NOVEMBEB 5, 1009] 



SCIENCE 



653 



how great or oven what may be the injury to 

 vegetation. Bearing in mind that leaves are 

 the parts of the more highly developed plants 

 in which food is made under the influence of 

 light, and through which that exchange of 

 gases takes place which corresponds with the 

 more mechanical part of the process of respira- 

 tion in our ovni bodies, we see at once that 

 these functions of leaves may be interfered 

 with by dust. The exchanges of gases in food- 

 manufacture and in respiration take place 

 mainly through the openings, known as sto- 

 mata, in the epidermis of leaves. If these 

 openings are stopped or are closely covered, 

 obviously the passage of gases through them 

 will be correspondingly more difficult, slower 

 and less adequate. This will be possible, how- 

 ever, only if the particles of dust correspond 

 in shape, size and position to the stomata, or 

 are so compacted on the surface of the leaves 

 as to cover them. Examination shows that 

 some of the particles are small enough to clog 

 the stomata and that they do so on the leaves 

 of oak (Quercus lohata), fruit-trees and grape- 

 vines. The coarser particles form more or 

 less extensive crusts, thus covering over the 

 stomata. Although naturally most of the 

 dust settles on the upper side of the leaves, 

 the lower side does not entirely escape the 

 clogging or crusting over of its pores. But on 

 neither the upper nor the lower surface is the 

 covering of dust so thick and opaque as greatly 

 to interfere with the passage of light to the 

 inner tissues of the leaves. 



The effect, then, of an accumulation of dust 

 on the surfaces of leaves constitutes mainly a 

 mechanical interference with the proper ex- 

 change of those gases concerned in respiration 

 and in food-manufacture in plants. There is 

 no evidence that the dust on these leaves has 

 exercised any corrosive or otherwise poisonous 

 influence on the tissues, and so far as my pre- 

 liminary tests indicate, there is no evidence 

 of the presence of injurious substances in the 

 dust. The effect of the dust is mainly, if not 

 wholly, mechanical. But, interfering with the 

 supply of food-materials and with the proper 

 aeration of the plant-body, it must be more or 

 less injurious. 



Furthermore, whatever the effect of the dust 

 may be on leaves already grown and developed, 

 it is certain to be greater on young and grow- 

 ing leaves. The cement plant in question has 

 been in operation only a few months, since the 

 season's foliage was developed. The effect on 

 young parts is not known. It may be antici- 

 pated, however, that the setting of this ma- 

 terial on the rough or hairy surfaces of young 

 and growing leaves would not only interfere 

 with the exchange of gases above mentioned 

 and absolutely necessary to the health of 

 plants, but would offer a mechanical hindrance 

 to growth which would lead to distortions 

 more or less serious. 



Perhaps it will be objected that what I have 

 said is not probable, in the light of experience 

 elsewhere. For example, Haselhoff and Lin- 

 dau express the opinion that cement dust does 

 no harm to vegetation :^ but they speak of the 

 rains which wash off the foliage at frequent 

 intervals during the German summer. In this 

 part of California, on the other hand, condi- 

 tions are quite different. There is practically 

 no rain after the leaves of deciduous plants 

 have developed, although there may have been 

 abundant rain before then. The leaves are 

 not washed off frequently; usually they are 

 not washed off at all throughout the season. 

 In this part of California there are frequent 

 summer fogs which give very valuable mois- 

 ture to vegetation.^ These fogs would affect 

 ordinary dust very little, if at all, but they 

 would tend to set dust containing or largely 

 composed of cement. In dry weather cement 

 dust will tend to blow away, especially in high 

 wind, but it will be held more or less firmly, 

 in spite of wind, on leaves roughened by hairs 

 or made sticky by the wounds of aphids. Any 

 moisture, whether from the plants themselves 

 or from the air, which does not come with 

 sufficient force to ivash, will tend to fix the 

 dust, forming a more or less permanent crust. 

 Indeed, the rain which fell with considerable 

 force on one occasion between my two visits 

 removed far less dust from the surface of 



' Haselhoff & Lindau, " Die Bescbadigung der 

 Vegetation durch Rauch," Leipzig, 1903. 



' See a forthcoming paper by me in The Plant 

 ^yorld on the botanical conditions of this region. 



