54 INFLUENCE OF CHEMICALS 



When ready to start the test the seedlings are fastened to the cir- 

 cumference of this cork by means of pins thrust through small 

 split corks holding the seedlings, and the glass rod is pushed up 

 or down to allow the roots to be immersed over a greater part of 

 their length. Have the beaker half full of a solution of a strength 

 of ^-gVfr g ram equivalent per liter. Make up a second solution 

 of half of the above concentration, also a third and fourth of the 

 same strength. Fasten several seedlings in each beaker. Make 

 a fine India ink mark by means of a thread saturated with the 

 fluid and held taut by a pair of calipers, at a distance exactly 

 15 mm. from the tip of each root. Measure the amount of elon- 

 gation or growth of this apical portion of the root daily. The 

 data thus obtained will determine the effect of the acid upon 

 protoplasm, and will also fix the concentration producing fatal 

 results. 1 



80. Toxic Effect of Silver Nitrate. Make up fractional normal 

 solutions of silver nitrate and determine the degree of concentra- 

 tion in which the roots of Zea, or Phaseolus may survive (78). 



81. Effect of Oxalic Acid. Repeat 79 with oxalic acid as an ex- 

 ample of the organic acids. 



82. Toxic Effect of Potassium Hydrate. Repeat 79 with solu- 

 tions of potassium hydrate made up as above. 2 



83. Substitution Poisons. Certain substances may be classed 

 as substitution poisons, and comprise some of the sulphur com- 

 pounds and many nitrogenous compounds. The substitution 

 poisons attack chiefly the amido and aldehyde groups in living 

 matter. Many of these substances bear special relations to the 

 higher animals, by affecting specially differentiated masses of 

 tissue. An example of this is afforded by hydrocyanic acid, 

 which is much more highly poisonous to the higher animals than 

 to the lower forms. 



1 Heald, F. D. On the toxic effect of dilute solutions of acids and salts upon 

 plants. Bot. Gazette, 22 : 125. 1896. 



2 Kahlenberg and True. On the toxic action of dissolved salts and their electro- 

 lytic dissociation. Bot. Gazette, 22: 81. 1896. 



