Crystals in Storage
This ESEM image shows a few radial parenchyma cells
in common hazel and some rather beautiful crystals
that have formed inside them. These are not starch
grains, which have a rounded shape—so what are they?
A literature search shows that crystals in woody plants
have hardly been researched at all. Scurfield et al. (1973)
found such crystals to consist of calcium carbonate or
calcium oxalate. For conjectural purposes, if these are
crystals of a calcium compound, why would this hazel
tree store that up for later use? The production of hazel
nuts involves the use of significant amounts of calcium;
we are, perhaps, seeing nut-growing resources stored
up in these cells, ready for use when they are needed.
The Scalariform Plate
Where a vessel element meets another vessel element,
there is typically a restriction or barrier, in the form
of a sieve plate, also known as a scalariform plate.
Trees need to be clever about how they do this: they
cannot have a significant restriction to sap flow through
their vessels, but they need to avoid embolisms,
where air seeds into a vessel, rendering it useless.
This combination of needs results in vessel elements
with squeezed ends, or with ladder-like plates—as
with the [slightly broken] scalariform plate shown—
or ones that have a pore-like structure. These
end-types represent only a minor restriction to sap
flow when the vessel is functional, but can prevent
an air bubble expanding into the next vessel element
when one element becomes dysfunctional. What
amazing and ancient 'tech' this innovation represents—
like water pipes with many discrete air valves!
Some authors have suggested that vessel ends act as
a barrier to fungal decay too. Well, just look at that
scalariform plate—is a fungal thread unable to pass
through that? Vessel ends will seem to act as a barrier
to decay because the next vessel element is full of sap,
and a fungus typically cannot enter a vessel at 100%
moisture content. In addition, many trees and shrubs
have secondary mechanisms performed by adjacent
cells that fill dysfunctional vessel elements with gums,
resins, or tyloses. It is really these two mechanisms
that are the key barriers to decay passing up through
a vessel—not the scalariform/sieve plates, which most
often represent little obstruction to a hungry fungus.
24 CityTREES
