182 
ON THE DESCENT OF THE SAP. 
therefore, though believing that the presence of sap, after being 
elaborated in the leaf, is necessary to vigorous accretion, there is no 
anomaly in perceiving a slight advance upwards from the under side of 
a ringed stem, since we know that the bark possesses in itself an 
appropriating elaborating principle, as upon removing a piece of bark 
from one plant, and filing the space accurately with a piece from 
another of a different variety, although it be destitute of any bud, the 
wood formed under it in every successive year would be of the same 
quality as the identical plant from which it was taken. 
After mentioning these facts, I would not ask you to believe that 
the fresh layers of liber and alburnum are formed fortuitously from 
the descending sap; but I would ask you, granting meantime the full 
benefit of your indusium , if you do not consider that sap necessary to 
its vigorous development ? If not, then I desire to know upon what 
principle you can account for the facts, that the under side of a ringed 
stem increases not equally with the upper ; that upon making two 
annular incisions on a stem, and leaving a space between them 
destitute of a twig or a bud, this space will make most trifling advances 
either upward or downward; that trees bleed freely in spring before 
their leaves are expanded, and in the end of autumn, when they are 
falling or withering; that a twig stripped of its leaves in the growing 
season, if it does not die, makes no addition to its substance; that a leaf 
and lateral shoot together, taken from a vine-branch, tend to render the 
buds in their axil abortive in a succeeding year; and, not to be tedious, 
that I have witnessed the industrious cottager obtain few tarts from his 
rhubarb plants this season, from having pulled the leaves too closely 
in the last ? The determination of these points I conceive to be of 
more importance, as respects the practices of gardening, than the 
establishment of the indusial membrane. I have as yet had no proofs 
of its existence so far as my organs of vision are concerned, and, there¬ 
fore, for the present, I must remain rather sceptical, unless I identify 
its existence with the inner layer of the liber. Besides, though plausible 
at first sight, I am not convinced that granting the existence of such a 
member throws any additional light upon the secret processes of 
vegetation. I think you must grant that the descending fluid must be 
present, even upon your own system, to enable the indusium to develop 
the fresh layer of alburnum and liber. Grant this, and we need little 
more concession. It is true that some represent the regular mutations 
and accretions of plants as merely accidental, but, abjuring such a 
system, I see nothing more unphilosophical in supposing that the plant 
has an inherent property, by virtue of its vital principle, of appropriating 
a portion of the descending sap to the formation or expansion of what 
