450 
WRIGHT : FOLIAR PERIODICITY 
climate is not always so moist and dull, and during the 
remaining part of the year certain changes may occur. At 
Buitenzorg, with a relatively equable climate and trans¬ 
piration throughout the year, the trees are more or less 
evergreen, and foliar periodicity is mainly determined by 
internal forces. 
In a tropical district like Peradeniya, where a marked dry 
season sets in, a large number of trees drop their leaves 
during the hot dry season between January and March, 
probably because they find it impossible to obtain the 
amount of water in the soil equal to that which would be 
lost by transpiration if they retained the full complement 
of foliage. Certain forms retain their full amount of foliage, 
and others are provided with adaptations which allow them 
to pass through a period of drought unimpaired. 
In a temperate zone the roots cannot absorb the required 
amount of water during the cold months, and the trees 
therefore drop their leaves for the winter and remain 
in that state until the warm spring weather arrives. 
We are therefore led to conclude that foliar periodicity is 
associated with transpiration, and that the latter varies 
according to the temperature and moisture of the air and 
soil. It remains to be shown how far the results ot‘ the 
observations made in Ceylon agree with these conclusions, 
and the questions connected therewith can now be con¬ 
sidered. 
Periodicity and Transpiration. 
From the foregoing considerations several interesting 
questions arise. The first is a question as to the extent of 
the reduction in transpiration which is effected by species 
passing through their leafless phase during the hot period 
in Ceylon. 
That defoliation in deciduous trees is associated with the 
hottest and driest period, and therefore probably with reduc¬ 
tion of transpiration, seems obvious from the facts : (1) that 
