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The pollen wall protects the sperm while the pollen grain is moving from the anther to the stigma; it protects the vital genetic material from drying out and solar radiation. The pollen grain surface is covered with waxes and proteins, which are held in place by structures called sculpture elements on the surface of the grain.
Stigma can vary from long and slender to globe shaped to feathery. [4] Pollen is typically highly desiccated when it leaves an anther. Stigma have been shown to assist in the rehydration of pollen and in promoting germination of the pollen tube. [5] Stigma also ensure proper adhesion of the correct species of pollen.
In angiosperms the ovule is enclosed in the carpel, requiring a specialised structure, the stigma, to receive the pollen. On the surface of the stigma, the pollen germinates; that is, the male gametophyte penetrates the pollen wall into the stigma, and a pollen tube, an extension of the pollen grain, extends towards the carpel, carrying with it ...
The stigma (from Ancient Greek στίγμα, stigma, meaning mark or puncture) is usually found at the tip of the style, the portion of the carpel(s) that receives pollen (male gametophytes). It is commonly sticky or feathery to capture pollen. The word "pistil" comes from Latin pistillum meaning pestle.
The stigma has been found to be covered by hydrophilic proteins on the outer wall; these are probably the ones that act in the recognition of suitable pollen and in the reactions of self-incompatibility, in which case sometimes callose is deposited to stop the germination of incompatible pollen.
Xenogamy (Greek xenos=stranger, gamos=marriage) is the transfer of pollen grains from the anther to the stigma of a different plant. This is the only type of cross pollination which during pollination brings genetically different types of pollen grains to the stigma. [1]
A pollen grain sticks to the stigma at the top of the pistil, germinates, and grows a long pollen tube. A haploid generative cell travels down the tube behind the tube nucleus. The generative cell divides by mitosis to produce two haploid (n) sperm cells. The pollen tube grows from the stigma, down the style and into the ovary.
Reverse herkogamy - (called "thrum flowers") is displayed when the stigma is recessed below the level of the anthers. This arrangement causes floral visitors to first contact the anthers before the stigma. For this reason, reverse herkogamy is believed to facilitate greater pollen export than approach herkogamy.