420 
WRIGHT : FOLIAR PERIODICITY 
and the growth in length of roots, and concludes that there 
is no season which does not set certain functions of the plant 
in activity. There is however a foliar periodicity of tem¬ 
perate zone vegetation which is obviously directly associated 
with the seasons. 
Periodicity in the Tropics . 
In the tropics there may or may not be a climatic periodi¬ 
city analogous to the seasons of temperate zones. In all 
tropical areas heat and light are intense, and these, together 
with the heavy rainfall of many places, result in a conspicuous 
growth of vegetation at most times of the year. Schimper 
stated that his observations in tropical districts, with an 
abundant rainfall at all seasons, convinced him that the life 
occurrences of the plant show a rhythmical change of periods 
of rest and activity. 
The investigations of Sachs and Muller-Thurgau have 
proved that there is no such thing as a resting period for the 
whole of the life occurrences of the plant, but only resting 
periods for certain functions. 
The tropical plants are just as subject to periodical changes 
of rest and activity as the plants of cooler zones. 
Schimper asserted that where a marked climatic periodi¬ 
city prevails the functions of the plants are influenced 
thereby, the dry seasons operating in many respects like 
cold ones. He contended that where the climate is nearly 
uniform the internal causes are chiefly or alone influential 
for the change of rest and activity. Such periodicity is 
founded on the existence of the organism and not on external 
conditions. Schimper believed that their connection with 
the latter is quite secondary and an adaptation. He also 
stated that the difference between such a vegetation and one 
subject to the influence of alternating favourable and un¬ 
favourable seasons is that in the first case the amount of 
rest and activity in every period or generation remains 
approximately the same, while in the second case it has a 
periodic increase and decrease. 
