Tristyly refers to the existence in a plant population of three different floral morphological types.  The differences between the types are manifested in the relative heights of the reproductive structures of the flower.  Anthers (male reproductive organs) and the stigma (female reproductive organ) may occur at three different heights within a flower, as shown by the figure.  In any flower, the stigma occurs at one height, while anthers occur at the other two heights.  For example, in the L (long) morph,
the stigma occurs at the highest position, while anthers occur at the middle and lowest positions.  In the M (medium) and S (short) morphs, the stigma occurs at the middle and lowest positions, respectively, while anthers occur at the other positions.

    In many tristylous plant species, floral morph is determined genetically.  Typically, two loci govern heights of the reproductive structures.  At the S locus, an individual plant that is of genotype Ss has short-morph (S in figure) flowers regardles of genotype at the M locus, while an individual of genotype ss has stigma height determined by the M locus.  Note that there are no SS individuals.  At the M locus, individuals that are ssMM or ssMm have stigmas at the middle position (M in figure), while individuals that are ssmm have stigmas at the tallest position (L in figure).

    Most tristylous species also exhbitig within-morph incompatibility: flowers of a particular morph can not be fertilized by pollen of the same morph.  This incompatibility is genetically based and means, for example, that L-morph individuals can be pollinated only by M- and S-morph individuals.  The difference in anther and stigma heights helps avoid pollen wastage in this type of system.  Pollen from the tall anthers of M and S morphs is placed on a specific portion of the pollinator's body that tends to contact only tall L-morph stigmas, and hence is only transfered to L morphs, etc.
 

Figure from S.C.H. Barrett.  1988.  Evolution of breeding systems in Eichornia (Pontederiaceae): a review.  Annals of the
    Missouri Botanical Garden 75: 741-760.
 
 

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