Re: How come delicate toadstools manage to break up the ground?

Mushrooms are delicate because of the way their tissues are assembled, but
they can break through hard ground because the individual fibers in their
tissues are very tough. In order to understand this contradiction, it is
necessary to know a bit about the fascinating life cycle of Basidiomycete
fungi: mushrooms are the fruiting bodies of this large and economically
important class of fungi. 

For most of its life, a basidiomycete fungus lives as a loosely organized
collection of strands called hyphae which invade the fungus’s habitat –
soil, rotting wood, the body of another organism, etc. – and gather
nutrients. These hyphae are haploid (i.e.,
each cell has only one copy of each chromosome), and they are only one cell
thick. The strands are very tough, because their cell walls are composed of
the polymer chitin,
a durable compound also found in the exoskeletons of arthropods. This
allows them to break through the substrate surrounding them.

When two hyphae of compatible mating types encounter each other, the cell
walls fuse together. The two nuclei remain independent inside the cells in
what is called a dikaryotic state (from the Greek “di” for “two” and
“karyos” for nucleus). The dikaryotic hyphae begin to organize and form a
fruiting body – a mushroom; the only part of the fungal life cycle we
usually get to see. (Click here
for an image of the basidiomycete life cycle.)

If the individual strands that make up a mushroom are so tough, why do
mushrooms fall apart so easily? The answer lies in the way their tissues
are constructed. In animals and plants, the other kingdoms with
multicellular organisms, cell division can occur in any plane. The
three-dimensional tissues produced from this type of division are called parenchyma.
Parenchyma tissues do not break up easily: since the cells were produced by
divisions in so many different planes, there are no readily available lines
of cleavage where the tissues can break. Fungal fruiting bodies, on the
other hand, are composed of a tissue type called pseudoparenchyma.
It resembles parenchyma in that the cells are rather tightly packed in a
three-dimensional array, but it is produced from cell division in only one
plane: the hyphal strands can only elongate by dividing at the tip. Because
it is made up of fibers only one cell layer thick, pseudoparenchyma is much
less resistant to tearing than true parenchyma.

So as a mushroom develops, the individual strands that compose the fruiting
body force their way through the soil, breaking it up. But since the
aggregation of hyphal strands is not as tightly connected as true
parenchyma, it breaks up easily when force is applied to it. And next time
you eat a mushroom, remember that you’re chowing down on dikaryotic
pseudoparenchmya!