Published in Fremontia-the journal of the California Native Plant Society. October 1994 issue.
By Christine Leigh Elder, M.A. candidate, Humboldt State University
This is an update on the progress of my research generously funded by the California Native Plant Society. I am currently completing my Master's Degree in Biology at Humboldt State University where my thesis research has focused on the reproductive biology of the California pitcher plant, Darlingtonia californica.
Darlingtonia is a monotypic member of the small insectivorous pitcher plant family, Sarraceniaceae. Also known as the cobra lily, it is endemic to northern California and southern Oregon inhabiting montane and coastal serpentine seeps and is the family's only west coast representative.
Its carnivorous habit, exotic cobra-shaped leaves and showy blossoms have made it a popular research subject, with work focused primarily on carnivory, leaf morphology and arthropod prey and associates. However, few have delved into other equally fascinating aspects of its' life history, especially that related to reproduction and pollination biology. For example, all carnivorous plants must face the quandary of their simultaneous need for insects both as prey and as pollinators.
The most puzzling unanswered question that most fascinated me and propelled my research was that despite nearly 150 years of observations by various workers, none have reported discovering a pollinator-a seemingly simple task for a species that produces such picturesque meadows full of large, colorful blossoms.
My first task then, was to determine whether the lack of pollinator observations were due to the possibility that the pitcher plant relies on self-pollination. I ruled this out by discovering that blossoms experimentally deprived of would-be pollinators matured few fruits, and those that did mature, contained few seeds. In addition, I removed the pollen-producing stamens from another set of blossoms and found that these matured a statistically similar number of seeds as unmanipulated 'control' blossoms. This is a second line of evidence showing that self-pollination is not a major form of reproduction, and that it therefore relies on animal pollination.
Unmanipulated blossoms matured prodigious quantities of seeds, averaging 1,160 3 mm long seeds per fruit capsule. However, this was significantly less than the seed set of blossoms to which I had experimentally added extra pollen, which yielded 1,275-2,147 seeds each. This demonstrates that although a pollinator exists, it is a limiting resource.
Like other workers before me, I observed few pitcher plant blossom visitors identifiable as pollinators, despite my spending a majority of the blooming season observing the pitcher plants at all hours of the day. Candidates I observed include bees belonging to the andrenid, bombid and vespid families, and several as yet unidentified beetles.
Nearly every mature blossom is inhabited by a spider that builds a web around the sepals, capturing large numbers of flying insects. Several of the insects are likely pollinators and I questioned what effect the spider had on the reproductive potential of the pitcher plant. Thus I collected, and am in the process of identifying, the spiders and their prey inhabiting twenty blossoms.
A second focus of my research involved gathering base-line data on blossom morphology and phenology. I found that pitcher plants are among the earliest spring bloomers, possibly an adaptation to avoid competition with the myriad lilies, orchids and violets that share their wetland habitat. Their blossoms cover the sphagnum moss carpet in densities up to one per square foot. The solitary blossom is held aloft on a peduncle that averages 16 inches high.
The blossoms avoid possible competition for insects with the carnivorous pitcher leaves by the fact that the blossoms are held so high above them and that they do not become attractive to insects until well into the season, after the blossoms have matured. I found the average number of anthers and bracts to be higher than previously reported in the literature (17 and 9 respectively) . The anthers mature as soon as the blossoms open and continue dehiscing pollen for 11-23 days, a relatively extended period. My preliminary studies suggest the species is protandrous, however, the fact that some self-pollination occurs shows that there is an as yet undetermined period of overlap between pollen dehiscense and stigma receptivity.
I determined the age structure of the population of blossoms and found that most were of a similar age. This suggests a 'mass blooming' strategy, often interpreted as an adaptation to an unpredictable pollinator.
One of the most exciting findings of my research thus far was the discovery of a population of albino blossoms. Instead of having the normal maroon-purple petals, these possess greenish-yellow petals similar in color to the sepals. The 30 individuals were found concentrated primarily in one corner of the discovery site, interspersed among the normal blossom color variety. The two color varieties were indistinguishable from each other in all measurements including height, number of bracts, number of anthers, and number of seeds produced per fruit capsule. Such color polymorphisms, usually due to genetic mutations, are not uncommon among flowering plants but unreported in this species until now.
Lines of inquiry I hope to pursue in the upcoming field season include the identity of the pollinator and its' interaction with the blossom's arachnid inhabitants and the detection of possible blossom scent glands, nectaries, and ultraviolet patterns that serve to attract pollinators.
I'll keep CNPS informed of my progress on this most fascinating subject. If any fellow carnivorous plant enthusiasts have questions or comments on my work, they are invited to contact me at the following address.
Sincerely,
Christine Leigh ElderAustin, R. M. 1875-1877. Letters to W. M. Canby on Darlingtonia . The Society of Natural History of Delaware, Winmington, DE: Wilmington Institute Free Library. (cited in Joel.)
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