418 
WRIGHT : FOLIAR PERIODICITY 
functions in plants, the characters and periodicities of which 
are to a great extent controlled by the adversities of the 
climate in which the plants and their ancestors have lived. 
In this paper an account of observed foliar activities of 
known endemic and indigenous arborescent species in 
Ceylon, and their variation in the same or different climates 
is given; the adaptability of introduced species will be 
dealt with in a subsequent paper. Exact information of 
the foliar periodicities of trees in the tropics has been 
repeatedly asked for by Ursprung, Schimper, and others who 
have worked at the subject of tropical vegetation. It is only 
by the possession of such information that the question of 
internal factors in plant life in equable or tropical areas can 
be determined. 
The investigation has also been carried out to determine 
how far external environment in a tropical area affects the 
annually recurring period of leaflessness. The information 
on this point has been gleaned from a study of the same 
species in various parts of Ceylon, Java, and countries men¬ 
tioned by Ursprung. 
The subject has also been studied with the view of throw¬ 
ing some light on the time-value of rings of growth in the 
stems of trees which are evergreen, partially or completely 
deciduous, and the general relationship of foliar periodicity 
to xylem differentiations in tropical or equable countries. 
Some palaeontologists have contended that the rings of 
growth in the stems of fossil plants indicated the existence 
of a temperate-zone climate ; and their absence, a more 
equable or tropical climate. The facts on this part of the 
subject will probably throw some doubt on such hypotheses. 
It was generally thought that there were very few trees 
which pass through a leafless stage in Ceylon. In Trimen’s 
“ Flora ” 16 species are described as being deciduous. The 
number of deciduous species herein described (excluding 
foreign ones) is only about ninety. 
The number of species now known to be deciduous will 
probably be greatly increased at a later date, when the 
