Genus of plants
Streptocarpus ("twisted fruit" from Greek στρεπτός ("streptos") "twisted" and καρπός ("carpos") "fruit") is an Afrotropical genus of flowering plants in the family Gesneriaceae. The genus is native to Afromontane biotopes from central, eastern and southern Africa, including Madagascar and the Comoro Islands. The flowers are five-petalled, salverform tubes, almost orchid-like in appearance, and hover or arch over the plant, while the pointed, elongate fruit is of a helical form similar to that of the "tusk" of a narwhal. In the wild, species can be found growing on shaded rocky hillsides or cliffs, on the ground, in rock crevices, and almost anywhere the seed can germinate and grow. For the home, there are now many hybrids of various colours and forms available.
Although generally referred to simply as "Streptocarpus" or "Streps", the common name for subgenus "Streptocarpus" is Cape primrose, referring to the nativity of several species to South Africa and their superficial resemblance to the unrelated genus "Primula". The common name for subgenus "Streptocarpella" is nodding violet. "Streptocarpus" sect. "Saintpaulia" ("African violet") is a separate section within "Streptocarpus" subgenus "Streptocarpella".
DNA studies have shown that, despite not having a twisted fruit, African violets evolved from within the Tanzanian "Streptocarpus" subgenus "Streptocarpella".
There are a few Asian species that have recently been removed from the genus, most notably "Streptocarpus orientalis", now reclassified as "Damrongia orientalis." Molecular systematics has shown conclusively that they are not true "Streptocarpus".
Description.
There are two subgenera within "Streptocarpus", namely subgenus "Streptocarpus" and subgenus "Streptocarpella".
Subgenus "Streptocarpus".
Within the "Streptocarpus" subgenus "Streptocarpus", there are two main forms, the plurifoliates and the unifoliates.
"Streptocarpus" with more than one leaf are called "plurifoliates", and there are two main types of these. First is the rosulate form, which is perennial. Rosulates are made up of a basal rosette of leaves. Flower stems sprout from the upper surfaces of the bases of these leaves. The most common "Streptocarpus" houseplant is of the rosulate type. The flowers of modern rosulate hybrids are generally three to several centimeters in diameter. The other plurifoliate type is essentially somewhere in between a rosulate and a unifoliate. In this group, two or three leaves grow in addition to the first leaf. The plurifoliates are perennial. Examples are "Streptocarpus prolixus", and "Streptocarpus polyanthus" ssp "polyanthus".
The second form is the unifoliate, which only has one leaf. In the unifoliates group, one single leaf grows continuously from the base. Many unifoliates are also monocarpic, which means that they will flower once, set seed, then die. But they may take a few years to reach this stage. In other unifoliates, the original leaf may die, but one or two new leaves will sprout from it, and the plant continues to grow.
This subgenus (comprising plurifoliates and unifoliates) is unique in that, in winter, they can form abscission lines part way down the leaf (see photo gallery below). The leaf then dies back to this point, whilst the proximal part of the leaf stays alive and healthy. In unifoliate species, the remaining healthy portion of that leaf will start to grow again from the base.
Subgenus "Streptocarpella".
Members of the "Streptocarpus" subgenus "Streptocarpella" are very different in form to those mentioned above.
The flowers and seed pods are similar, but the leaves and stems are caulescent (have stems). "Streptocarpellas" are generally clump-forming or trailing plants. Their flowers are only about 2.5-3.5 cm in diameter, and their colour range seems to be limited to mid-purples, pale pinks, and white.
"Streptocarpella" leaves can be decussate in arrangement (each pair of leaves at a node is at 90 degrees to the ones preceding or following it), or ternate (whorls of 3 leaves at each node). Some specimens may exhibit both on the same plant. Streptocarpella are grown as houseplants, hanging plants, and sometimes as bedding plants.
These two "Streptocarpus" subgenera do not interbreed.
