CSA-Journal-2016-04

lights (10 h/d) at around 20 °C. The protocorms
of Cym. hookerianum develop a shoot
tip and roots, and by six months have been
reflasked on similar medium (Figure 4).
Reflasking is also necessary for Cym.
goeringii which need a much longer development
time in the flasks. But with
them there is no sign of shoots or roots.
Instead an elongated rhizome with knobs
and hairy tufts has formed and looks not
unlike a green caterpillar (Figure 5). This
is a mycorrhizome, a structure of variable
morphology produced by several mycoheterotrophic
plant groups including ferns
(their mycorrhizomes are already known
from fossils from the Carboniferous Era),
Gentianaceae, Ericaceae (subfam. Pyroloideae)
and Orchidaceae. Hans BURGEFF
(1932) has extensively worked on orchids
with such structures, though he mentions
the term mycorrhizome only briefly; cymbidiums
were not treated by him. In orchids
the term was long used as a substitute for
protocorm. The term protocorm for the
germination body (in German ‘Keimachse’
or the later term ‘Keimkörper’ in the papers
of Otto MÖLLER) of orchids was
introduced by the French botanist Noel
BERNARD (1899) in analogy to the mycotrophic
prothallium of horsetail ferns
(Equisetaceae) and club mosses (Lycopodiaceae)
for which according to ARDITTI
and KRIKORIAN (1996) and CHANG et
al (2005) the term protocorm was coined
Fig. 5 Mycorrhizome of Cym. goeringii on
MSO 1c medium 1 year after sowing.
by the Dutch botanist Melchior TREUB in
1890.
The term protocorm is based on the
Latin pro, meaning before, and the classic
Greek term ‘kormos’, meaning ‘tree
trunk’ or in general botanic terms stem
axis or caulom, so protocorm designates
the structure that comes before the stem,
or, in case of many orchids, rhizome. The
rhizome is a creeping, i.e. horizontal stem,
not a root structure! As the body which
is formed right after the germination of
the orchid seed, it anatomically does not
conform with a rhizome but, as LUCKE
(2003) has shown, is dominated by a derived
cotyledon, it is most appropriate to
call it protocorm, i.e. ‘the body that comes
before the stem’. The term is fully and long
established internationally in scientific and
horticultural circles, and for good reason
so. In most orchids the first root and shoot
with leaf initials will soon develop from the
protocorm’s meristematic part (or better
the totipotent cell region of the protocorm,
because the term meristem is restricted to
totipotent cells of stems and roots which
are both not developed yet). At this point,
the structure can be called a young seedling.
In several terrestrial orchids, however,
whether they are temperate or tropical, a
more or less long prostrate stem (i.e. rhizome)
without functional roots and no
leaves grows from the protocorm instead.
The rhizome is usually closely connected to
a mycorrhizal fungus and thus the term mycorrhizome
is derived, though in the past
it was used interchangeably with the term
protocorm, e.g. by ZIEGENSPECK (1936).
Hanne RASSMUSSEN (1995) defines the
term mycorrhizome as the transitional rhizoferous
structure found in orchids, deriving
from the protocorm and changing to a
mature rhizome in further ontogenesis (i.e.
the individual life history) of the plant.
Through the mycorrhizome young orchids
of several terrestrial genera can live
below ground for a long time, even for
20