Not all plants use seeds. Plants use more than one method of reproduction. The first life forms on Earth were photosynthetic. They used light from the sun to make sugar, and then consumed the sugar to reproduce. The leap from single cell organisms to multi cell organisms is unknown, and there are many theories. 

I think the plants you refer to have roots and leaves and reproductive organs that produce seeds. Evolution states that these plants evolved from simpler plants, but nobody knows the exact evolutionary path from single cells, to plants without seeds, to plants with seeds. The link below gives a lot more data about each phase.

 

I hope this helps.

 

“Plants” is a pretty broad term covering a wide range of organisms, not all of which reproduce sexually.The simplest plants are single-celled green algae.  They don’t have separate seeds; they’re just a single cell.  Some algae have just a single set of chromosomes; others chromosome pairs.  You get double chromosomes when two algae merge, giving you a unique organism that can the reproduce.  Sometimes it will reproduce by simple fission, giving you two new organisms with unique single-chromosome genomes.

The history of plants goes from single-celled algae to simple clusters of algae (with different genomes) to multicellular algae with a single genome but different cells performing different tasks called Charophytes.  They spawn special cells with half genomes (like our own sperm and egg cells) that go out and find other gametes from other charophytes, but they don’t distinguish separate male and female genders. 

You can think of these as alternating between half-genome generations (“haploid”) and full-genone generations (“diploid”).  Looked at that broadly, animals do the same thing: you give rise to a generation of haploid cells (sperm or eggs) that merge with somebody else’s next generation to form the subsequent generation of diploid humans.  It’s only our conceit that makes the human generations more important than the haploid generations; as far as the sperm and egg are concerned, humans are just ways to make more sperms and eggs.

Actual seeds and pollen came even later, with the seed plants called “spermatophytes”.  That involved several different evolutionary steps, but it started with the charophytes developing specialized male and female gametes.  Later, after the development of the other characteristics of true plants (embryophytes). 

The embroyphytes are actually relatively recent; they evolved on land rather than in the sea. There are some aquatic embryophytes, but in fact they’ve moved into the sea from land rather than the other way around. They have specialized sex organs, unlike the charophytes, but they don’t have separate egg and pollen cells.

That was yet another evolutionary step, the spermatophytes, which is what you think of when you say “plant”.  That happened when some of the plant’s reproductive system (the rest of the organs) began to encase the fertilized seeds for greater durability, enabling them to spread a lot further inland.  They conquered the world.

That’s why you think of them as “plants”, even though there are so many plants without seeds.  The charophytes had other descendants that don’t produce seeds, like ferns and mosses.  These may actually have derived from the seed plants.  Tracing down the evolutionary path is best done with gene sequencing, and while they’re sequencing things as fast as they can it’s still an expensive and time-consuming process.

The upshot is that no, “seeds” didn’t just mysteriously appear one day.  The evolutionary history of plants is long and complicated, with a lot of side journeys and dead ends along the way.  I’ve only barely touched on it, what you’d find in any introductory biology textbook.  There’s a lot more to know about it, but I’ve let this answer go on long enough.

Well first you have to define “Plant”.   Let’s go with a living thing that uses chlorophyll to capture energy.The first “plants” did not look like your average Dandelion.  They were probably derived from blue-green algea,  which are very very very old.  Eventually they added more of the chlorophyll and depended more on sunlight.  Then they stumbled on the idea that  it was beneficial to stick together into larger and larger groups, like colonies.  Eventually some specialized into stronger cells for plant strength, others specialized into root hairs to get more water.  Up to here they’ve been reproducing asexually-by splitting or budding.   Eventually they stumbled upon sequestering the DNA into certain cells and spilling those around.  Some even found an advantage into splitting the genetic material into two parts, that was the invention of sex!