138 Mr. Knight on the office 
appears, however, to be purely mechanical, and not to be in 
any degree dependent upon the vital power of the tree ; and 
some writers on vegetable physiology have regarded it as a 
wholly lifeless substance. This opinion I have always re- 
jected, though I was unable to adduce any decisive evidence 
in opposition to it ; but I have now reason to believe that the 
heart wood becomes, during winter, in common with the 
alburnum and bark, a reservoir of the organizable matter 
which the tree expends in germination in the spring ; and 
that the fluid sap passes abundantly into it laterally, though 
it does not ascend through it. 
I had long previously been perfectly satisfied that every 
species of tree, and perennial plant, contains within itself, 
during winter, all the organizable matter which it employs in 
the formation of its first foliage, and succulent shoots, in the 
spring ; and that it is owing to the presence or absence of 
such reservoir, that the lives of plants become annual, bien- 
nial, and perennial. The annual plant exhausts itself wholly 
in feeding its flowers and seeds : it forms no reservoir, and 
consequently perishes. Its vital powers are not expended, for 
detached parts of the same plant, and obviously possessing 
the same life, become perennial, if planted in such season 
that they cannot exhaust themselves by the production of 
flowers before winter. A biennial plant (the common turnip 
affords a good and familiar example) fills its reservoir in one 
season, and wholly expends it in the following, when it con- 
sequently dies, like the annual plant. In the tree, as in the 
biennial plant, a part of the reserved sap descends early in 
the spring to form new roots, whilst another portion ascends 
to feed its buds ; but the tree also discharges laterally a large 
