The " Anthophyte Theory " was based on the observation that a gymnospermic family vessels as found in the angiosperms, and the megasporangium is covered by three envelopes, like the ovary structure of angiosperm flowers.
12.
A Middle Devonian precursor to seed plants from Belgium has been identified predating the earliest seed plants by about 20 million years . " Runcaria ", small and radially symmetrical, is an integumented megasporangium surrounded by a cupule.
13.
A middle Devonian precursor to seed plants from Belgium has been identified predating the earliest seed plants by about 20 million years . " Runcaria ", small and radially symmetrical, is an integumented megasporangium surrounded by a cupule.
14.
The pollen enters a pollen chamber close to the nucellus, and there it may wait for a year before it germinates and forms a pollen tube that grows through the wall of the megasporangium ( = nucellus ) where fertilisation takes place.
15.
This system paved the way for ovules and seeds : taken to the extreme, the megasporangia could bear only a single megaspore tetrad, and to complete the transition to true ovules, three of the megaspores in the original tetrad could be aborted, leaving one megaspore per megasporangium.
16.
The pollen grains are dispersed by the wind to the female, ovulate cone that is made up of many overlapping scales ( sporophylls, and thus megasporophylls ), each protecting two ovules, each of which consists of a megasporangium ( the nucellus ) wrapped in two layers of tissue, the integument and the cupule, that were derived from highly modified branches of ancestral gymnosperms.
