from one another. Indeed a substance may be 

 put into one of these classes on the basis of experiments made with one 

 plant alone and into another when a different plant is used, while it 

 is quite conceivable that further experiments with other plants may 

 abolish the division between the two groups altogether. It is even 

 impossible to speak rigidly of toxicity. The addition of the inorganic 

 food salts to solutions of a poison reduces the toxicity of the latter, so 

 that the plant makes good growth in the presence of far more poison than 

 it can withstand in the absence of the nutrients. This masking effect 

 of the inorganic food salts upon the toxicity of inorganic plant poisons 

 is paralleled by a similar action on organic toxic agents. Schreiner and 

 Reed (1908) found that the addition of a second solute to a solution 

 decreases the toxicity of that solution; further the plant itself may 

 exercise a modifying influence upon the toxic agent. Water culture 

 experiments were made upon the toxicity of certain organic compounds, 

 with and without the addition of other inorganic salts. Arbutin, 

 vanillin, and cumarin were definitely toxic and the toxicity decidedly 

 fell off after the addition of sodium nitrate and calcium carbonate, 

 especially with the weaker solutions of the toxins. Curiously enough, 

 while weaker solutions of vanillin alone produced stimulation, the 

 stimulating effect of this toxic agent disappeared entirely on the addi- 

 tion of the inorganic substances. The results showed that the addition 

 of certain inorganic salts to solutions of toxic organic compounds was 

 decidedly beneficial to the plant 



Another important problem has come to the front with regard to 

 these toxic substances — How do these substances get into the plant ? 

 Are they all absorbed if they occur in the soil, or is there any 

 discriminatory power on the part of the root ? In other words, do the 



