HORMONES, AUXIMONES, V IT AMINES, AND TOXINS 247 



Acetic acid must be about five times as, concentrated as this, and 

 other organic acids may be much more concentrated still before 

 they produce the same injurious effects. The toxic effect of all 

 these acids is greatly reduced in soil cultures, or if particles of sand, 

 graphite, clay, filter paper, etc., are suspended in the solutions 

 containing the acids, the reduction in toxic effect being probably 

 due to the adsorption of the acids upon the solid particles. Hence, 

 the concentrations which limit the toxic effects of these acids in 

 water solutions cannot be taken as representing the condition 

 with which the same plant will have to contend when growing 

 under normal cultural conditions. 



Alkalies. The caustic alkalies must usually be present in 

 from five to ten times as great concentrations as those of the 

 mineral acids, in order to produce the same injurious effects upon 

 the rootlets of common plants. The so-called " alkali " of soils is 

 not alkali at all, but is neutral soluble salts present in sufficient 

 concentration to exert a toxic effect. 



Salts of the heavy metals are especially toxic to rootlets of 

 plants. Salts of copper, mercury, and silver, have been found to kill 

 the roots of seedlings immersed in them for twenty-four hours when 

 present in proportions of less than three parts per ten million, 

 while salts of many other heavy metals are toxic when present in 

 concentrations of less than one part per million. The salts of 

 the alkali metals are considerable less injurious than are those 

 of the heavy metals, but even these exert their familiar injurious 

 effect if present in concentrations which, measured by the ordinary 

 standards, would still be regarded as very dilute solutions. 



Illuminating gas, and similar hydrocarbon gases, kill plants 

 when present in the atmosphere in as little as one part per million. 

 Leaves, buds, and roots are all alike sensitive to this toxic effect, 

 the nature of which is not yet understood. 



Formalin, or formaldehyde, is a penetrating toxic agent for 

 nearly all plant cells, and is commonly used as a fungicide for the 

 destruction of parasitic fungi. It probably affects the qplloidal 

 condition in some way similar to its hardening effect upon gela- 

 tin, etc. 



The toxic effect of many different organic compounds is so 

 varied in its nature and extent that it is impossible to give any 

 satisfactory brief review of its manifestations. Recent investiga- 

 tions appear to indicate that organic products of decomposition 



