Sori on the underside of fronds of the common polypody fern,Polypodium vulgare. The life cycle of a fern consists of alternating generations of which the green fern "plant" is the asexual phase (the sporophyte generation). Mature fronds develop spore-bearing structures ( sporangia ),here seen as circular groups of yellow or brown particles. The groups are called sori ( singular,sorus ). The yellow sporangia are at an early stage of development. Eventually the spores within each sporangium are released,and on falling to moist ground,germinate to form a tiny green plant ( the gametophyte ),which goes on to produce male and female gametes. When these combine,a new fern plant ( sporophyte) is produced,and the cycle completed. P. vulgare is a widespread species found throughout N Europe,temperate Asia and western N. America. It grows on rocks,walls,and as an epiphyte on trees |