THE EFFECT OF CEMENT DUST 



65 



Table VI gives the results of tests in which the concentration 

 of the dust solution was decreased to 1-300. Even here there 

 was very little germination. 



Calcium a toxic agent. It has been mentioned previously that 

 in the writer's opinion it is the alkaline calcium salt which pre- 

 \'ents germination. That such is the case is very strongly indi- 

 cated b\^ the following experiment : When a drop of the cement 

 dust solution is permitted to remain exposed to the air for an 

 hour or two, the greater part of the calcium crystallizes out in 

 the form of calcium carbonate. If tested when exposed, the 

 drop gives an immediate alkaline reaction with phenolphthalein; 

 several hours later there is no reaction, showing that the carbon 

 dioxide of tlie air has completely neutralized the alkaline salt. 



TABLE VI 



Tests with various concentrations of the cement dust solutions. All solutions of 

 10% saccharose. Checks same as the 10% solution in the preceding table 



Now if the pollen grains are put to germinate at once there is 

 practically no germination, as given in the table above. If, how- 

 ever, the drop is allowed to stand until neutralization occurs and 

 the pollen then put in, it germinates almost as well as the checks. 

 Numerous tests of the substance which crystallized out failed to 

 show anything present except calcium carbonate. If then this 

 is the only substance which it is necessary to remove in order to 

 get germination, the conclusion is inevitable that this is the toxic 

 substance. It must not be understood, however, that all the 

 calcium crystallized out. Even after remaining exposed to the 

 air for a long time, some calcium in the solution can be brought 

 down with ammonium oxalate. This calcium is probably in the 

 form of bi-carbonate and therefore gives no reaction with phenol- 



