Seeds are specialized structure resultant of gamete fusion. It has several advantages including late rejuvenation, safety besides others.

Now if we see the lineage, seeds have emerged late in evolutionary history. This means that primitive plants do not have the capacity to produce seed. Hence evolution has taken a direction and all advanced plant now bear seeds. Spore plants did not disappear but as seeds have so many advantages compared to spores they became the dominant plants. Archaeopteris is generally considered the first known tree ( with all the characteristics of a modern tree, especially a wooden trunk ) and lived probably between 385 and 323 million years ago.

a) The earliest seed plants, “progymnosperms”, emerged in the late Devonian . Progymnoperm fossils show vegetative morphologies to seed plants. But all proaymnosperms did not have seeds or seed-like structures (ovules or ore-ovules). Archaeopteris spp. was the first modern tree. But it produced spores rather than seeds. However, it exhibited an advanced system of spore production called heterospory. Heterosporous plants produce two sets of specialized spores: megaspores (haploid female-like megaspores) and microspores (haploid male-like microspores). Heterospory, has been evolved independently in several lineages. It is believed to be a precursor to seed reproduction. The progymnosperms are regarded as the ancestors of the seed plants.

b) Fossils  of  seed-bearing seed ferns (Lyginopteridopsida) exhibit a variety of seed and seed-like structures. The seed’. might have evolved once or several times during evolution.