For information on "Streptocarpus" subgenus "Streptocarpella" section "Saintpaulia", see "Streptocarpus" sect. "Saintpaulia".
Flower anatomy.
"Streptocarpus" flowers are five-lobed, salverform, zygomorphic tubes. The diagram at the right shows a labelled drawing of a vertical cross-section along the length of a "Streptocarpus" flower.
It is important to note that the two anthers (each arising from one filament on each side of the flower), are loosely joined together. As the anthers mature, they open slightly and act like a pepper shaker, which means that not all the pollen is released at once.
"Streptocarpus" flowers have evolved to be pollinated by birds, long-tongued flies, butterflies and probably long-tongued moths and bees. Although almost any small insect, animal, or breeze could potentially pollinate "Streptocarpus". "Streptocarpus" flowers often have nectar guidelines that guide would-be pollinators to the nectar (and anthers and stigmas). Self-pollination is also common.
Species.
184 species of "Streptocarpus" are currently recognized. "Streptocarpus rexii" is the first species described.
A complete list of the species and their synonyms can be found at the Smithsonian's World Checklist of Gesneriaceae.
A list of selected species from representative locales follows:
Cultivation.
Each grower will have their own preferences for cultivation. The details given below are a tested general guide, but "Streptocarpus" will do quite well on either side of these optimums.
The two main things to remember when growing "Streptocarpus" are that they do not like soil that is too wet, and they do not like it too hot.
Soil: Use an ordinary commercial potting mix with 1/8 to 1/4 perlite mixed in. This makes sure the soil will retain some moisture but not get boggy. Always have adequate drainage holes at the bottom of the pot you are planting in.
Temperature: . They can be taken down to or less in winter for a rest.
Light: Medium to bright indirect light is best. However, a bit of morning/late afternoon sun is more than okay. Even in dimmer light, they will flower - but less floriferously.
Water: Water only once the soil is almost dry. Some growers prefer to water only when the leaves have just started to wilt (or just before). They recover very well from dehydration, and this is one of the traits of the species. Make sure the pot has holes in the bottom to drain water, and never leave the pots sitting in a saucer of water.
Feeding: Feed occasionally with a "fruit and flower" or general fertilizer.
Seasons: Generally, "Streptocarpus" will flower from spring to autumn. In winter, they will stop flowering and may lose some leaves, which is normal. However, some varieties flower in winter.
Pruning of leaves &amp; flowers: You may slice off yellowing or browned leaves at the base - these will be the older leaves naturally dying off. If there is a healthy leaf with some blemishing, you can successfully cut off only the blemished parts and trim the leaf to a normal shape. With regards to flowers, snip off individuals as they finish, then snip the whole stem off at the base once the last flower on that stem is spent.
Cut flowers: "Streptocarpus" flowers also make excellent cut flowers, especially the long-stemmed varieties. They last well.
Pests and diseases: "Streptocarpus" are generally pest and disease-free. However, the most common afflictions are aphids and mealybugs. These are easily treatable with commercial insecticides or cultural pest removal methods.
Leaves and abscission: It is common for older leaves to die off occasionally, but especially in winter. They may be snipped off. New leaves will replace them.
The leaves of some perennial, but usually unifoliate "Streptocarpus", are unusual because, as winter approaches, they slowly die back to an abscission line midway down the leaf. The end portion of the leaf will gradually die back to this line. In most flowering plants, an abscission line forms at the base of the leaf, and the whole leaf will fall off (e.g. the leaves of deciduous trees like oak).
Propagation.
Propagation is usually either by seed or leaf cuttings. Some species produce plantlets from the roots, which can be used to propagate the plant. Mature clumps of plants can also be divided up and repotted.
"Streptocarpus" species seed that has been self-pollinated will grow true to type.
