34 VEGETATION OF THE PEAK DISTRICT [CH. 
The importance of wind velocity as regards vegetation 1s 
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 lowést 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. 
