440 Plant Physiology 



Many species of the lower algae are particularly sensi- 

 tive to certain toxic agents, such as the salts of copper and 

 other heavy metals. Zoospores of fungi and some species 

 of bacteria pathogenic in animals may be equally sensitive. 



265. Toxic action and the substratum. Much of the 

 literature of toxic action is confusing, owing to the fact that 

 the results are not comparable. Substances usually ex- 

 hibit their greatest toxicity in distilled water. Any nearly 

 neutral nutrient solution reduces toxic action even in cases 

 where molecular readjustments would not seem to be im- 

 portant. In the soil complex physical and chemical con- 

 ditions prevail, and these further modify toxic action. 



Solid particles, such as pure sand, graphite, and filter 

 paper, may reduce toxic action to a considerable extent. 

 True and Oglevee found that twice as much sand as solu- 

 tion may reduce the toxic action of CuSCh for Lupinus 

 albus as much as thirty-two times (Fig. 124). The method 

 of reducing toxicity by solid particles is usually denoted ad- 

 sorption. It is a phenomenon explained upon the hypoth- 

 esis that many molecules or ions of the toxic substance are 

 physically held by the surfaces of the particles of the inert 

 material, and are, for the time, removed from the possi- 

 bility of chemical action. Another explanation is that the 

 solid substances offer obstacles to the free movement of 

 the solvent particles. Possibly both views are important. 

 Many of the so-called absorptive properties of soils both 

 respecting fertilizers and deleterious agents are in reality 

 adsorptive. 1 



1 The table from Jensen, on the opposite page, affords a comparison of 

 toxic action in sand and in solution cultures. 



