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or
S. magnicamporum
from western Illinois. Observations of other species of
Spiranthes
maintained in insect-proof cages indicated that
S. laciniata
,
S. lacera
var
gracilis
, and
S. tuberosa
were also uniformly sexual (Catling
1980b, 1982
).
Experimental self-pollination, geitonogamous pollination, and cross-pollination
in examined species all usually led to full ovary expansion. However, when com-
pared to self- and geitonogamous pollinations, cross-pollinations produced a higher
percentage of ovaries with seeds (Catling
1980b, 1982
). Catling attributed the dif-
ference to partial self-incompatibility or postzygotic embryo abortion (e.g., Brink
and Cooper
1939, 1947
).
Embryo development followed a similar pattern. Although highly variable, the
percentage of seeds with well-developed embryos was reduced in cases of self-
pollination and geitonogamous pollination when compared to cross-pollination
(Catling
1980b, 1982
). Catling (
1980b, 1982
) suggested that this might be related to
the rate of pollen-tube growth and/or a reduced rate of pollen germination in intra-
plant pollinations (e.g., Weller and Ornduff
1977
). Seed development and capsule
dehiscence occurred up to 5 days earlier in cross-pollinated as compared to self-
pollinated individuals (Catling
1980b
). In addition, seed development only occurred
in the upper portion of the ovary in cases of geitonogamous and self-pollination,
especially in populations of
S. lacera
var.
lacera
and
S. lucida
(Catling
1980b,
1982
). Thus, it appears that some interference with the development of the pollen
tubes occurred in these treatments. Inbreeding depression in
S. lucida
was also sug-
gested by a reduction in the size of the embryo in many seeds resulting from self- or
geitonogamous pollinations, often to one-quarter or less of the usual size (Catling
1982
). No small embryos were produced in cross-pollinated ovaries: all seed coats
contained either large embryos or no embryos at all (Catling
1980b, 1982
).
Unlike
S. ovalis
var.
ovalis
(an outcrossing or geitonogamous plant), the absence
of a rostellum in
S. ovalis
var.
erostellata
permits the pollinia to develop in direct
contact with the stigmatic surface (Catling
1983b
). The seeds are monoembryonic,
and routine experimental tests suggest that the flowers are autogamous, although
pseudogamy cannot be entirely excluded (Catling
1980b, 1983b
). In addition, the
flowers are partly open, and a limited amount of outcrossing might be possible
(Catling
1983b
; Sheviak and Brown
2002
).
Autogamy can confer advantages pertinent to plants colonizing disturbed or suc-
cessional sites, and
S. ovalis
var.
erostellata
has recently become more common in
old-field and second growth woodland habitats in some northern parts of its distri-
bution (Sheviak
1974
). Like agamospermy, the level of autogamy can be responsive
to both genetic and environmental factors (Uphof
1938
; Jain
1976
; Frankel and
Galun
1977
) and is, therefore, subject to selection (Catling
1983a
). Mixed mating
systems may be highly adaptive to characteristics of the local environment, such as
the availability of pollinators. As in facultative agamospermy, protracted adjustment
of autogamy and outcrossing may reach equilibrium, balancing short-term fitness
with the advantages of genetic recombination (Catling
1982
). In addition, distinct
modifications in floral morphology, associated with restricted gene flow, accom-
pany autogamy (see Ornduff
1969
) and may be related to the development or aug-
mentation of ethological isolating mechanisms (Levin
1971
).
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