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The Plant World 



longer, but their moisture is then less needed owing to the rains 

 which they commonly follow. The fogs of ocean origin come as 

 streams from the great bank of fog, fifty miles wide and over 

 one thousand miles long, which lies along the Pacific coast most 

 noticeably in summer. * These fog streams, coming generally 

 in the afternoon, flowing over the passes and through the can- 

 yons, spread out on the valley floor to varying distances, and 

 becoming confluent may there attain a depth which at times 

 equals the height of the passes. The check which fog offers to 

 evaporation, from soil and vegetation, is obvious; but beyond 

 this conservation of moisture, very valuable in itself, there is 

 an actual contribution of moisture which we may someday be 

 able to reckon in inches of precipitation. There are many plants 

 in this region which absorb moisture from fog and become wet, 

 for example lichens, * * mosses, etc. Other plants seem ac- 

 tually to comb out moisture, which falls in drops from the leaves 

 and branches. I have heard of two instances where men have 

 made use of this phenomenon. One settler in the mountains 

 between here and the sea-coast, having an otherwise inadequate 

 supply, collects the fog product of a certain group of trees and 

 thus secures enough water for his stock. This case is extreme, 

 but it fairly indicates the value of fog to plants which thrive 

 where no rain falls during four or more months of each year. 



At the same time that I thus emphasize the value of fog to 

 the perennial plants of this region, I must also speak of the 

 amount of light and sunshine. It is greatly to be regretted that 

 there are no adequate instruments for measuring and recording 

 the amounts and qualities of light reaching the earth's surface. 

 The various "sunshine recorders'* leave very much to be de- 

 sired. The verbal communications of experiences of photogra- 

 phers confirm my own impression regarding the amounts and 

 qualities of sunlight in different regions The far-famed clearness 

 of the summer air in the Sierra Nevada Mountains results in 

 "over-exposures when the photographer in that region does not 

 adjust his camera accordingly. The impression, as one travels 

 eastward from coast to coast, is not of increasing cloudiness so 

 much as of decreasing brightness; the sunlight of the Mississippi 



*Mc Adie. A. G. Ice. cii., p. 239 et seq. 



**Peirce. G. J. Nature of the association of alga and fungus in lichens. Pro. Calif. 



Acad. Science. 3rd Ser.. Bot.. Vol. I, 1899. 



