Gesneriaceae:
Exceptional Morphologies (part 2)

 

Epithematoid Gesneriaceae

In this small group of mainly SE Asian plants, each genus exhibits its own strange morphology. The most peculiar genus of this alliance is Monophyllaea. Plants of this Malesian genus consist of a single, huge foliar organ (macrocotyledon), and a long stalk (hypocotyl). The plants thus comprise the same elements as the unifoliate species of Streptocarpus. However, apart from the obvious difference of the elongated hyopcotyl, there are strong differences in the formation of the inflorescences. While in Streptocarpus the inflorescences are produced in a series in the axil of the macrocotyledon, those of Monophyllaea emerge in an alternate sequence, forming two ranks in an almost distichous arrangement. The cymes are associated with tiny bracts at their base (Weber 1975a). From the species arrangement of the bracts and the axillarly cymes it can be inferred that the ancestors of the genus must have been a Whytockia-like plant (Weber 1976a). Indeed, recent molecular analyses show that Whytockia and Monophyllea are sister genera (Mayer et al. 2003).

Perhaps the most peculiar species of Monophyllaea is M. singularis from Borneo. In this species the flowers are arranged in numerous tiny groups along one side of the stalk (sometimes also along the midrib of the macrocotyledon). Here a mesocotyl is formed as in Streptocarpus. The flower groups are produced in a most peculiar manner: mature epi- and hypodermal cells divide secondarily and form meristematic spots from which the flower groups emerge (Weber 1990; Imaichi et al. 2001). The groups correspond to the inflorescence aggregates of other Monophyllaea species, but comprise only few and epedunculate cymes with few flower pairs.

Also noteworthy are some Monophyllaea species which produce several leaves and exhibit a phyllomorphic structure (Burtt 1978)

Whytockia is a small genus of S China and Taiwan. It is a rhizomatous plant with aerial shoots. The shoots bear pairs of strongly asymmetrical leaves which are, however, very unequal. One is a normal foliage leaf, the other reduced to a tiny, stipule-like structure. The leaves alternate and fall roughly into a single plain. The inflorescences (unbranched, pair-flowered cymes) emerge only from the foliage leaves. By the alternate arrangement of the laves they also from two ranks.

Similar in habit to Whytockia are Loxonia and Stauranthera. In these genera the inflorescences, however, do not emerge from the axils of the large leaves, but stand opposite and seemingly emerge from the axils of the rudimentary leaves. Closer analysis shows that they represent terminal and complex structures, comprising two to several pair-flowered cymes in alternate position. The whole flowering region of Loxonia and Stauranthera is composed of sympodial units, each comprising a strongly anisophyllous leaf pair and the terminal complex inflorescence. By the growth of the subsequent units the terminal inflorescences are bent to the opposite side of the large leaves (Weber 1977).

In Rhynchoglossum, anisophylly has proceeded to its extreme: the small leaves are completely reduced and only the strongly asymmetrical foliage leaves remain, being placed in two almost opposed ranks. As in Loxonia and Stauranthera, the inflorescences are terminal, but the cymes are reduced to single flowers. This results in unilateral racemes with two flower rows, often (especially in the small-flowered annual species) with highly increased flower number (Weber 1978).

The genus Epithema is unique in several respects. Above the strongly unequal cotyledons (placed at the very base of the plant and decaying soon) a single foliar leaf is produced, recalling an additional macrocotyledon. It is followed by one to several opposite pairs of ± equal foliar leaves. The stem and the axillary shoots are terminated by an inflorescence consisting of a globose “head” (an extremely contracted pair-flowered cyme) and a large, cucullate bract (Weber 1976b).

The last genus of the alliance is the ill-known Chinese genus Gyrogyne (monotypic with G. subaequifolia). By the plicate calyx it is said to be closely related to Stauranthera. Its subequal leaves may represent an ancestral condition to the strongly unequal leaves of Stauranthera.

 

Sinningia tuberosa and Lembocarpus amoenus

These species belong to the morphologically most remarkable species of neotropical Gesneriaceae. Both plants possess a storage organ in the form of a tuber and produce usually only one leaf per season. In Sinningia tuberosa the tuber is perennial and produces one (rarely two or more) tiny vegetative shoot(s) at its top. This shoot produces first a few pairs of tiny leaflets and then a strongly unequal leaf pair, with one leaf developing into a large foliar leaf. The flowers are produced on separate short shoots. The two leaves in the illustration are probably not an accurate reflection of the model plant, although the brilliant red flowers growing directly from the tuber definitely are.

In Lembocarpus amoenus the tuber is annual and a new tuber is formed each year on top of the old one. The tuber produces also a tiny shoot with an extremely anisophyllous leaf pair, but - in contrast to Sinningia - no separate floral shoots are developed and the inflorescence is produced in the axil of the foliar leaf.

 

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