462 
WRIGHT : FOLIAR PERIODICITY 
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This is an important point to notice, as it suggests that the 
difference between a conspicuously deciduous tree and one 
which undergoes partial defoliation at intervals may be due 
to the difference in the extent of exposure to light and wind. 
It certainly shows that by a slight change in the external 
environment the completeness and duration of defoliation is 
considerably altered. 
Another point of some importance is the direction of 
defoliation, which in most cases is probably influenced by 
exposure to light. Trees of Berrya Ammonilla, Spondias 
mangifera, and Bertholletia excelsa, growing on open ground 
at Peradeniya are invariably defoliated from above down¬ 
wards. 
Trees of Ficus altissima, F. bengalensis, and Lecythis 
Pohlii, growing under unequal shade are, on the other hand, 
defoliated first on one side only. The order of appearance of 
new leaf usually follows the same sequence as the fall of the 
old, the twigs which are first leafless receiving the first set of 
new leaves independent of their age. 
7.—External and internal Factors. 
Haberlandt, when discussing the question of internal 
forces, stated that in those tropical districts where there is 
abundant moisture throughout the year and where the 
evergreen “ rain forests ” are seen at their best, periodicity 
in leaf production is not to be observed, the trees throwing 
off their old leaves and developing new ones the whole year 
round. In such cases foliar phenomena can only have their 
origin in forces other that those of climate. He also points 
out, however, that in those districts of the tropical zone 
where there is a rainy season and a dry one the winter of 
the temperate zones is represented by the latter. 
Sachs, when dealing with the periodic alternation of 
vegetative activity and rest, stated that this matter must 
depend chiefly upon changes which take place in the resting 
plant, independently of external influences, or only in¬ 
directly affected by them. 
