Stigma Surfaces
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Stigma surface characteristics vary widely, but
these four examples, all
from different families,
illustrate two fairly
typical surfaces and two
with a slightly more
unusual format.
The stigma surfaces of L. longiflorum and V. speciosa are both clearly divided into three sub-surfaces which correspond to the three carpels of the ovary. The extended focus micrograph of the very small stigma (ca. 1 mm) of V. speciosa shows a deeply papillate surface (complete with a few pollen grains attached). The papillae are the ends of individual elongated cells, all aligned end-on across the whole surface. The stigma of L. longiflorum is much larger (ΒΌ"/6 mm across) than that of V. speciosa, and unlike some lilies it does not exude large quantities of sticky liquid to trap pollen grains. However, the stigma is slightly sticky to the touch and there is a quantity of orange/brown pollen grains stuck around the edges, all of which suggest that it has exuded a small amount of liquid.
Across the plant kingdom as a whole, the stigma of T. candicans is a odd structure, though less so if considered purely within the Cactaceae: it hangs well clear of the mouth of the perianth, and represents the tip of a very long style (ca. 5"/125 mm). The whole stigma is ca. 3/8"/10 mm across, so that its 17 individual limbs (this number seems to vary from plant to plant) give it a large surface area. This may represent an evolutionary response to its arid, sub-tropical environment, in which a flower that lasts less than 24 hrs. needs to gather as much wind and/or insect born pollen as possible, as quickly as possible.
The stigma of P. orientale is also multi-limbed (19 in the example shown, but this also varies from plant to plant) but in this case the style is absent, leaving a sessile stigma sitting directly on top of the ovary. The stigma is dry, but has a deeply papillate surface - rather like a fine brush. In common with T. candicans, the flower of P. orientale is quite short lived (2 - 3 days, depending on the weather), so the very large stigma may again be a mechanism for gathering large amounts of pollen quickly.
