Streptocarpus:
Growth Forms

 

 

The genus Streptocarpus has traditionally been divided into the subgenera Streptocarpus and Streptocarpella.  Subgenus Streptocarpus is comprised of plants with typically strap-like leaves, which grow either in an irregular rosette (rosulate) or as a single leaf (unifoliate).  Subgenus Streptocarpella is comprised of conventional-looking herbs (called herbaceous or caulescent), with stems and leaves, and with flowers produced from the leaf axils.  The Streptocarpellas can vary in growth habit from very short to quite tall herbs of up to one metre.

In common with all members of the Coronantheroid and Gesnerioid gesneriads, all members of the genus Streptocarpus begin life as seedlings possessed of two tiny cotyledons or seed leaves, only one of which grows beyond its initial tiny size.  This pattern is referred to as "anisocotely", and is discussed in the "Seedlings" chapter of Weber's Gesneriaceae, a Scientific Perspective on this website.   In the case of the Streptocarpella  group, normal development of stems and leaves proceeds after initial enlargement of the single cotyledon. 

In subgenus Streptocarpus, on the other hand, the active cotyledon continues to grow without production of stems.  This leaf is referred to as a phyllomorph.  In the unifoliate Streptocarpus the phyllomorph produced from the cotyledon becomes the only leaf the plant ever has.  Unifoliate species develop flowers and produce seed from this single leaf, which can reach a considerable size, in some species 75 cm long and 30 cm wide. Many of the unifoliate Streptocarpus are also monocarpic (= single-fruiting), as the single phylomorph dies after flowering and setting seed. In some of the unifoliates the original phylomorph dies, but one or two new leaf phylomorphs sprout from it and the plant survives for another season in this way.

In contrast, the rosulate species also start out with the development of one cotyledon into a phyllomorph, but then another leaf phyllomorph develops from the first leaf and eventually a rosette shaped plant with numerous leaves arises. Rosulate species are also referred to as plurifoliate and can as a result continue growing for many years and are therefore perennial. Their leaf sizes also vary from tiny leaves a few millimetres long to 40 cm long, and up to 6 cm wide. The unifoliate and rosulate species are quite similar, however, and some species have forms which are unifoliate and monocarpic, with other forms of the same species being plurifoliate and perennial.

A characteristic of the unifoliate and rosulate species distinguishing them from the herbaceous species is the formation of leaf abscission zones during winter. These abscission zones will develop on a leaf after the first winter chill, often resulting in the die back of the distal two thirds of the leaf . The remaining leaf stub will then grow out in the next spring and extend again to become a large leaf.

This mechanism most probably evolved as a measure to counter moisture loss during winter. The living surface of the leaf always transpires a certain amount of moisture, and substantially reducing the living leaf surface reduces transpiration, enabling the roots and remaining leaf to survive.  Many Streptocarpus occur in areas which have a summer rainfall and dry winters and as a result are exposed to drought conditions during winter.

 

 

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