of the Fall of the Leaf 239 
the amount of denudation will have a faithful correspondence 
with the phenomena which favour radiation ; and I am persua- 
ded, from repeated observation, that the leaves which strew the 
brooks are more numerous in the evening and at night than 
during day, when the calorific impressions of the sunbeam play 
on the leafy surface. 
There is a phenomenon which I do not remember to have 
seen any where estimated, though it seems to me to add force 
to the opinion presumed. In the leech and oak , &c. particu- 
larly in the former, the leaves begin to fall from the tips of the 
twigs or branches, and are shed gradually downward: now 
this is in exact conformity with what should happen in conse- 
quence of radiation ; the lower foliage is protected from its effects 
by that above it, the last, being lowered in temperature, falls, 
and exposes that leafage which before was comparatively con- 
cealed and sheltered ; it falls in turn, and is succeeded by the 
next, &c. We cannot suppose this phenomenon connected with 
the gradual retirement of the sap from the extremities of the 
branches. The twig can never be wholly exhausted of sap 
the circulation is comparatively languid, and its action torpid ; 
but were it to evacuate the branch entirely, it must cease to live. 
In torpid animals the circulation continues, though reduced to 
its lowest ebb. Experiment has determined this, and the ana- 
logy holds good. 
In spring, the temperature acquired by absorption more than 
counterbalances the loss sustained by radiation , while in autumn 
this last is a maximum. In very early spring, the premature 
blossom is often cut off by the sudden contraction of the vessels 
which supply it. 
By watering the early bloom before sunrise, the fatal conse- 
quence referred to is perhaps prevented ; because the effects of 
radiation are modified by a film of aqueous vapour; or the 
previously contracted vessels are softened and expanded to the 
more gradual reception of the temperature shed upon the flower 
by the solar ray. 
The corolla is a very interesting subject of inquiry. The 
celebrated Linnaeus conceived that the petals acted as wings 
to waft the flower up and down in the air. Others have believed 
