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14. Plectranthus petiolaris E.Mey. ex Benth.
Habit, habitat, distribution and phenology
Plectranthus petiolaris is a sprawling branched herb with ascending and descending
stems, reaching 1 m in height. It grows on forest margins and in scree below cliffs
covered by scarp forest. It occurs in the Eastern Cape (Transkei) northwards along the
coast through KZN and inland to Mpumalanga. It flowers from December to May.
Inflorescence and floral morphology
Flowers are produced in lax, racemose synflorescences (that occasionally branch at
the base), 100 – 250 mm in height. At Umtamvuna the flowers were deep purple
(violet), with the corolla lips often tinged with blue; at Oribi Gorge the flowers were pale
pink. The corolla tube is laterally compressed, 7 – 11 mm long and sigmoid in shape;
from the narrow base it ascends for 3 mm, then deflexes and expands to 3 mm wide at
the corolla mouth. The upper lip is 6 – 8 mm and the shallowly boat-shaped lower lip is
7 – 9 mm long. The four free stamens extend 4.5 mm (upper pair) and 5.5 mm (lower
pair) from the corolla mouth.
Study sites and observations
Pollinator observations were made at Umtamvuna and Oribi Gorge, where at least 18
hours were spent with different populations on various days over a number of years,
between 9.00 am and 5.30 pm. Some observations were also made on cultivated
plants in Pietermaritzburg.
Pollinators
The main pollinators of P. petiolaris were apinid bees of the genera Amegilla and
Xylocopa. At Umtamvuna Amegilla caelestina, A. mimadvena and Xylocopa hottentotta
were common visitors. At Oribi Gorge Amegilla bothai and A. caelestina were
commonly seen to visit the pink populations of P. petiolaris. At Pietermaritzburg
Amegilla caelestina was a frequent floral visitor to cultivated plants. These bees picked
up pollen ventrally on the thorax and abdomen. The flexible tip of the bee proboscis in
most cases corresponds to the bend in the corolla of P. petiolaris, and the full length of
the proboscis corresponds to the full tube length.
At Oribi Gorge one visit by the long-proboscid fly, Stenobasipteron wiedemanni, was
seen on P. petiolaris. The hovering fly bent its proboscis around the corolla bend to
Appendix/ 148
P. petiolaris