Petal scales

Smith and Downs (1974) considered the petal scale, also called a ligule, nectar scale or lateral fold, one of the premier indicators of generic-level relationship in Bromeliaceae (Fig. 3.1B). Appendaged corollas characterize about one-third of the family, including at least some members of 14 of the 27 genera comprising Bromelioideae, six more in Pitcairnioideae, and three in Tillandsioideae. However, recent ndings drawn from studies of ontogeny question its current broad application as a taxonomic marker, particularly in Tillandsioideae where presence or absence differentiates Vriesea from Tillandsia (Brown and Terry 1992). Even Smith and Downs granted overall similarity greater weight at least once by subsuming Tillandsia pabstiana under Vriesea drepanocarpa. Petal scales clearly lack diagnostic value elsewhere in the family, for example in Pitcairnia and Puya where several species (e.g., Pitcairnia pulverulenta, Puya hofstenii) include individuals with appendaged and naked corollas.

The bromeliad petal scale assumes a variety of forms at maturity following a more uniform beginning. Development starts with the emergence of a pair of adaxial excrescences on the base of the expanding petal on either side of the antipetalous stamen lament (Fig. 3.1B). Primordia of Pitcairnioideae and some Tillandsioideae fuse into a single, variously bi d or lobed, tongue-like ap with an entire margin. Six petal scales characterize the more elaborately appendaged owers of many Bromelioideae, one on each side of the three stamen laments opposed to the same number of corolla members. Final shape may be sac or pouch-like with distal loba-tions or fringes. Accessory lateral folds of undetermined homology embellish some scales. Conversely, those of typically ant-inhabited Aechmea bracteata remain much simpler, perhaps consistent with small, autogamous owers and the modest amounts of nectar they produce to encourage outcrossing (Fig. 3.2C).

Brown and Terry (1992) discovered that petal scales emerge shortly before the bud expands preparatory to anthesis. Initiation sometimes coincides with microsporogenesis, but usually occurs later along with the post-meiotic, tetrad stage. Most signi cantly, scales proved to be the last external multicellular structures to form during petal development. So timed, this organ can appear and disappear in the evolutionary sense without effecting fundamental change in corolla structure.

As a "terminal ontogenetic character , these delicate appendages probably represent recent, minor modi cations of more deeply seated oral patterns. If so, features determined earlier during ontogeny should provide superior markers for genera and higher taxa. A hybrid between Billbergia nutans (appendaged) and a Cryptanthus (unappendaged) species possessed petals with scales, suggesting genetic dominance at a single locus. Brown and Terry (1992) concluded that petal scales represent synapomorphies in some parts of the family (e.g., possibly several subgenera in Aechmea, Neoregelia subgenus Hylaeaicum) and homoplasies within groups containing more divergent populations (e.g., Tillandsioideae, Pitcairnia, Puya).

Most of the speculation about petal scale function has focused on intraoral nectar management. Frequent ornithophily and the occurrence of septal nectaries at the bases of elongated owers support this contention. Brown and Terry (1992), Ueno (1989) and Böhme (1988) demonstrated that the ducts and pore systems occur in different locations to assure product delivery into capillary rather than noncapillary space located near the base of the corolla. Nectar consistently exits from points on the gynoe-cium (superior-ovaried species) or the oor of the hypanthium (inferior-ovaried species) opposite the antipetalous stamens. Scales and other oral parts assume various shapes and juxtapositions to elevate columns of nectar within essentially tubular corollas and stabilize the nectar s sugar concentration and viscosity by retarding evaporation, or they guide the mouth parts of pollinators.

Varadarajan and Brown (1988) obtained information on two Pitcairnia species that bears on scale function. Speci cally, they noted how this organ appears to diminish without reinforcing selection. Pitcairnia brevicalycina features yellow owers with or without petal appendages. When present, the scale is simple to match the accompanying modest nectar production. Pitcairnia heterophylla (Fig. 2.12A), on the other hand, produces scarlet, nectar-rich owers bearing larger scales equipped with elaborate distal modi cations.

Bee pollination seems to explain corolla structure and oral reward in the rst species; birds or large moths with comparably high caloric demands service the second population (Varadarajan and Brown 1988). Although petal scales assist nectar presentation in Pitcairnia heterophylla, they appear to be vestigial in Puya brevicalycina. However, nectar scale structure can be deceptive; even a pair of simple, vertical folds with no discernible glandular lining held nectar in the corollas of some Puya floccosa populations. Other modi cations of the bromeliad ower, such as the coherent, swollen lament bases in some Dyckia, may help maintain reservoirs of nectar in the absence of scales.

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