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Morphological characters and their use in species delimitation
In the following section we discuss the range of characters which have proved useful in species delimitation and have indicated some of the pitfalls in their use. Taxonomic decisions often have to be made using incomplete material. Many species of Ipomoea are extremely localized in their distribution and many of their morphological characters are unknown, particularly the roots and the fruit characters, which are unknown for perhaps a third of species.
Habit and lifeform
Species of Ipomoea may be annual or perennial, herbaceous or woody, twining (or at least scrambling), erect, decumbent or prostrate. All of these characters are potentially useful in species delimitation and are used in the keys. It is useful, for example, to distinguish between lianas and scandent herbs or between prostrate or erect herbs but the distinctions need to be treated with caution. Many species have a woody rootstock and herbaceous stems, which may or may not be woody at the base. Stems may become somewhat woody with age. Twining plants may be trailing in the absence of shrubs to climb on. We have also avoided the use of the term vine as it is sometimes used to mean a woody climber (like the grape vine), so almost a synonym of liana, and sometimes to mean a relatively slender twining plant.
Annual species are characterized by having fibrous roots and typically flower in the late rainy season (tropical summer) as they require sufficient time to reach maturity after the onset of rains. In the herbarium, in the absence of roots, annuals can often be identified by their slender habit and the presence of mature capsules on flowering specimens. Perennial species, in contrast, are relatively stout and often lack mature capsules on flowering specimens or are almost entirely without corollas on fruiting specimens. It is possible that some normally annual species perenniate under suitable circumstances, especially in areas with no distinct dry season. There are no known erect annual species. Annual species are not found in Clades A1 or A2. In contrast they are well-represented in the Batatas (A3 in part), Pharbitis (B1 in part), Quamoclit (B2 in part) and the Pedatisecta Clades (B2 in part).
The majority of species are twining perennial herbs or lianas with petiolate, ovate, cordate leaves. The inflorescence is formed of pedunculate axillary cymes, the cymose structure usually being very obvious, although the cymes are sometimes reduced to single flowers. There is a tendency for some of the lianas to develop inflorescences on short leafy branchlets, rather than from the axils of the stem leaves.
Somewhat similar is a less well-defined assembly of essentially trailing plants. At one extreme these species root at the nodes and form extensive mats, in one case (Ipomoea aquatica) extending its stems to float on shallow water. Two widespread submaritime species, I. pes-caprae and I. imperati, are good examples of this growth form. More common are trailing species that do not root at the nodes. They usually grow in open, often sandy inland habitats. These trailing species often have shortly petiolate, elliptic leaves rounded to truncate at the base combined with axillary cymose inflorescences, these sometimes being shortly pedunculate. These trailing plants are, thus, apparently intermediate morphologically between the true climbers and the erect species. Some trailing species are morphologically indistinguishable from the climbers, the prostrate habit apparently the consequence of the absence of suitable plants to climb. Ipomoea maurandioides, a South American species principally of rock outcrops, is one such example.
The erect habit is usually associated with subsessile, oblong, lanceolate, or oblong-elliptic cuneate-based leaves with a terminal inflorescence, the upper leaves clearly bract-like and the pedicels and peduncles reduced so the inflorescence is subracemose or even subspicate in form. Species with this habit occur mostly in open grasslands and especially in the cerrados of South America. Most species produce annual stems from a tough woody perennial subterranean xylopodium, which is resistant to fire, a characteristic and perhaps defining feature of these habitats. Erect species are found in many different clades but are unknown in the Batatas, Quamoclit and Pharbitis Clades and rare in Clade B.
The erect habit is also associated with a number of shrubs and small trees often treated as Section Arborescens. These usually (always?) have white latex and often flower when leafless or nearly leafless. The inflorescence often develops on short branchlets and is not obviously axillary and cymose in structure. The corolla is white with a dark centre, subcampanulate to funnel-form in shape and possibly bat-pollinated (Felger and Austin 2005). Species with these characteristics mostly occur in very dry forest along the mountain chains of Mexico, Central America and the Andes and are completely absent from Brazil and the Caribbean.
Much the most widespread and common erect species, Ipomoea carnea subsp. fistulosa fits none of the above characteristics, having ovate cordate leaves and pink flowers in axillary cymes but its uniqueness is perhaps a consequence of its close relationship with Ipomoea carnea subsp. carnea which is a characteristic climbing species, from which it is presumably diverged.
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