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

Corolla (Figure 8)

The corolla is most commonly funnel-shaped, but is quite often campanulate, or hypocrateriform, or sometimes suburceolate, the limb usually prominent, entire or shallowly lobed but occasionally deeply lobed, or much reduced and present only as five indistinct teeth. The corolla exterior has five prominent midpetaline bands, which may be more darkly coloured and/or more pubescent than other parts of the corolla exterior. The corolla is very variable in size from less than 1 cm long in species like I. eriocarpa or I. minutiflora to over 10 cm in length in species like I. jalapa, I. megalantha, I. parvibracteolata, I. subalata and I. pterocaulis. Size is an unsatisfactory character at one level because of its variability within individual species, but is nonetheless often characteristic of a particular species.
Corolla shape is usually, perhaps always, related to pollination. The commonest corolla shape consists of a very short subcylindrical basal tube which is then gradually widened to the mouth. Corollas of this type are described as funnel-shaped, are usually, pink, sometimes blue or white, in colour and are apparently pollinated by bees. The limb is entire, undulate or shallowly (very rarely deeply) lobed. When the corolla is very short, the tube is more abruptly widened from the base and is campanulate in form. This is characteristic of some species in the Batatas Clade and also of small-flowered species with a cream corolla, such as Ipomoea reticulata, I. corymbosa and I. syringiifolia. This kind of corolla tends to intergrade with the common funnel-shaped corolla. The corolla of the Arborescens Clade and some other, mostly woody liana species is shortly funnel-shaped (almost campanulate), white or white with a dark purple centre. These flowers may be bat-pollinated (McDonald 1991: 73, Felger and Austin 2005, Queiroz et al. 2015) but confirmation is needed in most cases.
Other corolla shapes are less common. A hypocrateriform or salver-shaped corolla in which the nearly cylindrical corolla tube is only slightly widened at the mouth is associated with red flowers, exserted stamens and bird pollination. This corolla type is characteristic of the Quamoclit Clade but is also fairly common in the Clade A2 in South America (Ipomoea exserta, I. longistaminea, I. ana-mariae, I. verruculosa), and especially the Caribbean (I. argentifolia, I. digitata, I. microdactyla, I. steudelii). In Mexico and northern South America it is more commonly associated with Clade B in the Pharbitis Clade (I. jamaicensis) and elsewhere (I. bracteata, I. dumosa, I. chenopodiifolia, I. retropilosa, I. tubulata). Occasionally the corolla limb is very deeply lobed as in I. repanda, I. hastigera, I. electrina (which is orange, rather than red). An occasional variation is the suburceolate corolla, in which the corolla tube is essentially cylindrical but somewhat swollen in the middle and with a short corolla limb consisting of small teeth. Ipomoea suburceolata from Bolivia, I. lobata and I. tehuantepecensis from Mexico and I. praecox from Cuba have flowers of this kind. nother variation is found in plants with a white or pale blue corolla in which the tube is exceptionally long. This type of corolla is associated with night-flowering hawk moth pollinated species. The best-known species of this type is I. alba but there are various others with similar corollas including I. habeliana, I. violacea, I. tuboides, I. scopulorum, I. riparum, I. santillanii, I. chiriquensis, I. ampullacea, I. macdonaldii and I. lottiae. Species with this kind of corolla are notably more common on oceanic islands and in Mesoamerica and Mexico than elsewhere.
Corolla colour. Field and herbarium observations of flower colour need to be treated with caution. Flowers change colour during the course of the day, most obviously in the case of Ipomoea nil, which is blue when fresh but turns pink as it ages and appears pink in herbarium specimens. Equally, one collector’s purple is another collector’s pink or lilac or even red. Although the great majority of species have a corolla colour that is generally described as pink, there are many exceptions. White flowers (often with a dark centre) are characteristic of the Arborescens Clade and of several other woody liana species, such as I. magna, I. longibracteolata, I. brasiliana and I. paradae, and are in some cases pollinated by bats. Night-flowering moth pollinated species typically with a hypocrateriform corolla, such as I. alba, I. santillanii, I. habeliana, I. violacea, I. ampullacea have pure white corollas. Campanulate or funnel-shaped white flowers are noted for many different species in different clades but are more common in the Batatas Clade (I. lactifera, I. lacunosa), Clade A1 (I. cerradoensis, I. macrorhiza, I. langsdorfii, I. vivianae, for example) and Clade A2 (I. proxima, I. suaveolens, I. pruinosa) but occasionally occur elsewhere (I. imperati). Many usually pink-flowered species are recorded as sometimes being white-flowered (I. acanthocarpa, I. bahiensis, I. carnea). Slightly different are those species with creamy or violet-tinged flowers such as I. lindenii, I. corymbosa, I. saopaulista, I. minutiflora and I. syringiifolia. Truly yellow flowers are rare in American Ipomoea but include I. ochracea, I. longeramosa and I. lutea. There are many subtle variations between red and pink. Red flowers being principally a feature of the Quamoclit Clade, some Caribbean species (I. montecristina, I. microdactyla, I. repanda and a few South American species notably I. cavalcantei). Some corollas are described as purple and include forms of I. indica, I. cuzcoensis and I. magnifolia. Blue flowers also occur and are often associated with a white corolla tube. I. hederacea, I. nil, I. aristolochiifolia, I. tricolor, I. marginisepala and I. cardiophylla are species with this corolla colour.
Corolla indumentum. The indumentum of the corolla exterior is best observed on buds as there is some evidence that hairs are caducous in some species as the corolla matures. Hairs are often difficult to see on open corollas but are best searched for at the tips of the midpetaline bands. Although previous studies have not seen corolla indumentum as particularly important taxonomically, we have found it of great significance both at species and clade level. It is nearly always constant in a particular species, exceptions being very rare and their existence raising doubts about the circumscription of the species in the few cases where it has been noted (Ipomoea lindenii, I. wolcottiana, I. brasiliana). All species of the Quamoclit and Batatas Clades have corollas glabrous on the exterior. All species in Clade A2 have coriaceous sepals and glabrous corollas (except I. discolor). All species in the very large Jalapa radiation (Species 1–83) have pubescent corollas.