246 CHEMISTRY OF PLANT LIFE 



Toxic substances may act injuriously upon plant tissues in a 

 variety of ways. Many electrolytes, especially the salts of the 

 heavy metals of high valency, coagulate protein material and the 

 entrance of such substances into the protoplasm causes disturb- 

 ances in the colloidal condition which cannot be otherwise than 

 injurious to its normal activities. Similarly, formaldehyde and 

 many other organic compounds may affect the colloidal properties 

 of the protoplasmic gel in such a way as to injure the plant tissues. 



The same substance is sometimes much more injurious to the 

 tissues of one part of a plant than it is to those of another part 

 of the same plant. Thus, the rootlets of a young growing plant are 

 much more susceptible to injury by many mineral salts than are 

 the vegetative parts of the same plants; while anaesthetics of 

 various kinds generally exhibit their greatest injurious effects upon 

 the leaves, or synthetizing cells. Again, the mycelia of fungi are 

 much more easily killed by toxic agents used as fungicides than are 

 the spores of the same fungi. Some of these observed differences 

 in toxicity may be due to differences in the physiological effect of 

 the substance upon the protoplasm of the tissues which it enters, 

 and others may be due to differences in the resistance of the pro- 

 toplasm, or of its protective coverings, to penetration by the toxic 

 material. Indeed, the possibilities of different types of toxic 

 action, and of resistance to it by individual plants and species, 

 are so varied that it is not possible to divide toxic agents into spe- 

 cific groups according to the nature of their injurious action upon 

 the plant cell. They are, therefore, more commonly grouped 

 into classes according to their chemical nature and economic 

 significance as fungicides, as follows: inorganic and organic acids; 

 caustic alkalies; salts of the heavy metals; hydro-carbon gases; 

 formaldehyde; alcohols and anaesthetics; nitrogenous organic 

 compounds; and miscellaneous decomposition productions of 

 organic origin. The following brief review of some of the results 

 of the experimental studies of the toxicity of different compounds 

 belonging to these several groups will serve to indicate the general 

 trend of the investigations of these matters which have thus far 

 been made. 



Acids. The common inorganic acids (hydrochloric, nitric, and 

 sulfuric) kill the rootlets of common farm crops when the latter 

 are immersed for twenty to twenty-four hours in solutions of these 

 acids containing from three to five parts per million of free acid. 



