248 CHEMISTRY OF PLANT LIFE 



of plant residues in the soil may exert powerfully toxic effects 

 upon succeeding generations of the same, or of different, plants 

 growing on the land. But the experimental data and conclusions 

 concerning these matters are not yet accepted without question 

 by all students of plant science or of the problems of the productiv- 

 ity of the soil. In fact, it is yet an open question whether toxic 

 soil constituents are really an important factor in the so-called 

 " unproductivity " of certain soils. 



Alkaloids, and even the amino-acids which are produced in the 

 tissues of some species of plants, while not toxic to the plants or 

 organs which elaborate them, sometimes exhibit strikingly toxic 

 action upon other plant organs with which they are brought into 

 contact. There is, as yet, no satisfactory explanation of this 

 difference in behavior between plant tissues toward various organic 

 toxic substances. 



In fact, the whole subject of the toxic action of various sub- 

 stances upon plants needs much more study before it is brought to 

 the, point where it will afford definite knowledge of either the 

 physiological problems involved or of their practical applications 

 in questions of soil productivity, etc. 



