[Vol. 2 



508 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



cytolysis or death of the cells (though it should he noted here 

 that excessive exosmosis from the roots may result even when 

 those tissues are in an apparently normal condition), and at 

 the lower end of the scale the curve of slight exosmosis that 

 is in the region of the normal curve of exosmosis for un- 

 treated plants placed from full nutrient solution into distilled 

 water. Between these two extremes lie various gradations 

 depending on conditions. 



II. Historical Review 



The work that has been done on the problem of excretions 

 from the roots of plants is very interesting from several stand- 

 points and has been considered by various workers to be of 

 great practical importance. Nearly a century ago De Candolle 

 ( '32) advocated a theory of crop rotation on the basis of root 

 excretions in which he claimed that certain plants excreted 

 from their roots substances which are harmful to succeeding 

 crops of closely related plants, but not so to plants less closely 

 related. This theory was based partly on his own observations 

 and partly on the statements of earlier workers. 



At De Candolle 's suggestion Macaire ('32) performed some 

 experimental work pertaining to root excretions. He took 

 plants from the soil, washed the roots carefully, and placed 

 them in rain water. After several days, during which the 

 water was frequently changed, the water was yellow and had 

 odor, taste, and chemical reactions indicative of contained 

 exuded materials. By placing one part of the roots of a plant 

 in a vessel of pure water and another part in a second vessel 

 containing a solution of lead acetate and later finding the salt 

 in the pure water, he concluded that a plant can excrete a 

 poison which it has absorbed. The results of Macaire 's ex- 

 periments with water cultures led him to favor the theory of 

 crop rotation on the basis of the excretions from the roots of 

 plants, as advanced by De Candolle. 



Braconnot ('39) repeated many of Macaire 's experiments 

 but was unable to convince himself that plants excrete toxic 

 substances from their roots, and hence he did not look with 

 favor upon De Candolle 's theory. Braconnot believed that 



