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The majority of species are low, perennial herbs. The
spectrum of life forms is, however, much broader and includes a range from
small annuals to trees reaching 13 m or more in height. Here are some brief
comments about the main types:
Annual herbs (plants
germinating, flowering, fruiting and dying in one year or season). True
annuals are rare and are only found in genera which also include perennial
species, e.g. in the South African genus Streptocarpus and the Asian
genus Chirita. The plants occur in (sub)tropical areas with a
pronounced seasonal climate. With the possible exception of Sinningia
nordestina the annual habit is virtually unknown among the New World
gesneriads.
Perennial herbs.
This pattern is by far the dominant one in Gesneriaceae and essentially represents
an adaptation to a seasonal climate. Two forms can be distinguished: monocarpic perennials (flowering and fruiting once, after one or several
years of vegetative growth) and polycarpic perennials (flowering and
fruiting periodically). In the aseasonal (ever-wet) parts of the tropics
this distinction becomes blurred as flowering and fruiting is often
continuous, lasting over many years without interruption. This pattern can
be referred to as “prolonged monocarpy”; it is found, for
instance, in many species of Monophyllaea; these rain-forest plants
enter the reproductive stage soon after germination, and this stage lasts
without interruption several to many years until the plants die; death is
often by external causes such as damage, desiccation etc., while undisturbed
plants appear almost immortal.
Most common are polycarpic
perennials. They represent the typical growth form in subtropical,
warm-temperate and temperate areas around the globe, and are also found in
tropical regions with a slightly to pronounced seasonal climate. Perennation
serves to survive a period with adverse conditions, either a dry or a cold
(winter) season.
A striking array of
different perennation strategies can be observed in the family. These
include survival by
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Ground stems. These are
distinguished from proper rhizomes by their unspecialized habit: the basal
parts of the stems lie on the ground or are somewhat buried in the ground,
producing roots for anchorage and water uptake. However, they lack nutrients stored in
the stem or in cataphylls. Such ground stems are found in most ground herbs
of ever-wet rain forests, e.g., most Cyrtandra species.
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Stolons. These are
axillary side branches with markedly elongated internodes, creeping over the
ground and producing adventitious roots. They serve for perennation,
migration and multiplication. Conspicuous examples include species of
Alsobia (with one stolon per node and monochasial branching), and
Episcia (with two stolons per node and dichasial branching). Some species names
allude to the possession of stolons (Henckelia stolonifera, Boeica
stolonifera, Chirita stolonifera).
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Rhizomes and rootstocks.
Typical rhizomes (= distinctly thickened horizontal underground shoots) are
found in some Gesneriaceae of seasonal sub-tropical or temperate zones.
Their function is manifold: (1) perennation = survival of unfavourable
periods: the aerial plant parts die back at the beginning of the
unfavourable season, and new aerial shoots are produced from the rhizome
when favourable conditions return, (2) storage of nutrients (and to some
extent water), (3) migration: by the elongation of the rhizome the plant
migrates into areas hitherto not exploited by the roots, (4) multiplication:
branching of the rhizomes leads to multiplication of the aerial shoots and
thus vegetative propagation, (5) anchorage: with the aid of the
rhizome-borne roots the aerial shoots are mechanically anchored in the
ground. Rootstocks are shorter, much contracted and grow
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vertically, but a clear demarcation from rhizomes is not possible.
Rootstocks and transitional forms
to rhizomes occur in most rosette plants.
Examples are Saintpaulia, Petrocosmea, Corallodiscus, Didymocarpus,
Ramonda, Haberlea, Jancaea etc. Very instructive are the rhizomes of Platystemma.
They consist of a linear
succession of swellings, each
corresponding the production of an aerial shoot
consisting of a single leaf
with one or two axillary flowers.
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Scaly
rhizomes. This special type of
rhizome is found only in neotropical Gesneriaceae (especially in
the Gloxinieae). They represent underground appendages of the
shoot axis which serve as survival and propagation structures.
They are one to several centimeters long and consist of a thin
central axis and closely packed pairs or whorls of small, fleshy
leaf scales. They survive in the ground when the above-ground
plant parts die back in the dry period and sprout when
favourable conditions return. As one plant usually produces
several rhizomes, multiplication also takes place. A well-known
example is Achimenes, in which aerial scaly rhizomes are
sometimes also found. Intermediate forms between stolons and
scaly rhizomes are the so-called “stringy rhizomes” produced
above-ground in some species of Gloxinia.
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Tubers. Like scaly
rhizomes, tubers are found exclusively in the New World Gesneriaceae (Sinningia,
Chrysothemis, Lembocarpus, Rhoogeton and some species
of Nautilocalyx). The tuber may be buried in the ground, but (in
Sinningia) is more often exposed, with only the base and the roots
penetrating the ground. An above-ground portion of a tuber is sometimes
referred to as a “caudex”, and this general habit as “caudiciform”. Either
underground or caudiciform tubers may be globose, pear-shaped or
flat. The size depends on the species and the age of the plant. The largest
tubers have been found in Sinningia macrostachya, reaching a diameter
of up to 1 m.
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