Self-pollinated hybrid seed will not grow true to type. The only way to propagate hybrid plants and retain the characteristics of the hybrid, is by leaf propagation (or other types of vegetative cloning)
For "Streptocarpus" subgenus "Streptocarpus":
By seed: "Streptocarpus" seed is generally very fine (see image gallery below). To germinate, the seed must be scattered thinly on top of potting mix, as they require light to germinate. The pot they are sown in must be covered in clear plastic “cling film” to keep up the humidity. Keep the sown seed where it will get bright, indirect light, and remain about 18-20 degrees Celsius. Keep them out of direct sunlight.
By leaf:
A "Streptocarpus" leaf can be severed at the base, taking some petiole (but above where flowers arise), and potted, base-down, in a few centimetres of potting mix. Place a clear plastic bag over the pot and secure with a rubber band to keep up the humidity.
Leaf segments, cut either horizontally across the leaf, or length-wise along the leaf (removing the midrib), can be used as cuttings in much the same way. Place the segments cut-side down in soil, as above. For unifoliates, this method is apparently less successful, but not impossible. However, it must be done before the plant flowers.
"Streptocarpus" leaves have a high concentration of cytokinin (a type of rooting hormone), so the use of artificial rooting hormones is unnecessary.
By root plantlet: Un-pot a plant that you know is susceptible to producing root plantlets (e.g. "Streptocarpus johannis", and its sports ‘Falling Stars’ and 'Gloria'. You will see, once the roots are exposed, whether any plantlets have formed between the soil and pot.
Sometimes, these plantlets will be evident growing out of the holes at the bottom of the pot. You can then snip these plantlets off (preferably with their attached roots), and plant up as for leaf propagation above).
By clump division: You can divide a multi-crown clump into pieces (each with a root system), and plant up as for leaf propagation above.
For "Streptocarpus" subgenus "Streptocarpella":
By seed: "Streptocarpus" seed is generally very fine. To germinate, the seed must be scattered thinly on top of potting mix, as they require light to germinate. The pot they are sown in must be covered in clear plastic “cling film” to keep up the humidity. Keep the sown seed where it will get bright, indirect light, and remain about 18-20 degrees Celsius. Keep them out of direct sunlight.
By stem cuttings: Cuttings of about 5–10 cm can be taken beneath a leaf node. When the cutting is placed in clean water, it will sprout roots. Keep the cuttings in bright, indirect light at about 18-20 degrees Celsius. Once the roots are about 5 cm long, you can pot up the cutting into the soil mixture mentioned above.
Hybridisation.
Hybridisation of "Streptocarpus" is conceptually very simple. The pollen of one plant (therefore the father) is placed onto the tip of the stigma of another plant (the mother). To prevent unwanted self-pollination of the mother plant, carefully remove its anthers beforehand. To help prevent unwanted cross-pollination, cover the newly fertilized stigma with a small plastic bag. If fertilization was successful, the seed pod will start to elongate within a few days. As the pod grows, it will start showing its namesake twisted form. Once the pod is mature, it will turn brown, dry off, and split open along the spiralled seams to release the seeds.
Another method that is used to create hybrids is to use radiation or chemicals to alter the genes. For example, irradiation may be used (e.g. xrays, gamma rays) to induce mutations that may give rise to plants with new characteristics. Another example is the use of colchicine to induce polyploidy (multiplying number of chromosomes) in plants, also to introduce new traits.
Over the years, numerous hybrids have been produced. "S. rexii" was used in many of the early hybrids, and its form is one that we most recognize in modern-day hybrids. But more recent hybrids may contain the genes of other species or hybrids. When making a cross, hybridizers keep in mind which traits they wish to bring out or improve in the progeny.
Much hybridizing work has been done to produce modern hybrids with an increased range of flower colours and forms, leaf variations, increased flowering periods, and more recently to introduce scent. Thanks to the work of hybridizers, "Streptocarpus" now come in a range of colours. These include reds, pinks, purples, blues, yellows, whites, and near-blacks. The only colour that is so-far not available is