[gourds] Gourd shell composition
Owen Kibel (gourds@globalgarden.com)
Wed, 14 Jan 1998 16:13:48 -0800
david wrote:
> Now, I am
> still pondering how well they last in this hot, humid and rainy
> climate. I
> know they will eventually decay out in the open, but they sure do
> resist it
> for a long time.
>
> Any ideas as to the make-up of the shell that contributes to this>
David,
There are at least three plausible reasons why thick shelled Lagenaria
gourds may last a long time even under humid conditions. I have just
done a little qualitative analysis to report on your question, but
please note that my method of tasting on occasion the results of
experiments arises only when I am convinced after reading science books
that it is safe to do so!
Mostly structural components of flowering plants are cellulose or
lignin. Cellulose is a polymer of glucose units, and is fairly resistant
to decay from microorganisms and fungi but eventually will succumb.
Lignin is a polymer of 9 carbon units that include aromatic rings, and
is found in wood. Presumably wood-like things like gourd shell and peach
seed walls are also likely to have lignin. Does anyone know how we can
tell lignin from cellulose?
Lignin is no infallible deterrent against attack by fungi, but certain
woods are less affected , presumably because they contain certain resins
- redwood, for example is widely used as a building material in
California and lasts longer than other woods. People who pyroengrave
woods report on resins in gourd shell, and these may contribute to the
effect.
Certain grasses and woods contain high concentrations of silica, which
makes them hard on tools. However gourd wood is not known for this and I
doubt that silica is a significant reason for resistance to decay. My
experiment was to take a small piece of gourd shell and burn it fully to
ashes. It was from a Bushel gourd, about a quarter inch in thickness,
and fairly dense, strong and good quality as far as gourd shells go. The
gourd had been green-skinned and left to dry so that moisture containing
solutes had evaporated on its surface.
If silica had been a major constituent of the gourd shell then there
would have been a copious quantity of insoluble ash left behind. There
was plenty of light gray ash after burning, however after dissolving the
soluble components in water there was only a tiny quantity of gritty
stuff left which would be probably be silica. This does not mean that
one should not wear masks with gourd dust, as Old Jim suggests, only
that silica is probably not the reason for gourd shell longevity. The
ash tasted very strongly - probably of potassium carbonate - and Becky
has suggested adding potassium sulfate towards the end of the gourd
season for healthy plants and big fruit. The large quantity of salts in
the gourd shell are likely the results of the moisture evaporating on
the surface.
Obviously there is not likely to be a strong chemical fungal inhibitor
in gourd shell, because numerous molds grow on the surface during
curing. These fungi are probably aerobic and put out their spores on the
surface; however it would be interesting to know how much their hyphal
filaments penetrate the shell. Certain woods are frequently attacked by
fungi. However these have vascular bundles, which contain plenty of air,
and may also allow for more penetration. A microscopic examination of
gourd shell may show structures that are either too dense to be easily
penetrated; contain too little air for profuse fungal growth, or are the
incorrect shape for penetration. All this raises more questions than
asked previously - but as a major traditional use of gourds was as
liquid vessels that kept their contents cool through evaporation it
would be worthwhile to find the answers!
Owen