OBSERVATIONS ON REPRODUCTION IN TRIPHORA-TRIANTHOPHORA (ORCHIDACEAE)

14 Pages·1994·7 MB·English

by  S A Williams

RHODORA,
Vol. 96, No. 885, pp. 30-43, 1994
ON
OBSERVATIONS REPRODUCTION
IN
TRIPHORA TRIANTHOPHORA (ORCHIDACEAE)
Susan Williams
A.
ABSTRACT
A
small population of the orchid Triphora trianthophora in western Massachu-
setts was investigated over a six-year period (1988 through 1993). Observations
on the flowering habits and seed capsule production arc described, as well as
vegetative reproduction by means of tuberoids. Triphora produces an abundance
The
of short-lived flowers yet very few capsules are initiated. majority of Tri-
phora\ existence spent underground reproducing ascxually by means of new
is
tuberoids.
Key Words: Triphora trianthophora, orchid, tuberoids
INTRODUCTION
Triphora trianthophora a small woodland orchid, elusive and
is
secretive, appearing abundantly one year and rare or absent for
many
Ames,
succeeding years (Lownes, 1920; 1948; Sheviak,
1974; Brackley, 1981). Triphora trianthophora also shows syn-
chronous flowering, the majority of short-lived flowers appear
its
48 hours temperature drop Keenan,
after a (Brackley, 1981; 986,
1
1988; Sheviak, 1974). In species of Triphora which are
fact, all
not self-pollinating exhibit gregarious flowering (Dressier, 1981)
apparently to increase their chances of cross-pollination (Luer,
Even
1975). so, Triphora trianthophora rarely sets seed (Lownes,
Keenan,
1920; 1992).
The most
unusual of however,
feature this orchid, exis-
is its
time
tence for years at a in a subterranean, tuberous condition
Ames,
(Lownes, 1920; 1948; Zavitz and Gaiser, 1956). Although
the tribe Triphoreae appears to be a group with no close
relic
members
allies, does share a unique feature with of the tribes
it
Orchideae, Diseae and Diurideae in having these root-stem tuber-
oids (Dressier, 198 started observing the underground tuber-
I
1).
made
oids of Triphora in part because of a statement by Oakes
Ames maximum
1948 he supposed was
in in that there a ".
. .
size for the tubers that bear flowering stems." Every plant of
T.
trianthophora has from one numerous tuberoids attached by
to
stolons. These tuberoids are thickened underground storage struc-
As
tures superficially similar to tubers, but structurally
different.
Dressier (1981) points out, true tubers are not found in orchids.
30
1
Williams—
Triphora
3
1994]
may
The primary bud which form
tuberoid contains the apical
new
a shoot in the growing season. Axillary buds form secondary
tuberoids at the end of slender stolons. These secondary tuberoids
continue to increase in size with the age of the plant, along with
The grow down-
increasing length of the stolon. stolon tends to
wards into the leaf so that with accumulating accounts
litter litter
for the depth of the primary tuberoids. These secondary tuberoids
on becoming detached from the parent plant form primary tuber-
new same
oids of plants with the genotype as the parent.
Most
however, do not form flowering shoots but remain
plants,
and of primary
in the leaf litter (pers. obs.), these plants consist
and secondary tuberoids as seen in Figure 4a and 4b.
This paper presents the observations gathered over six years
on: sexual reproduction in Triphora trianthophora, in particular
1)
the reason for low capsule and vegetative reproduction in
set; 2)
underground growth form with
Triphora, particularly the possible
and
correlations to population fluctuations, the relationship be-
tween primary tuberoid size and the plant's reproductive status.
METHODS
when
became
acquainted with Triphora trianthophora in 1988,
I
began studies on a small population in western Massachusetts
I
for the Massachusetts Natural Heritage and Endangered Species
Program.
The study comprises an area of approximately two acres
site
of northern hardwood dominated by Fagus and
forest grandifolia
The
Acer saccharum. site on a southeast facing slope at an
is
elevation of 900-1 100 midslope being approximately
feet. It is
300 above River
the Deerfield basin.
feet
The was where
study area subdivided into distinct sites the
were 1988 were 25 and each
plants located. In there sites in
succeeding year found additional sites until there were a total
I
The
of 72 sites in 1993, all within the general area. cumulative
of vegetative and reproductive plants observed over the
total six
was
year period 1448.
observed on above-
the plants a daily basis for their entire
I
ground of approximately one month, from
existence their
first
mon-
emergence through the capsule Each was
litter, to set. site
number number
itored for the total of plants; of vegetative plants;
Rhodora
32 96
[Vol.
which produced buds but blossom cases
plants failed to (in all
was due some type of herbivory); plants which blossomed
this to
and which
but did not initiate a seedpod; plants ripened seedpods.
