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Good question my son, define "seed"
A seed is an integumented indehiscent megasporangium with one functional megaspore.
It doesn't have an ambiguous definition, and we know, without any uncertainty, that it evolved precisely once.
Can you translate that to English
Somewhat, but keep in mind, its a half decade of study to develop the understanding. Also, trying to create parallels between how plants do sex and how animals do sex, thats going to throw you off. Plants do sex in a fundamentally different way than how animals do sex.
The basic trajectory in the evolution of land plants has between towards additional layers around the gametophytic generation, and additional investment in that generation. Animals, like us, have a unicellular gametic generation (sperm and eggs). Plants, well, its complicated.. Basically, when plants first came onto land, the haploid, gametic generation was the "big obvious plant" thing, but that switched at a certain point. So its just not possible to map plant evolution onto animal evolution.
Early land plants invested very very little into the next generation. It was all spores, single cells, which then had to establish themselves without any support from the parent generation. But the haploid generation was the dominant plant part. These plants are still with us today in the form of mosses and liverworts.
In liverworts and mosses, its still the N generation that is the dominant plant part, and the 2N generation is totally dependent on the N generation. This all got flipped on its head when plants developed vascularization, and the 2N generation became the dominant plant part.
PLANT EVOLUTIONARY TIMELINE FOR SEED COMPONENTS | MYA | Evolutionary Step | Seed Component | Definition | Dominant Plant Body | |-------|----------------------------------|------------------------|------------------------------------------------------------------|----------------------| | ~470 | Earliest land plants | — | Non-vascular; liverwort-like | N (haploid) | | ~430 | Vascular tissue appears | — | Enables upright growth, fluid transport | 2N (diploid) | | ~420 | Sporangia | Megasporangium begins | Spore-producing structures (seen in Rhyniophytes, Lycophytes) | 2N | | ~410 | Heterospory | Functional megaspore | Plants make large (mega) and small (micro) spores | 2N | | ~385 | Runcaria | Integument precursor | Fossil shows integumented megasporangium, no fertilization yet | 2N | | ~365 | Seed ferns (Pteridosperms) | Ovule (true seed) | Integumented, indehiscent megasporangium with 1 megaspore | 2N | | ~360 | Early gymnosperms | Full seed | Retained embryo + full protective tissue | 2N | | ~320 | Gymnosperm radiation | — | Conifers, cycads diversify | 2N | | ~140 | Angiosperms (flowering plants) | — | Double fertilization, fruit, enclosed ovules | 2N |
This, and your explanation below is fantastic. I had no idea that this was known and thought it plausible to have evolved many times like crabs.
Also, name checks out
Sigh *unzips*
( ͡° ͜ʖ ͡°)
:-\
ಠ_ಠ
What? I'm just giving a practical demonstration.