An Effect ofCement Dust on Orange Trees. 287 



indicated by the difference in the amounts of starch in the two 

 halves of the leaf. 



This decrease in the amount of food made daily in the leaves 

 of orange and lemon and grape fruit has resulted in a decreased 

 yield, as well as in a decreased growth of the trees. I am told 

 that tlie yield decreases in spite of cultivation being continued 

 with care, skill, and thoroughness. Indeed, the growers, finding 

 their crops decreasing, appear to have emploved everv possible 

 means of cultivation, fertilization, etc., to maintain the pro- 

 ductiveness of their crops, but in this they appear, without ex- 

 ception, to have failed. 



In a note published in Science in November, 1909, I spoke 

 of the possible effect of cement dust on plants growing in one 

 of the fertile valleys not far from San Francisco. The major 

 part of the vegetation, both native and cultivated, of the valley 

 described in that paper is composed of annuals or of plants which 

 lose their leaves annually. In that paper, also, I spoke of the 

 fineness of the particles of dust which coated all exposed surfaces 

 in the neighborhood of the cement works there. I was very much 

 interested to compare with the varied and deciduous vegetation 

 of the San Ramon Valley, near vSan Francisco, the peculiar and 

 homogeneous evergreen vegetation of the San Bernardino Val- 

 ley in Southern California, both as to the vegetation itself and 

 also as to the effect of dust upon it. The more or less permanent 

 coating of leaves which last for two or three years, or possiblv 

 longer, with an opaque crust, will necessarily produce a greater 

 and more continuous effect than a covering which must be re- 

 newed with the annual production of leaves. 



In the case of deciduous leafed plants in the San Ramon 

 Valley, near San Francisco, I was not able to carry on anv ex- 

 periments, owing to the lateness of the season, but my repeated 

 visits to the orange groves of the San Bernardino Valley have 

 made it possible to show, as I have above described, one result 

 of the interference of one industry with another. There mav be 

 other effects produced by the dust which escapes from cement 

 manufactories, but the result which I have just now described 

 is both easily demonstrated and greatly to be regretted. It 

 would seem to be necessary so to modify the method of manu- 

 facturing cement that this necessary commercial enterprise can 

 be carried on without interference with the rights of others al- 



