464 WRIGHT : FOLIAR PERIODICITY 
tiens. The curves on plate XXIX. show that one or more 
species may be defoliated during every month in the year. 
In the third place it is of interest to note that plants of 
the same species, e.g., Cassia Fistula, may undergo defolia¬ 
tion and foliar renewal at approximately the same time 
of the year, though growing in the dissimilar climates of 
Peradeniya, Colombo, Vavuniya, and Mannar; this suggests 
that the foliar periodicity is inherent and will make itself 
evident even under a variety of exacting climatic con¬ 
ditions. 
Another point which indicates the power of internal 
forces is that outwardly similar plants of the same species, 
within a score or two of yards from each other, are deciduous 
at periods varying by many weeks and months. Haberlandt 
quotes Palaquium macrophyllum as a species which at 
Buitenzorg shows an independence of external meteorologi¬ 
cal conditions in so far that individual trees growing in the 
same neighbourhood throw off their old leaves and produce 
new ones during different months. 
If one selects trees of Lageï strœmia Flos-reginæ, Retz., 
Bridelia retusa, Spreng., or species of Canarium or Palaquium, 
he will find that though they exist alongside each other 
under conditions which are physically identical, yet the 
time of fall of old leaf, production of new leaf, and of flowers 
differs considerably in specimens of the same species. In 
the case of Bridelia retusa the variation is one of months, 
and to see individuals of this species on the same plot, 
some dropping their leaves, others quite bare, many in 
full old leaf, and others in full new leaf and flower impresses 
one with the significance of the internal forces. 
Another consideration which may be put forward to 
indicate independence of external forces is the behaviour of 
the trees in respect to the dry period of the year. If there 
is one part of the year when the climatic conditions are 
likely to exercise their power it is the relatively dry period 
during February and March. It cannot be questioned that 
conditions favour excessive transpiration during that period, 
