IN CEYLON. 
471 
for complete renewal of foliage. Trees classed as evergreens 
may undergo the same change within the year and the question 
often resolves itself, in the tropics, into one of the possible 
advantages which accrue from throwing off the old worn- 
out leaf tissues quickly. 
There is, in this phenomenon, a withdrawal of materials 
from the leaves before they are allowed to fall : this is 
followed by a leafless period during which the assimilation 
of carbon compounds, circulation of materials and construc¬ 
tive metabolism generally, are at the minimum. This semi- 
stagnant period of relative rest is followed by one of great 
activity, first in the using up of store materials for production 
of new leaves, then in transpiration and root activity, due 
to the exposure of the numerous thin leaves and the absorp¬ 
tion of water and plant food. The alternation of rest and 
activity is the principal feature in these phenomena, and 
species which exhibit it may thereby obtain certain advan¬ 
tages, though what the latter may be is by no means 
always clear. 
There appear to be in Ceylon three possible sources of 
advantage, which may be of importance in this phenomenon, 
viz.—(1) checking the loss by transpiration during the 
drought ; (2) throwing oft* worn-out tissues and useless 
ingredients ; (3) functional changes during the different 
periods. With respect to the first question it has been 
pointed out, when discussing transpiration and foliar perio¬ 
dicity, that a reduction in the amount of water lost by 
transpiration is effected by species which behave similarly to 
Bombax malabaricum, Spondias mangifera, &c., but that in 
other cases the loss by transpiration may not be reduced but 
actually increased. 
In the second case it is suggested that the defoliation of 
tropical species in a climate which is comparatively regular 
throughout the year is probably more a question of casting 
off useless ingredients and worn-out cells and tissues than 
anything else. It is well known that the leaves of nearly 
all our tropical trees do not possess a cambium, and they can 