Percent capsule was determined by the ratio of capsules set
set
buds produced by the population.
to total
While data on each any pollinators or
collecting floral
site,
were and whether had undergone
visitors noted, also the plants
any type of herbivory.
A
few plants that did not produce capsules were carefully re-
moved
from the and the length of their primary tuberoid
litter
was measured. This group included small primary tuberoids which
had never produced any stem; tuberoids with hyaline stems
(frag-
translucent stems remaining under the not producing
litter
ile,
more
leaves or flowers but with one or secondary tuberoids at-
having above-ground stems with
tached); vegetative plants (those
one small but no flowers); and plants with one, two, three,
leaflet
or four flowers.
many
As removing noted other tuberoids
started also
I litter I
same had produced nor
the vicinity that neither leafy flowering
in
many
stems. These consisted of a primary tuberoid and one to
Some
secondary had hyaline stems with secondary
tuberoids.
Many
tuberoids but had never produced above-ground stems.
large tuberoids were also noted which had previously produced
above-ground stems but had not produced any the current
in
Due
These tuberoids were not measured. to the invasive
year.
smaU
nature of measuring primary tuberoid length, only a sample
was measured each
year.
A
made number
yearly comparison was of the total of plants
comparing
for the original 25 sites as well as fluctuations within
individual
sites.
RESULTS
Observations on Sexual Reproduction
Thphora
Throughout the six-year study period, the majority of
10%
trianthophora plant produced flower-bearing stems; only or
Most
less failed to flower in any given year. (Figure of the
1).
plants bore one or two flowers; those having three of four were
much 10%
less frequent totaling less than in any given year. This
pattem remained relatively constant over six years.
Williams—
1994] Triphora 33
1988
o
[11989
OJ
1990
Q.
O
1991
15
1992
O
D
1993
o
0)
o
^
sterile
flower 2 flowers 3 flowers 4 flowers
1
Reproductive
Plant Status
Figure 1. Percentages of plants of differing reproductive status in a Triphora
population over
a six-year period.
Figure 2 illustrates on a yearly basis the percentages of bud
produced and
loss, flowers capsules ripened out of the bud
total
population.
The
large percentage of buds was caused by
lost several
factors.
Rarely, a few buds would be underdeveloped and and
tiny these
would
always drop
off prior to flowering. Occasionally,
entire
consumed
clumps
most
destructive
Stems
Sometimes
were chewed up and
in half higher occasionally the buds were
many
half eaten. I noted slugs in the upper litter layers times and
observed on
Triphora
slugs plants (including the tuberoids) sev-
eral times.
Also
illustrated in Figure 2 that capsule production
relative
is
buds produced very However,
to total is low. out of 10 capsules
1
Rhodora
96
34 [Vol.
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'90
'89 '91 '92 '93
Year
produced and seedpods ripened out
Figure Percentages of buds lost, flowers
2.
compared
bud
of the total population, yearly.
104 ripened, which 95%. The other 6 did not ripen
initiated is
because of herbivory
los
appears that few of the flowers are actually pollinated, sup-
It
ported by the fact that on only two occasions in six years did
I
One
note any floral visitors. appeared to be a small species of
bumblebee (Bombus) which blossom few
entered the for a sec-
The
onds, then backed out with pollinia attached. only other
much
observed was smaller and slender bee
flower visitor a
I
probably belonging to the genus Hylaeus. did not observe any
I
removal with but whether the bee was too small
pollinia its visit,
removed
or the pollinia had already been could not be deter-
mined.
Another factor contributing to low capsule set the fact that
is
blossom one day
the Triphora available to pollinators for only.
is
The day following a bloom shows noticeable fading and drooping
Williams—
Triphora 35
1994]
2.8-
1
2.6 1
1 1
1 1
1
1
2.4-
1
1
1
2.2 •
-
11
1
2.0 •
-
1
1
1
+
1.8-
- 1
1
• •
•
*
1.6- • •
• •
1.4
• * •
1
1
1
1.2 • • •
2
• • • •
*
1.0 •
-
•
• •
-
S
0.8 • • • • • • •
• • • •
-
•
0.6 4c
• • •
• •
0.4 • • • • • •
•• •
* •
0.2
•
NS
HyS
StP 1FI 2FI 3F1 4FI
a b b c d d —
Reproductive Status
Figure Relationship of primary- tuberoid length (cm) reproductive
3. to status
NS-No
HyS-Hyaline
of plant. Stem; Stem; StP-Sterile Fl-one
Plant; flower;
1
— — —
two
2 Fl flowers; 3 Fl three flowers; 4 Fl four flowers. Each dot represents an
mean
individual plant. Asterisks represent the of the group. Different
letters in-
=
dicate a significant difference between the means. (Bonferroni adjustment, P
.05)
an average of blossom
until, after five days, the off the plant.
falls
If the flower fertilized, the ovary starts to swell and the dried
is
remains
corolla
intact.
