476 
WRIGHT : FOLIAR PERIODICITY 
in his Forest Flora of India, particularly Gmelina arborea, 
Erythrina indica, Chloroxylon Swietenia, Careya arborea, 
Bombax malabaricum, Cassia fistula, &c. 
10.—Foliar Periodicity of Fossil Plants. 
Information concerning the foliar periodicity of trees 
when grown in dissimilar climates is of great value. It 
may help one to interpret the significance of structures in 
plants which have existed in palaeozoic and mesozoic times. 
We have accurate knowledge of the periodicities of trees of 
the same species in different climates and the behaviour of 
the same species when introduced to different countries. 
The foliar periodicity, due in many cases to climatic forces, 
is to some extent responsible for the differentiation of rings 
of growth in the stem. The xylem differentiations met 
with in various fossil plants have been variously interpreted 
as indicating the equable or variable nature of the climates 
in which the plants have grown. Generally speaking, 
palaeontologists have concluded that the complete and 
distinct differentiation of rings of growth in the stems 
of fossil plants indicated periodic foliar activity, and therefore 
a climatic periodicity, analogous to that in temperate zones ; 
and also that the absence of the rings of growth was more or 
less associated with an evergreen type of vegetation such as 
one may expect to obtain in an equable or tropical climate. 
Many statements have been made apparently under the 
impression that foliar periodicity was inseparable from 
climatic periodicity, and the degree of distinctness of rings 
of growth in recent fossil woods has been frequently 
associated with a change from a relatively equable and 
tropical climate to one having a periodicity similar to that in 
middle Europe at the present day. 
It is hardly necessary to draw attention to the erroneous 
idea that a period of foliar rest and activity is undiscernible in 
arborescent tropical plants in various parts of the world, or 
that the tropical vegetation is characterized by uninterrupted 
