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Winter 2017 Sepals and Petals and Stamens—Oh, My! Or, a brief discourse on putative homologies of perianth elements of Common Black Cohosh W. John Hayden University of Richmond, [email protected]
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Recommended Citation Hayden, W. John. "Sepals and Petals and Stamens—Oh, My! Or, a brief discourse on putative homologies of perianth elements of Common Black Cohosh." Sempervirens Quarterly, Winter 2017, 8-9.
This Article is brought to you for free and open access by the Biology at UR Scholarship Repository. It has been accepted for inclusion in Biology Faculty Publications by an authorized administrator of UR Scholarship Repository. For more information, please contact [email protected]. 8 Sempervirens, Winter 2017 Sepals and Petals and Stamens—Oh, My! Or, a brief discourse on putative homologies of perianth elements of Common Black Cohosh By W. John Hayden, Botany Chair
encountered some contradictory became larger and pigmented; these Iinformation while preparing to write modifications mark a shift away from the 2017 Wildflower of the Year bro- a direct role in reproduction to the chure: some sources describe flowers equally important supporting role of of Actaea racemosa, Common Black enhancing pollinator attraction. Flow- Figure 1 Stamen (left), petal/staminode (right front), and sepal of Actaea Cohosh, as having petals, while others ers believed to have produced petals racemosa. Illustration by Sheila Hayden. say petals are absent. How can that be? via sterilization of stamens are said to How could there be such uncertainty possess andropetals. This model in- could be valid; some plants may have about this common plant, one known volves a somewhat round-about path: andropetals while others may have to science since before the time of Lin- leaflike organs bearing anthers and bracteopetals. naeus? After a little research, I decided pollen first became stamens, and then Multiple tools are available for to describe Black Cohosh flowers as some stamens became petals of this addressing which model of petal origin having a series of organs interpretable sort. An alternative model suggests applies for a given species. From the either as staminodes (nonfunctional that petals originated more directly realm of morphology, an andropetal stamens) or as petals located be- from a leaflike ancestral condition originates as a slender bump (resem- tween its sepals and stamens (Figure simply by loss of chlorophyll and bling the first visible stages of a stamen 1). Frankly, I waffled on the petal enhancement of other pigments; such primordium), and at maturity it has issue, and this article explores why. petals are termed bracteopetals. For a narrow base and a single vascular Petals are the floral organs situated any species, either the andropetal or trace, just like a stamen. In contrast, between sepals and stamens, usually the bracteopetal model could be cor- a bracteopetal originates on the floral distinctively pigmented, and function- rect, but not both. For flowering plants meristem as an arclike bulge, retains ing to attract potential pollinators. as a whole, however, both models a relatively wide base at maturity, and At some fundamental level, all floral organs are interpreted to be modified leaves attached to the end (receptacle) of the flower-bearing stem (pedicel)— an idea first articulated by the German poet Goethe (1790). But the diversity of flowering plants is profound, and there are lots of variations in floral organography. Certain flowers chal- lenge simplistic interpretation, and the structural details of petals and petallike organs have led botanists to ponder whether the petals of all flow- ers are fundamentally the same. Comparative morphologists have developed two models for the origin Figure 2 Myosurus minimus; 3a. Helleborus foetidus, flower, longitudinal section;3b. of petals. One model posits that petals Helleborus foetidus, petal; 4a. Xanthorhiza simplicissima flower, top view;4b. Xanthorhiza represent stamens that lost the capac- simplicissima, petal; 5. Aquilegia vulgaris, flower, longitudinal section. Images from H. ity to form anthers and pollen as they Baillon. 1867–1869. Histoire des plantes, vol. 1. Hachette, Paris. Sempervirens, Winter 2017 9
