*3.3.2 Morphological dormancy (MD) or class B*

Class B dormancy is evident in seeds whose embryos are under developed in terms of size but well differentiated cotyledon and hypocotyl-radical. The embryos are not physiologically dormant but simply needs time to grow before seed germinates, i.e., growth period of embryo = period of dormancy.

#### *3.3.3 Morpho-physiological dormancy (MPD) or class C*

It is common in seeds that have underdeveloped embryos 'in terms of size', supplemented with physiological component to their dormancy. These seeds therefore need a dormancy-breaking treatment that will take in to account both morphological and physiological constraints. For example a defined combination of warm and/or cold stratification which in some cases can be replaced by GA treatment. In MPD-seed, embryo growth/emergence requires a considerable longer period of time than in MD-seeds.

## *3.3.4 Physical dormancy (PY) or class D*

PY is known to occur only in angiosperms [17]. Seeds with physical dormancy possess water impermeable fruit coats 'pericarps' or seed coats 'testa'. Impermeability might be enhanced by many factors among which are several layers of tightly spaced, thick-walled cells in the pericarp and testa, the presence of waxes, lignins, and pectins, or a combination of these factors, all impeding water uptake during imbibition [17]. As far as these impermeable layers remain intact, dormancy is maintained until when subjected to natural or artificial conditions that will disintegrate the impervious layers. Wide temperature variations, fire, drying, and passage through gut of animals are among natural factors that can disintegrate impervious layers of the pericarp. Mechanical or chemical scarification is common practices used in seed technology to break PY dormancy. In several cases, specialized structures are associated with the control of water-impermeability, for example, in PY-dormant *Anacardiaceae* species (Rosids), the endocarp consist of three water impermeable palisade layers (macrosclereids, osteosclereid and brachysclereids) and the outer crystalliferous layer which block water entrance to the carpellary micropyle in dormant seeds [73]. Once the impervious layers becomes permeable following natural or artificial scarification methods, the later can no longer return to their initial stage of absolute impermeability. Thus the timing natural release of dormancy is a more significant in the life cycle of plants with PY, than it is in plants with PD [74].

#### *3.3.5 Combinational dormancy (PY + PD) of class E*

Class C dormancy is found in seeds with water impermeable seed or fruit coat (PY) synchronized with physiological embryo dormancy (PD none deep). Release from PY and PD of PY + PD-dormant seeds appears to be independent events and the timely order can be species specific [64].
