IN CEYLON. 
419 
hrubby vegetation has been more closely studied. Shrubby 
plants are, on account of their being so small and having so 
few leaves, liable to be overlooked. It is only when they 
attain the size of a mature plant of Pavetta indica, or Brucea 
sumatrana that a heavy fall of leaf is likely to be noticed. 
2n— Periodicity in Growth. 
Though there are great differences in the periodicities of 
vegetative growth in all climates, they can usually be traced 
to external forces such as moisture, food supplies, temper¬ 
ature, &c., or they are the expression of apparently indepen¬ 
dent activities of the whole or parts of the plant or plants 
under consideration. We are here concerned with the 
periodicity of defoliation and the production of new leaves. 
Periodicity in Temperate Zones . 
In temperate zones, where seasons are pronounced, the 
leaves of certain trees discolour and fall during autumn and 
new leaf production is delayed until the warm spring time 
arrives. This is characteristic of trees which have thin and 
tender leaves. Those trees the leaves of which are tough 
and leathery, reduced in area, or covered with a thick cuticle 
may retain their leaf throughout the year. Examples of the 
former class are the Birch, Beech, Hawthorn, &c., and of the 
latter, Holly, Horse, Conifers, &c. It is usual to regard the 
leafless period of these plants as one of rest, and in so far 
that there is a cessation, almost entirely in species with 
woody twigs, of the work of assimilation and the processes 
associated with this function, a period of relative inactivity 
of these functions may be acknowledged. Complete rest, 
however, would involve the cessation of all functional acti¬ 
vity, and this cannot occur until the death of the plant. 
Schimper stated that during the winter months of temperate 
climates various changes may be taking place in the plants, 
such as conversion of starch into oil, chlorophyll into red 
colouring matter, production of cyanophyll in the epidermis, 
