4/U WRIGHT : FOLIAR PERIODICITY 
8. -Advantages of Periodical Phenomena. 
It is impossible to assign one reason only to explain why 
the various species undergo a leafless phase once or twice each 
year. In parts of Ceylon the conditions are favourable to 
growth throughout the whole year. The flora is mainly of 
an evergreen type, but many of the deciduous species select 
the driest period for their leafless phase. In the temperate 
zones only those trees the leaves of which are toogh and 
leathery, or are surrounded with a thick cuticle, or are so 
constructed that transpiration can never be rapid, retain 
their leaves throughout the winter months. The advantages 
which accrue from a leafless period in Europe are obvious, 
and on the defoliation may depend the life of the species 
n such distncts. The trees throw off their relatively thin 
t: rr sets in; if ** did »«* ^ 
* -w—— 
zame necessity for such a’phJsta^a , 6 9 . PerhaPB n<>t ““ 
humidity are always mow.« 1 . th tem P er atare and 
Ceyl.^ p^tfont^ , the fl d6<!id "onz species in 
time, but during any month ^ l Peri0d a ‘ the ““>* 
district. The tropical species mu ^ eT6n io the same 
few days or weeksor longer for dm ° 08ea f* mited P eri »d a 
ger tor droppmg their oM leave3 md 
