19a. Ericaceae Jussieu subfam. Monotropoideae Arnott in M. Napier, Encycl. Brit. ed. 7. 5: 118. 1832.
Gordon C. Tucker, Craig C. Freeman, Gary D. Wallace
Herbs (rarely subshrubs), chlorophyllous and autotrophic or achlorophyllous and heterotrophic, mycotrophic, multicellular hairs present or absent; bark absent. Stems absent, or erect or decumbent. Leaves absent or persistent, sometimes scalelike, often much reduced, cauline and alternate or in basal rosettes or pseudoverticillate; petiole present or absent; blade plane or acicular, abaxial groove absent. Inflorescences terminal racemes, umbels, corymbs, cymes, or solitary flowers; perulae absent; bracts shorter than, as long as, or longer than sepals. Flowers radially symmetric to slightly bilaterally symmetric, horizontal, nodding, or spreading to erect; sepals absent or 2-5(-6); petals (3-)4-5(-6), connate or distinct, corolla deciduous or persistent, rotate, crateriform, cylindric, urceolate, or campanulate, if petals connate, lobes shorter than tube; intrastaminal nectary disc present or absent; stamens 8-10(-14); anthers dehiscent by slits or pores; ovary 1- or (4-)5(-6)-locular; placentation axile or parietal; style straight or declinate. Fruits capsular, dehiscence loculicidal or indehiscent to irregularly dehiscent, or baccate and indehiscent. Seeds 25-1000+, distinct, fusiform or ovoid, winged or not.
Genera 14, species ca. 50 (12 genera, 21 species in the flora): North America, Mexico, West Indies (Hispaniola), Central America, n South America, Europe, Asia (including Sumatra).
Ericaceae subfam. Pyroloideae Kosteletzky; Monotropaceae Nuttall; Pyrolaceae Lindley
Two strictly Asiatic genera in the subfamily, Cheilotheca Hooker f. and Monotropastrum Andres, each contain two species.
A persistent error, that heterotrophic members of the Monotropoideae are saprophytic, was disproved by E. Björkman (1960), supported by T. E. Furman and J. Trappe (1971). These plants are considered epiparasitic, deriving their nutrition from coniferous or fagaceous hosts through mutually shared fungal associates. This mode of nutrition also is called mycoheterotrophy. Mycorrhizal associates of several genera have been investigated by K. W. Cullings et al. (1996) and M. I. Bidartondo and T. D. Bruns (2001, 2002).
Inflorescences, the only above-ground structures for these heterotrophic plants (excluding achlorophyllous forms of Pyrola), emerge from soil each year. One or more inflorescences develop from each perennial root system. Because the root systems may branch, it is generally not possible to determine whether any two inflorescences in proximity are derived from the same system. Thus, counting inflorescences as a measure of population sizes is essentially meaningless but may be an indication of the overall health of the plants and recent environmental conditions.
Conclusive distinctions among bracts and similar-appearing appendages without flowers in their axils are generally lacking. The latter are here called sterile bracts. Bracts and sterile bracts are generally similar in their sizes and shapes, but in some taxa sterile bracts are more appressed to the inflorescence axes or are conspicuously more succulent. Because similar structures occur in inflorescences of other Ericaceae and are uniformly sessile, it is difficult to justify calling these structures leaves.
Chlorophyllous members of the subfamily produce three types of similar, bractlike structures. Persistent bud scales, produced in winter buds at the end of each growing season, usually alternate with leaves at the base of the peduncle (H. F. Copeland 1947). Peduncles in all genera except Chimaphila bear up to five bracts, which sometimes subtend aborted flower buds. Individuals of nearly all species occasionally lack peduncular bracts. The pedicel of each flower (absent in Moneses) typically is subtended by a single bract. These inflorescence bracts are free from the pedicel in Orthilia and Pyrola; they are adnate proximally to the pedicel in Chimaphila, the bract thus appearing to arise from the pedicel. Bracteoles are absent.
Pollination studies of some species indicate that chlorophyllous members are largely outcrossing and entomophilous (H. B. Lovell and J. H. Lovell 1936; L. C. W. Jensen 1961; J. T. Knudsen and J. M. Olesen 1993). Knudsen and Olesen found the floral morphology of Chimaphila to be optimized for pollination by large, nectar-gathering insects, especially bumblebees. Moneses, which produces no nectar, is buzz-pollinated by pollen-gathering insects. Orthilia produces nectar and is visited by both nectar- and pollen-gathering insects. Pyrola produces no nectar and is buzz-pollinated.
SELECTED REFERENCES Copeland, H. F. 1947. Observations on the structure and classification of the Pyroleae. Madroño 9: 65-102. Wallace, G. D. 1975. Studies of the Monotropoideae (Ericaceae): Taxonomy and distribution. Wasmann J. Biol. 33: 1-88.