Tripartite Features

Mature pollen of conifers, such as Abies, Larix and Pseudotsuga, often shows proxi-mally a Y-shaped bulge, comparable to a tetrad mark, which is called an impression mark (HARLEY 1999). The mark results from the close proximity of the four pollen grains at the post-meiotic tetrad phase and is retained afterwards. Impression marks are also found in palm pollen. Note: the term tetrad mark is restricted to spores, where it is the germination feature, the impression mark of pollen grains is no germination feature.

Superficiallysimilarfeaturesinangiosperms are not comparable to that in gymnosperms. In recent and fossil Sapindaceae a three-armed feature (more precisely a triangle] is found. Cardiospermum has a narrow triangle (tenuitas) proximally, whereas other

Tripartite features

Abies cephalonica Pinaceae proximal polar view indistinct impression mark

Larix sp. Pinaceae, fossil proximal polar view Y-shaped impression mark

Larix sp. Pinaceae, fossil

Y-shaped impression mark

Tripartite features

Tripartite features

Cardiospermum corindum

Sapindaceae tricolporate left: equatorial view right: proximal pole with triangular area

Abies cephalonica Pinaceae proximal polar view indistinct impression mark

Larix sp. Pinaceae, fossil proximal polar view Y-shaped impression mark

Larix sp. Pinaceae, fossil

Y-shaped impression mark

Tripartite features

Cardiospermum corindum

Sapindaceae tricolporate left: equatorial view right: proximal pole with triangular area

Tripartite features synaperturate pollen

Melaleuca armillaris Myrtaceae syncolporate left: polar view right: polar area syncolpate left:

Primula denticulata Primulaceae polar view right:

Primula farinosa Primulaceae dry pollen

Tripartite features trichotomosulcus

Dianella tasmanica Phormiaceae dry pollen

Tripartite features synaperturate pollen

Melaleuca armillaris Myrtaceae syncolporate left: polar view right: polar area syncolpate left:

Primula denticulata Primulaceae polar view right:

Primula farinosa Primulaceae dry pollen

Tripartite features trichotomosulcus

Dianella tasmanica Phormiaceae dry pollen

recent and fossil Sapindaceae show such a feature at both poles.

The triangular pollen as found in Myrtaceae, some Primulaceae (Primula farinosa or P. denticulata) or in some Loranthaceae is characterized by a triangular field in both polar areas. The angles elongate to meridional rays (colpi) directed to the roundish pollen tips, the rays crossing the equator and forming an equivalent triangle on the antipodal polar area. Pollen is synaperturate (syncolpate, syncolporate).

Another tripartite feature is thetrichotomo-sulcus (HARLEY 2004), a three-armed sulcus found exclusively distally, as, e.g., in Dianella and Cretaceous fossils. Trichotomosulcate pollen has been discussed in relation to the evolution of the tricolpate dicot condition, but so far without success.

In contrast to the trichotomosulcus also trisulcate pollen is found. The angiosperm-like pollen of the fossil genus Eucommiidites has a distal sulcus and, at angles of ca 120° seen from the sulcus, two folds on the proximal side of the pollen. This feature was erroneously misinterpreted as more-or-less tricolpate pollen (with "colpi" equatorially situated, which has proved to be incorrect). A similar arrangement of a distal sulcus and two small additional "sulci" on the proximal face was described, for example, in some species of Tulipa (Liliaceae) and Tinantia (formerly Commelinantia, Commelinaceae), but these cases were never interpreted as equivalent to a tricolpate condition (HARLEY 2004). In some cases the three sulci are of quite equal size. The aperture condition is very similar to a tricolpate one. The interpretation "trisulcate" is possible and of use only in context with a morphological series.

Another three-armed feature is the triradiate aperture in Thesium alpinum (Santalaceae) pollen (an additional suggestive feature is the pollen shape forming a tetrahedron, the four triangular faces showing conspicuous reticulate areas that might be misinterpreted at first sight as four apertures). In fact Thesium alpinum pollen is three-aperturate, the apertures placed in the three tapered edges of the tetrahedron. Each aperture forms a very inconspicuous triradiate figure, which is situated equatorially. Two of the arms point towards the neighboring tetrahedron edge and are rather short; the third, elongated arm is directed to the rounded edge, which is probably the proximal pole.

Tripartite features

Thesium alpinum Santalaceae tricolpate, heteropolar

triradiate colpus polar view equatorial view polar view

Tripartite features

Thesium alpinum Santalaceae tricolpate, heteropolar

triradiate colpus polar view equatorial view polar view

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