THE WATER OF THE HABITAT 27 



pressure. Altitude has been thought to influence vegetation chiefly 

 by virtue of increased light and decreased heat. Recent studies of 

 the author seem to prove conclusively that light is of practically 

 no importance, and that the marked dwarfing of alpine plants is 

 due largely to the great rarefaction of air by which evaporation is 

 increased. Exposure, i.e., the position of a slope with respect to 

 the sun affects humidity through the action of sun and wind. 

 Slopes longest exposed to the sun's rays receive the most heat; 

 consequently slopes with a southern exposure regularly show some- 

 what lower humidities than those with northern exposures. The 

 effect of wind is most pronounced upon those slopes exposed to 

 prevailing dry winds. As a rule, these are southern or southwestern, 

 and for reasons both of temperature and wind these are usually the 

 driest slopes of hills and mountains. Cover increases humidity by 

 reducing the influence of temperature and wind. In addition a 

 living cover supplies moisture to the air in consequence of evapora- 

 tion from the plants that compose it. A similar effect is produced 

 by the water content of moist soils, particularly in forests and 

 thickets where the air is sheltered from sun and wind. 



35. Effect of climate and habitat. The general humidity of 

 a habitat depends upon climate and location with respect to bodies 

 of water. In comparison with each other forested regions show 

 high humidities, while deserts have low humidities. Coast regions 

 are moist, inland regions relatively dry, lowlands are more humid, 

 table-lands and mountains less humid as a rule. In a particular 

 habitat the relative humidity approaches or reaches saturation 

 during rain or fog, and then gradually decreases to a minimum 

 just before the next rain-storm. There is also a daily maximum 

 and minimum. The highest relative humidity, except when dis- 

 turbed by rain, usually falls at 3 or 4 a.m. It decreases slowly until 

 7 or 8 a.m. and then falls more rapidly to a minimum at about 4 p.m., 

 from which point it rises slowly to the maximum. Variations within 

 the habitat arise chiefly through differences in protection frr)m sun 

 and wind. For somewhat similar reasons the relative humidity is 

 greatest just above the surface of the soil; it is less at the level of 

 the vegetation, and still less a meter or more above the latter. 



36. Measurement of humidity. Humidity is measured by 

 means of a psychrometer. Of the latter there are three types: 

 the sling, the cog, and the stationary psychrometer. All consist 

 of a wet-bulb and a drv-bulb thermometer set in a case. The fii-st 



