Germinating seeds of wildflowers, an ecological perspective

Five kinds (classes) of seed dormancy are known: physiological (PD), morphological (MD), morphophysiological (MPD), physical (PY), and combinational (PY + PD). PD is the most common class in the major vegetation zones of the world followed by PY, MPD, MD, and (PY + PD). Each class is described, and a dichotomous key to identify them is presented. The environmental conditions required to break PD, MD, MPD, PY, and (PY + PD) and promote germination are discussed. To help determine which treatments to use for breaking dormancy in seeds with water-permeable seedcoats (PD, MD, MPD), a "move-along experiment" is recommended. Little or no convincing evidence for the role of microbes or mechanical abrasion by soil particles in breaking PY can be found in the literature. However, there is good evidence that the water plug or gap in the seed or fruit coat of seeds with PY responds to environmental cues that permit timing of imbibition and germination to be well controlled in nature. Seeds of many species remain viable after passing through the digestive tracts of animals, with varying effects on germination.