Based on morpho-anatomical features, three basic water-gap complexes (Types-I, -II and -III) were identified in species with PY in 16 families. Depending on the number of openings involved in initial imbibition, the water-gap complexes were sub-divided into simple and compound. The proposed classification system enables understanding of the relationships between the water-gap complexes of taxonomically unrelated species with PY.

Timing of seed germination with the onset of autumn can be explained by PY-breaking processes involving (a) two temperature-dependent steps in G. carolinianum and (b) one or two moisture-dependent step(s) along with the inability to germinate under high temperatures in G. dissectum. Geraniaceae is the third of 18 families with PY in which a two-step PY-breaking process has been demonstrated.

Acquisition of PY in seeds of G. carolinianum occurs after physiological maturity and is triggered by maturation drying. The micropyle and water gap cannot be considered as two separate entities, and thus it is more appropriate to consider them together as a 'micropyle--water-gap complex'.

Physical dormancy (PY) occurs in at least 18 angiosperm plant families and is caused by water-impermeable palisade cells in seed (or fruit) coats. Breaking of PY involves disruption or dislodgement of water-gap structures causing the seeds/fruits to become water permeable (non-dormant). The water-gap region is a morphologically distinct area of the seed or fruit coat that forms a water-gap complex. The location, anatomy, morphology and origin of water-gaps can differ between and even within families and genera. Water-gap structures sense environmental conditions that allow seeds with PY to become permeable just prior to the commencement of conditions favourable for germination and plant establishment. There are three basic water-gap morpho-anatomies characterized by the way the water-gap opens: Type-I, Type-II and Type-III. In Type-I water-gaps, specific kinds of cells pull apart to form a surface opening, while in Type-II a portion of the surface structure is pulled away from adjacent cells, opening the water-gap. Type-III is the least common type and has a circular, plug-like structure that is dislodged, whereby water entry occurs. In addition, water-gap complexes are either simple or compound, depending on whether only a single primary water-gap structure is involved in dormancy release or an additional secondary water-gap structure opens, permitting water entry.

Dislodgment of swollen 'hinged valve' palisade cells adjacent to the micropyle caused the water gap to open in physically dormant seeds of G. carolinianum, and it was clear that initial water uptake takes place through this gap and not via the chalazal opening as previously reported. This water gap ('hinged valve gap') differs from water gaps previously described for other families in morphology, anatomy and location in the seed coat.