Observations on Vegetative Reproduction
Although orodu
the stimulus for shoot
primary
primary
Rhodora
36 [Vol. 96
One-way The
Table analysis of variance of reproductive status. length data
1.
were log transformed to homogenize the variance and the log of length was then
used as the dependant variable.
Analysis of Variance
Mean-
Sum-of-
P
Source Squares df Square F-Ratio
Reproductive 29.766019 5.953204 47.560594 0.000000
status 5
8.511623 68 0.125171
Error
tuberoid with no shoot development had the smallest primary
tuberoids; those plants with either hyaline stems or sterile leaflets
had primary and primary
slightly larger tuberoids; tuberoid size
numbers
tended to increase with increasing of flowers per plant.
A
anova
one-way was done on
the data with the log of length as
The
the dependent variable. length data were log transformed in
homogenize As from anova
order to the variances. seen the table
(Table the developmental classes showed significant variation
1),
between
in tuberoid length. In order to analyze the variation
classes, a Bonferroni Adjustment was done. The results (shown
by the letters in Figure 3) indicated which classes varied signifi-
from which
cantly other
class.
The primary and
tuberoid generally the largest the deepest
is
down
the from which the flowering stem and secondary
in
litter
The
tuberoids Figure secondary tuberoid
arise {see closest to
4).
the primary tuberoid the largest with smaller tuberoids ap-
is
many
pearing up the stem. There are variations in the number
of secondary tuberoids and arrangement on any individual
their
Some
plant. plants have only one secondary tuberoid, in others
have counted up However, have
plants one.
to 17. at least
all
I
Occasionally the secondary tuberoids get quite large before be-
coming from main
separated the plant. Several secondary tuber-
oids were even slightly larger than the primary tuberoid. There
when
doesn't appear to be a specific size the stolon disintegrates
between main and secondary More
the plant the tuberoids.
likely,
of
they separate as a result physical forces since they are fragile
and near the litter surface. The tuberoids remain in the leaf litter
never reaching into the substrate.
soil
Another between primary and secondary
distinct difference tu-
beroids that the primary tuberoid a tan color whereas
is is all
waxy
secondary tuberoids are white. found tiny primary tuber-
I
Williams—
1994] Triphora 37
(b)
Figure
4. growth
stages of Triphora development, small primary tubcroids with secondary
(a) tuber-
oid development but no stem development; hyaline stem production;
(b)
(c) leaflet
of
a non-flowering shoot; (d) flowering plants (arrows show swellings near primary
tuberoid which will develop into secondary tuberoids).
»
Rhodora
96
[Vol.
38
300
•::::
250
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ri A
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100
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:•:
ii;ii=
'•:•::' '' * _ i _ p
• ' ^ * * * '
'88 '89 '90 '92 '93
'91
Year
on
population from 1988 through 1993 the
Figure Fluctuations in the total
5.
25
original
sites.
mm
some
lone,
new
producing secondary tuberoid.
a
Figure shows the yearly fluctuations in population size for
5
of
the original 25 sites found in 1988. It suggests a pattern in-
more needed
and decreasing population but data
creasing size, is
confirm
th
to
Table 2 shows the fluctuations of plants within each of the 25
over the six years. There not only a great deal of variation
sites is
numbers
What
same previous
reappear are not always in the location as in
that
years even though they are in the same site. This indicates that
pnmary
producing shoots for years at a time.
ery
many
and Their absence years
population vigor. for
assess size
mean confirmed by removing
does not they are not there. this
I
Williams—
Tn^/zora 39
1994]
Table 2. Actual numbers of vegetative and flowering plants for each of the
original 25 sites and their yearly fluctuations.
Number
Total of Plants
Site # 1988 1989 1990 1991 1992 1993
4
1
2 6 4
8 9 11
4
3
1
15
4
3
7 2
5 2
6 8 2
1
7 13
1 1
There no #8
8
is site
9 14 2 2
8 5
10
10 10 6 2
10
28 4
11 2 10
11
1
12
13 2
14
3 2
15 11 2
16 19 5 7 9 20 18
17 13
2 8 14
1
20
18 3 22 35 36
19 22 7 4 2
3
1
20 23 4
6 18 19 8
21 37 21 30 61 113 93
10 10
22 47
4
3 19
1
23
7 2 2 3 6
24
5
25
2
3 11 5
1
Total 300 51 65 148 243 240
observed
numerous
1990 these areas contained 21 flowering
plants.
DISCUSSION
Triphora trianthophora a species of the climax hardwood
is
forest always associated with Fagus grandifoUa (Lownes, 1920;
Zavitz and Gaiser, 1956; Sheviak, 1974; Crow and Stokes, 1980;
Brackley, 1981; Martin, 1983; Keenan, 1992). shoot
initiates
It
growth and flowers in August when the herbaceous environment
under a canopy characterized by very low
full is light levels,