April, 1928 
The Queensland Naturalist. 
63 
however, that the difference is not so great as to lead us 
to expect the difference in effect which is observed in 
plants. 
The important difference is in the amount of heat 
received by the plant during the day. In the morning 
the eastern side receives the direct rays of the sun and 
the temperature rises. Tim western side remains cooler 
until the afternoon when its temperature is raised. The 
eastern half' by this time shaded, does not cool down to 
the same temperature, as that of the western side in the 
morning^ so that taking the whole day into account the 
eastern side receives the greater degree of warmth. This 
hypothesis is supported by experimental evidence. In 
the case of Carnegiea gigantea, the Giant Cactus of 
Southern Arizona and Sonora, measurement has shown 
that 011 the eastern side the temperature is 2 deg. (I 
higher on the average than on the western. In this 
plant as it grows at Tucson, Johnson has observed that 
the flowers on the east side develop more rapidly and 
begin to open first. Those on the west side develop 
more slowly, and first begin to open many days after 
blooming lias started on the east side. Every plant has 
an optimum temperature which is most favourable for its 
growth. A small difference, such as that between the 
eastern and western sides of a plant may bring the tem- 
perature sufficiently near the optimum to cause very 
marked acceleration in growth. Molisch’s experiment 
shows that one part of a plant may be forced into action 
by heat while near-by parts remain dormant. lie im- 
mersed twigs of dormant branches in warm water (30 — 
35 deg. Q. or more) for ten or twelve hours and found 
that they soon commenced to shoot, whereas in the adjac- 
ent untreated parts the buds were unchanged in appear- 
ance. A. commercial application of growth acceleration 
by heat is the hot-house forcing of Cultivated plants. The 
smudging of the mango, which is adopted in some oriental 
countries to ensure the setting of the fruit, may have^ 
the same effect. Smoky fires are built round the trees, 
and it is often considered that the raising of the tem- 
perature is the stimulus to increased fruit production. 
The late Professor C. F. Baker, however, considered that 
the killing of sucking insects at the critical period was 
more likely, while other observers consider that the 
action may be due to, chemical stimulus from the smoke. 
Mosses, algae, and lichens are usually found on the 
southern side of rough barked trees in Brisbane. (The 
