34 VEGETATION OF THE PEAK DISTRICT [CH. 



The importance of wind velocity as regards vegetation is 

 that transpiration increases with the velocity of the wind. It 

 is not known whether the relation is in any degree precise, but 

 the general fact would appear to be incontestable. It will be 

 seen that there is a considerable increase in the velocity of the 

 wind as the higher altitudes are reached; and this fact, especially 

 when considered with the fact that the temperature decreases 

 at the higher altitudes, helps to explain, in no inconsiderable 

 degree, the paucity of the flora and the remarkable features of 

 the vegetation of the higher altitudes. It must be remembered, 

 also, that the peaty soil which characterizes nearly all the 

 higher summits of the district is usually regarded as being 

 physiologically dry ; and this, if the fact be so, further increases 

 the dangers of those plants which are exposed to excessive trans- 

 piration. The distribution of the days on which the velocity of 

 the wind reached 15 metres per second is shown on p. 33. 



Humidity of the Atmosphere 



Whilst the records for the two years indicate a mean 

 monthly humidity of the air of about 85 per cent., there are 

 remarkable fluctuations in the daily records which are inter- 

 esting when considered in relation to the conditions of plant 

 life. The important point, generally speaking, with regard to 

 atmospheric humidity, is that the lower the humidity the greater 

 the amount of evaporation or transpiration. Schimper (1903 : 4) 

 states that transpiration " constantly increases in proportion to 

 the dryness of the atmosphere." The daily records show that 

 the atmospheric humidity fluctuates considerably at the different 

 altitudes; and this variation is sometimes in the direction of 

 greater humidity at the higher altitudes and at other times in 

 the contrary direction. However, the lowest percentages occur 

 at the higher altitudes on the whole ; and, as plants, in order 

 to survive, must be adapted to the extreme conditions of their 

 habitats, it follows that plants at the higher altitudes are 

 disadvantageously situated not only as regards a lower tem- 

 perature and a greater wind velocity, but also, on the whole, as 

 regards a lower humidity of the atmosphere. Whether or 

 not this applies to altitudes much higher than 750 metres has 

 yet to be determined. 



