The Effect of Cement Dust on Citrus Trees. 289 



limited to within a radius of about one mile from the place of 

 its discharge, the deposit being much heavier within that area. 



Professor J. T. Norton ascertained the amount of dust 

 deposited in two orange orchards near the mills by exposing dur- 

 ing one week a number of glass plates, coated with vaseline and 

 placed horizontally on supports about three feet high. At 

 fixed intervals, the dust thus collected was removed, freed from 

 the vaseline, dried and accurately weighed. From data thus ob- 

 tained it was found that the annual deposit of dust per acre in 

 these two orchards was 2.39 and 3.18 tons respectively. 



So serious was the resultant injury to the groves that their 

 owners were obliged to apply to the Superior Court of the county 

 for an injunction restraining the cement company from contin- 

 uing to discharge dust from their works. The testimony of 

 more than 150 witnesses was taken at the trial, and in this num- 

 ber were many of the most experienced growers and packers of 

 citrus fruits, as well as several botanical experts for either side. 

 Some of the facts brought out are not without interest to the 

 plant physiologist. 



The cement dust is deposited mostly on the upper surfaces 

 of the leaves, and microscopic examinations showed that the 

 stomata, which are all situated on the lower surfaces, were not 

 clogged, so that there was no evidence of interference with 

 respiration or transpiration. Nor was any effort made to show- 

 that the dust in any way hindered the pollination of the flow- 

 ers, except that it was said to deter the visits of bees. But in 

 the present case this was unimportant, as most of the varieties 

 of oranges and lemons grown here are seedless varieties and pro- 

 duce their fruit without pollination. From a botanical point 

 of view, therefore, the whole case narrowed down to a considera- 

 tion of the effect of the dust in interfering with photosynthesis. 



Cement dust differs from other dust in that by absorbing 

 dew and the hygroscopic moisture of the atmosphere, it forms 

 on the leaf surface a hard, opaque, pellicle which is not removed 

 by rain, and, indeed, not without difficulty by the hand. The 

 life of an orange or lemon leaf is not less than three years, and 

 may exceed that period. It is evident that the coating of leaves 

 of long duration, such as these, with a scale constantly increasing 

 in thickness, is far more seriously injurious than a deposit on 

 leaves which are annuallv deciduous. This was verv evident in 



