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different powers which act on the adult tree, pro- 
duce different effects at different seasons. 
Thus in the early spring, before the buds expand, 
the variations of the temperature, and changes of 
the state of the atmosphere with regard to moisture 
and dryness, exert their great effects upon the 
expansions and contractions of the vessels ; and 
then the tree is in what is called by gardeners its 
bleeding season. 
When the leaves are fully expanded, the great 
determination of the sap is to these new organs. 
And hence a tree which emits sap copiously from 
a wound whilst the buds are opening, will no 
longer emit it in summer when the leaves are per- 
fect ; but in the variable weather, towards the 
end of autumn, when the leaves are falling, it will 
again possess the power of bleeding in a very slight 
degree in the warmest days: but at no other 
times. 
In all these circumstances there is nothing truly 
analogous to the irritable action of animal systems. 
In animal systems the heart and arteries are in 
constant pulsation. Their functions are unceas- 
ingly performed in all climates, and in all seasons •; 
in winter, as well as in spring; upon the arctic 
snows, and under the tropical suns. They neither 
cease in the periodical nocturnal sleep, common to 
most animals ; nor in the long sleep of winter, 
peculiar to a few species. The power is connected 
with animation, is limited to beings possessing the 
means of voluntary locomotion ; it co-exists with 
the first appearance of vitality ; it disappears only 
with the last spark of life. 
Q 
