Abstract:
Secondary pollen presentation is a well-known phenomenon in the Rubiaceae with particularly conspicuous pollen
presenters occurring in the tribe Vanguerieae. These knob-like structures are formed by a modification of the upper portion
of the style and stigma, together known as the stylar head complex. In the flower bud and shortly before anthesis, the
anthers surrounding the stylar head complex dehisce and release pollen grains which adhere to the pollen presenter. The
epidermal cells of the pollen presenter facing the anthers are radially elongated with a characteristic wall thickening
encircling the anticlinal walls of each cell towards the distal end. These cells were studied in the pollen presenter of
Vangueria infausta using electron and light microscopy in conjunction with histochemical tests and immunohistochemical
methods. Other prominent thickenings of the cell wall were also observed on the distal and proximal walls. All these
thickenings were found to be rich in pectin and possibly xyloglucan. The terms ‘‘thickenings of Igersheim’’ and ‘‘bands of
Igersheim’’ are proposed to refer, respectively, to these wall structures in general and those encircling the anticlinal walls of
each cell near the distal end. The epidermal cells have an intricate ultrastructure with an abundance of organelles, including
smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria and secretory vesicles. This indicates that these
cells are likely to have an active physiological role. The pollen grains possess prominent protruding onci and observations
were made on their structure and development. Walls of the protruding onci are also rich in pectin. Pectins are hydrophilic
and known to be involved in the dehydration and rehydration of pollen grains. We hypothesise that the thickenings of
Igersheim, as well as the protruding onci of the pollen grains, are functionally associated and part of the adaptive syndrome
of secondary pollen presentation, at least in the Vanguerieae.
