470 WRIGHT : FOLIAR PERIODICITY 
8 .— Advantages off Periodical Phenomena. 
It is impossible to assign one reason only to explain why 
the varions 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 tough 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 
in such districts. The trees throw off their relatively thin 
leaves before the winter sets in ; if they did not they would 
probably soon be destroyed by the melting of the ice crystals 
formed within their tissues ; furthermore they could not 
obtain the water required to make good that lost by slow 
transpiration during cold months, owing probably to the 
temperature of the soil being against root activity and the 
absorption of the necessary quantities of water. But it is 
obvious that such forces can be left out of consideration 
when dealing with trees in Ceylon 
A period of rest is characteristic of most temperate zone 
herbaceous plants, such as annuals which rest in the seed 
stage, perennials resting by corms, bulbs, &c. This resting 
feature is met with in many Ceylon herbaceous plants, such 
as Oxalis corniculata, L., species of Caladium and Amaryllis, 
Drosera peltata, Sm., &c., though there is perhaps not the 
same necessity for such a phase since the temperature and 
humidity are always more or less favourable to plant growth, 
and the soil of medium quality. 
It has been pointed out that the deciduous species in 
Ceylon do not pass through their leafless period at the same 
time, but during any month in the year even in the same 
district. The tropical species may choose a limited period of a 
few days or weeks or longer for dropping their old leaves and 
