Seed Plants

Seed plants evolved from a common ancestor making them a monophyletic clade, called the Spermatophytes, which includes all gymnosperms and the angiosperms. Sometime during the Devonian Period, a heterosporous spore-bearing group evolved features that created the first seed from a megasporangium. In particular, an ovule or seed is different from a sporangium by the following features. An ovule has a protective layer called the integument or seed coat surrounding it. The ovule has only one functional megaspore, compared to at least four in a spore-bearing plant. Lastly, the female gametophyte never leaves the megaspore, unlike spore-bearing plants, therefore fertilization occurs in the ovule for seed plants and it is external in spore-bearing plants.