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WRIGHT : FOLIAR PERIODICITY 
(5) Atmospheric Humidity. 
The humidity of the air is probably one of the most 
important factors in the defoliation of trees in the tropics. 
It can be shown by comparing Peradeniya, Hakgala, 
Colombo, Badulla, and Mannar, that the maximum yearly 
humidity obtains at Hakgala where the vegetation is 
more nearly evergreen than at any of the other places, and 
that the minimum humidity is at Peradeniya where the 
vegetation is perhaps more deciduous than in any of the places 
mentioned. The high humidity at Mannar—four degrees 
higher than at Peradeniya—is probably responsible for the 
stunted and often succulent nature of the vegetation and 
for the non-increase in the number of deciduous species 
compared with Peradeniya. 
It is important to notice that the Peradeniya, Hakgala, 
Colombo, and Badulla districts, which represent different 
climates, ranging from sea level, to over five thousand feet 
above sea level show a definite relationship between a 
particular stage of humidity and defoliation. In all these 
districts it is only when the humidity falls near to or helow 
70 degrees that the majority of the deciduous species drop 
their leaves. 
The best agreement is, however, between the varying 
humidity and number of deciduous trees at a given place, 
throughout the year. In each of the districts mentioned 
the number of deciduous species increases with increased 
dryness of the air and decreases with an increase in the 
dampness of the air. If the curves showing the relation¬ 
ship between humidity and number of deciduous species 
at Peradeniya are examined it will be seen that the 
minimum humidity is experienced in February and March, 
and these are the months when the maximum percentage 
number of deciduous species drop their leaves or become 
leafless. In April and May, when the temperature is higher, 
the number of leafless trees does not increase, but actually 
decreases on account of an increase in the dampness or 
