456 
WRIGHT : FOLIAR PERIODICITY 
temperature alone, or the variation in temperature on 
defoliation, is of minimum importance, and in comparison 
to the power of other factors may be almost neglected. It 
would be much more difficult and perhaps impossible to 
determine the effect of temperature on defoliation in Europe, 
where the great differences in seasonal climate are so powerful 
and all important. 
Of the three factors, temperature, humidity of the air, and 
rainfall, the variation in temperature is probably of least 
importance in determining whether a tree shall or shall not 
be deciduous. 
Moisture. 
The effect of changes of moisture on vegetative growth 
has been worked out for many plants in the temperate zones 
and for a few species in the tropics. 
Reinke worked with species of Helianthus, Datura, and 
others ; Baranetzky and Godlewski with Brassica, Phaseolus, 
&c.; Darwin with Cucurbita and Kraus, and Lock with 
tropical bamboos. 
In most cases a relationship between the rate of growth of 
various parts of plants and the hygrométrie state of the 
atmosphere was established, and in the results of Lock one is 
struck with the remarkable dependence of the rate of growth 
on the changes in the humidity of the air. The relationship 
between the production of new leaf on the tropical trees 
here considered and the change in hygrométrie conditions 
is equally striking, and furnishes another proof of the great 
importance of this factor in the growth of leaves in the 
tropics. 
(a) Rainfall. 
In a district like Peradeniya there is a coincidence 
between the period of drought and the number of deciduous 
species, the minimum monthly rainfall and the maximum 
number of species undergoing defoliation being in February 
and March. During August and September there is a 
marked decline in the monthly rainfall, but only one or 
two species are conspicuous in their leaf-fall during that 