derived from stamens. That is, petals that look like petals in Buttercups are in Ranunculaceae are andropetals fundamentally similar to the oddly and thus fundamentally different shaped organs illustrated in Figures from bracteopetals of core (i.e., most) 2–5 and to the odd organs located eudicots. Further, when present, petals between sepals and stamens in flowers in the Buttercup Family often bear of Actaea (Figure 1). nectar-secreting glands; classical-era My decision to waffle about the German morphologists called these petal/staminode organ of Actaea unusual nectar-bearing petals Hönig- racemosa emerged from within the blatter (honey-leaves) (Figures 2-5). framework of issues outlined above. These nectary-bearing petals can be Including both interpretations for Figure 6 Anemone nemorosa, flower downright odd: in Mousetail (Myo- this organ in the Wildflower of the longitudinal section. 7. Caltha palustris, surus, Figure 2), they are extremely Year brochure provided an opening flower, longitudinal section. Images from narrow, flexed structures with a for this article and the opportunity to H. Baillon. 1867–1869. Histoire des plantes, nectary located at the point of flexure; interpret floral morphology ofActaea vol. 1. Hachette, Paris. in Hellebores (Helleborus, Figures 3a, in the context of Ranunculaceae in has three (or more) vascular traces. 3b), they are tubular; in Yellowroot particular and eudicots at large. There In these features, bracteopetals are (Xanthorhiza, Figure 4), they are is not much that is petallike about the essentially like differently pigmented stubby and bilobed; and in Columbine petal/staminodes of Actaea racemosa versions of sepals. Further, determina- (Aquilegia, Figure 5), nectary-bearing (Figure 1). To interpret these organs as tion of which genes are active during petals form distinctive elongate spurs. staminodes emphasizes their similar- initiation and early development of In Buttercups (Ranunculus), nectary- ities with stamens: small size, narrow floral organ primordia can help resolve bearing petals look like ordinary pet- filamentlike base, and a bilobed apex the question. Finally, phylogenetic per- als that just happen to have a glandu- that suggests a pair of anther sacs. On spective provides insight to petal organ lar region toward their base. the other hand, though they would identity by mapping morphological Other familiar plants in the Butter be decidedly odd in any plant fami- characteristics on well-resolved evolu- cup Family possess just a single peri- ly other than Ranunculaceae, these tionary trees. For a long time, based anth whorl that is brightly pigmented; organs are not particularly unusual for only on morphological evidence, many further, these floral organs have rela- the nectary-bearing andropetals of the botanists subscribed to the andrope- tively wide bases and three vascular Buttercup Family. So, good reader, take tal model for all eudicots (traditional traces, but not the vestige of a nectary. your pick, call them petals, andrope- dicots minus basal angiosperms). But If we apply the criteria articulated tals, Hönigblatteren, or staminodes, this paradigm of petal origin has been above, these members of the Butter- but whatever you call them, know overturned by de Craene (2007, 2008), cup Family have sepals that have taken that the choice is a complicated one. whose analyses support the bracteope- on the pollinator-attraction function Who would have imagined that these tal model for core (i.e., most) eudicots. of petals. Examples include Anemone graceful forest herbs stood at the crux And this is where the story returns (Anemone, Figure 6), Marsh Marigold of such basic, yet complicated, issues to Actaea. Black Cohosh is classified (Caltha palustris, Figure 7), and Clem- of plant morphology? All petals are in Ranunculaceae, the Buttercup atis (Clematis). The showy sepals in not the same! v Family, which, along with Poppies, flowers of these members of Ranun- Barberries, and several other families, culaceae are commonly referred to as WORKS CITED constitutes the basal eudicots, distinct petaloid sepals, but they also fit well De Craene, L.P.R. 2007. Are petals sterile from most (or core) eudicots now the distinguishing features of bracteo- stamens or bracts? The origin and modeled to possess bracteopetals. So petals. The point is that, regardless of evolution of petals in the core eudicots. Annals of Botany 100: 621–630. what’s the story with petals in Ranun- what morphological terms we choose De Craene, L.P.R. 2008. Homology and culaceae—and in Actaea? Morphol- to apply, the things that look like evolution of petals in the core eudicots. ogy, development, developmental petals in flowers of Anemone, Marsh Systematic Botany 33: 301–325. genetics, and phylogenetic perspective Marigold, and Clematis are not the Goethe, J.W. 1790. Versuch die Meta- support the idea that petals, when same as the things that look like morphose der Pflanzen zu erklären. present in Ranunculaceae, were petals in Buttercups—and the things Ettinger, Gotha.