426 WRIGHT: FOLIAR PERIODICITY 
renewal are effected that the chief point of interest lies. 
A description has been given in another part of this paper 
of how the evergreen species may produce new leaf every 
month in the year, twice per year, or annually, and the 
behaviour of the deciduous species will now be described. 
The necessity for an annual defoliation, though obvious for 
many species in temperate zones, is far from apparent in 
tropical areas where there is no cold season analogous to a 
winter. In parts of the tropics where the monthly humidity 
and rainfall are relatively high throughout the year it is 
difficult to understand why the cycle of leaf-fall and renewal 
should coincide with that of a period of twelve months, and 
it would be natural to expect that the trees would become 
partially or completely defoliated at any time when the 
condition of the plant required either a change in the 
intensity of activity of certain functions or when the 
old leaves had become comparatively useless. In all parts 
of Ceylon, however, there is a more or less constant change 
of the climatic factors, and the behaviour of the plants is 
consequently more interesting. 
The Age of the Trees when they undergo their first 
Leafless Stage . 
This question is of the utmost importance in determining 
the time-value of rings of growth. It is obvious that the 
central cylinder of wood in the stem may be uninterrupted 
by rings of growth so long as the young tree continues to 
acquire new leaves and does not pass through a leafless 
phase. 
Generally speaking, once a tree has passed through a 
normal leafless phase it become® defoliated every succeed¬ 
ing year. It has also been pointed out that some trees are 
not defoliated until they are several years old— Bassia 
longifolia; and there are others which though leafless 
every year in their early life become more or less ever¬ 
green at a later date. This has been noticed for trees 
of Poinciana regia at Peradeniya and for Dalbergia Sissoo, 
