A foundation monograph of Ipomoea (Convolvulaceae) in the New World

Sepals (Figures 2–7)
The calyx is formed of five overlapping, free sepals. The two outer sepals are usually similar in size and form as are the two inner sepals, which often have relatively broad, scarious, glabrous margins. The middle sepal is intermediate in size and shape and is commonly asymmetrically scarious. The sepals are often of considerable taxonomic significance and constitute important conserved characters at the species level. The differences in size and shape between the inner and outer sepals are often of great significance. The apex is frequently especially diagnostic. Many species have mucronate sepals, but the mucros are often caducous so some or even all sepals may appear muticous or retuse. Also important is the abaxial surface of the outer sepals which may show all kinds of variation in indumentum, venation and surface which can be smooth, muricate or armed with soft spines. In a few species notably in the Arborescens Clade, the presence of hairs on the adaxial surface is significant. As observed by Hallier (1893a), the sepals are accrescent in fruit, more especially so in the lianas such as I. brasiliana or I. tiliifolia, sometimes doubling their size after anthesis and becoming wider so sepals which were lanceolate at anthesis may become ovate in fruit. They may also enclose or nearly enclose the capsule.
Many sepals display unusual features including:
• Very unequal sepals: I. anisomeres, I. cryptica, I. squamosa, I. asarifolia, I. paludicola, I. maurandioides, I. macedoi.
• Adaxial (inner) surface hirsute: Arborescens Clade, I. longibracteolata, I. magna.
• Subterminal awns: all species in the Quamoclit Clade.
• Sepals terminating in a long awn: I. alba, I. muricata, I. nil, I. hederacea. Sepals of some other species, such as I. incarnata, may be interpreted as terminating in an awn.
• Sepals with fleshy spine-like trichomes: I. crinicalyx, I. echinocalyx, I. altoamazonica, I. silvicola, I. setosa, I. tentaculifera, I. lozanii (smaller than in other species),
• Sepals with a prominent abaxial appendage, I. rosea, I. bahiensis; I. decemcornuta.
• Sepals with swollen abaxial tumour: I. appendiculata.
• Sepals with 1–2 prominent black abaxial glands: I. hieronymi, I. megapotamica.
• Sepals muricate: I. plummeae, I. capillacea, I. madrensis, I. aristolochiifolia, I. pedicellaris, I. obscura, I. ochracea, I. cairica, I. asarifolia, I. paludicola, I. procurrens, I. coriacea.
• Sepals with prominent longitudinal ribs: I. fimbriosepala, I. setifera, I. parvibracteolata, I panduata.
• Sepals with fimbriate margins: I. tenera, I. sidifolia (sometimes).
• Sepals with a prominent cordate base: I. macedoi, I. apodiensis, I. pantanalensis, I. pubescens, I. lindheimeri.
The great diversity of sepal form is curious and not easily explained. It has been suggested that the development of coriaceous and large sepals may have evolved in response to the need to protect nectar glands from robber insects. (McDonald 1991).