472 
WRIGHT : POLIAK PERIODICITY 
therefore only obtain new tissues by casting off the old 
members and producing entirely new leaves. It is obvious 
however that the same change can be obtained by a monthly 
fall and production of leaf. With respect to the possible 
necessity of throwing off the useless chemical ingredients 
when they have accumulated beyond the desirable degree of 
concentration, it may be stated that the yellow leaves of Eryth- 
rina indica as they fall from the tree contain 7 per cent, of 
lime. Nevertheless, it is erroneous to suppose that trees 
when throwing off their old yellow leaves are casting off 
members containing hardly any valuable food ingredients. 
Mr. Bruce analyzed several of the yellow fallen leaves of the 
common dadap (Erythrina indica) and found that they 
contained no less than 1*4 per cent, of nitrogen, 056 per cent, 
of potash, and 0*11 per cent, of phosphoric acid. These ingre¬ 
dients are not available in large quantities in Ceylon soils and 
the loss to the plant, temporary though in some cases it may 
be, is one which is not always seriously considered. 
The main advantages are, however, probably to be asso¬ 
ciated with the change in functional activity of parts of the 
plants. It is not necessary that the leaves be continually 
exposed to light in order to successfully carry on the 
various functions of the plant. Experiments "with various 
plants have proved that when light is supplied intermittently 
to the assimilatory organs the various functions are carried 
on as effectively as when light is continuously supplied. 
And the same may be said of our deciduous species, the 
leaves of which may be exposed to light for a varying 
number of months each year. By passing through the 
various stages of leaf-fall, leaflessness, and foliar renewal 
the assimilatory organs of the plant become subject intermit¬ 
tently to the stimulus of light, and on this depends the 
varying activity of many functions. 
The variation in the activity of certain functions due to 
annual defoliation may be attended with many advantages. 
The plants obtain a period of rest as far as assimilation 
of carbon compounds is concerned, and the check to the 
