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106
THE STOCK CULTURE LIBRARY
media.
Of all the mushrooms discussed in this book,
only strains of the Paddy Straw Mushroom,
Volvariella volvacea, should not be chilled. V
volvacea demonstrates poor recovery from cold
storage—both from simple refrigeration at 340
F. (2° C.) or immersion in liquid nitrogen at -
3000 F. C.). When the mycelium of this
tropical mushroom is exposed to temperatures
below 45° F. (7. 2° C.) drastic die-back occurs.
Strains of this mushroom should be stored at no
less than 50° F. (10° C.) and tested frequently
for viability. When cultures are to be preserved
for prolonged periods alt room temperature,
many mycologists cover the mycelium with liquid
paraffin. (For more information, consult
Jinxia and Chang, 1992).
When retrieving cultures from prolonged
storage, the appearance of the cultures can immediately
indicate potential viability or clear
inviability. If the mycelium is not aerial, but is
flat, with a highly reflective sheen over its surface,
then the culture has likely died. If the
culture caps have not been sealed, contaminants,
usually green molds, are often visible,
giving the mycelium a speckled appearance.
These cultures make re-isolation most difficult.
Generally speaking, success is most often seen
with cultures having aerial, cottony mycelium.
Ultimately however, cultivators can not determine
viability of stored cultures until they are
subcultured into new media and incubated for
one to three weeks.
The Stamets 'P" Value
System for Age
Determination of a Strain
The Stamets "P" value system is simply an
arithmetic scale I have devised for measuring
the expansion of mycelium through successive
Figure 85. An (Flammulina velutipes) culture
is labelled to indicate genus, species, variety,
"P" value, voucher collection number, and—if necessary—type
of medium.
inoculations from one 100 x 15 mm. petri dish
to the next.The number of cells divisions across
a petri dish is affected by the range of cell wall
lengths. Of the septate strains of fungi, some
have cells as short as 20 while others have
cells 200 and longer. The Stamets "P" Value
(SPV) benefits cultivators by indicating how
close to the origin their culture is at any point
in time by simply recording the number of petri
dishes the mycelium has grown across. When
a culture has been isolated from contaminants,
usually in one or two transfers, the first pure
culture is designated as P1. When the mycelium
has filled that dish, the next dish to receive the
mycelium is called P2. Each culture is labelled
with the date, species, collection number, strain
code, P-Value and medium (if necessary) .Thus,
a typical example from one of my culture dishes
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