RELATION OF CONDITIONS TO HABITAT DISTINCTIONS. 45 



The fog is of two sorts — a moving, wind-driven, relatively dry fog 

 seldom accompanied by rain, and stationary fog of high humidity and 

 often accompanied by drizzling rain or a heavy downpour. I have 

 observed on several occasions that the moving fog may pass without 

 influence on the humidity of the air. At Cinchona, on the late after- 

 noon of February 28, 1906, 1 obtained identical psychrometer readings 

 before, during, and after the passage of a wind-blown mass of fog, 

 the humidity being 94 per cent. 



The continual high humidity of the Windward Ravines is exhibited 

 in plate 24, figure B, and plate 25, figure B, both of which were secured 

 at the floor of ravines in the vicinity of Morce's Gap. Climatic and 

 topographic conditions join with the sheltering effect of the forest 

 itself and its immense evaporating surface to give to this habitat con- 

 dition of moistness which can hardly be exceeded in any locality on 

 the globe. The degree to which the surrounding vegetation and its 

 wet surfaces are accountable for the steady maintenance of these high 

 hiunidity conditions is revealed in the trace shown in plate 24, figure A, 

 which was taken in a tree top 38 feet from the ground and directly 

 above the spot in which the trace in plate 25, figure B, was secured two 

 weeks earlier. 



In similar fashion plate 24, figure A, exhibits the play of moisture 

 conditions on a ridge within 500 yards of the location for plate 24, 

 figure B. There was rain all day on Saturday and Sunday, giving the 

 ridge the conditions of a ravine, but on the earlier days of the week 

 fluctuations of humidity were recorded commensurate Avith those in the 

 tree top. The ridges are exposed to air movements which prevent the 

 attainment of the highest humidities and accelerate the drying of the 

 natural evaporating surfaces of the forest. 



The trace shown in plate 26, figure A, exemphfies well the average 

 conditions in Windward Slope forest, being intermediate between 

 ravine and ridge conditions. The greatest fall in humidity, coming 

 just at daybreak, is followed by either a sudden or a gradual rise which 

 is continued through the night. 



The humidity conditions of the Leeward Slopes may be judged from 

 plate 23, figure A, and plate 27, figure A. The former was taken in 

 November in the physiological laboratory at Cinchona, a small building 

 with windows and jalousies on all sides; the latter in April, in young 

 ruinate near Cinchona. Both traces exhibit rapid and continuous 

 fluctuations which carry the humidity to relatively low percentages 

 during a large portion of the day. The laboratory and ruinate curves 

 are comparable as respects the localities in which they were taken, but 

 not as respects the months of the year, for the humidity conditions in 

 November may be expected to exceed in every feature those of April 

 (see fig. 1). 



Two traces have already been published^ which go to show that the 



'Shreve, F. Studies on Jamaican Hymenophyllacese. Bot. Gaz. 51 : 184-209. Mar. 1911. 



