The result is a world where people become magical girls (the "girl" part is totally optional, I'm just calling them that because genre conventions) through the use of roses. Perhaps I should capitalize that to Roses or something, because these are not ordinary roses. You could think of them as magical parasites or symbiotes. They can only grow in humans; when separated, they enter a dormant state until the next time they find a host.
Needless to say, this explanation is going to get gruesome.
From the human host's point of view, it works like this: You pick up this weird-looking cut rose, and for whatever reason, hold it tightly enough by the stem that the thorns hurt you. The rose comes alive and coils around your arm until the flower is at the back of your hand; the end of the stem grows several new stems, all much livelier than ordinary plant life, and those burrow into your arm and keep growing and splitting until there are rose stems throughout most of your body. This doesn't cause you actual harm as such, but it does hurt quite a bit. Along the way, some of the stems re-emerge, and you end up artistically decorated with coiling rose stems that also helpfully provide a kind of swimsuit/bodysuit made of unreasonably durable rose-petal material. (This process may be incidentally rough on whatever you're wearing, but your existing clothes don't magically disappear or whatever.) Then any extraneous damage from the rose invasion heals over, and the stems/vines calm down and quit moving around, so continuing to be occupied by them is only moderately painful. Also, you now have cool magical powers! Congratulations. More on that later.
If you would like to stop being occupied by magical rosevines, just grab the flower head and start pulling on it and trying to unwrap it. It'll come off, and all the vines and the petal-bodysuit will extract themselves and wither away, which will hurt pretty much the same as getting them in the first place did, but once again the last thing they do will be to heal whatever damage they did you, and you will be left perfectly healthy and uninjured and holding a weird-looking cut rose, with all your magical powers gone.
Now, the cool magical powers: Roses come in eight different colour variations. Each one behaves slightly differently when uniting with a host, and each one gives slightly different cool magical powers, although some are more different than others. All roses grant the same baseline accelerated healing - rose hosts can recover from pretty much any injury that isn't immediately fatal.
- Red/black roses have red petals and black stems, with red-tipped black thorns. Their thorns are sharp, and tend to draw a lot of blood while uniting or disuniting with a host. They grant the host an automatic boost to speed and agility, as well as the ability to manifest edged weapons such as swords and knives; their primary magical focus is cutting. Secondary focuses include healing others and nullifying magic.
- Blue/purple and purple/blue roses have either blue petals and purple stems with blue-tipped purple thorns, or the reverse. Instead of physically burrowing into the host, the stems of these roses become partly insubstantial and sink into the host's body, causing bruises in all the flesh they pass through, as well as wherever the external vines touch. They grant the host an automatic boost to strength, as well as the ability to manifest blunt weapons such as clubs, maces, and baseball bats; their primary magical focus is telekinesis. Secondary focuses include magical shields.
- Orange/red roses have orange petals and brick-red stems, with orange-tipped red thorns. These are another kind that unite using insubstantial stems, but instead of bruising, these ones burn. They grant the host an automatic boost to the senses, as well as the ability to manifest ranged weapons like crossbows, longbows, or javelins; their primary magical focus is heat and fire. Secondary focuses include light.
- White/blue roses have white petals and pale blue stems, with white-tipped blue thorns. They unite using insubstantial stems that freeze the host, causing frostbite. They grant the host an automatic boost to the senses, as well as the ability to manifest ranged weapons like crossbows, longbows, or javelins, and their primary magical focus is cold and ice. Secondary focuses include light.
- Pink/yellow and yellow/pink roses have either pink petals and yellow stems with pink-tipped yellow thorns, or the reverse. They unite using stems that physically burrow into the host's body, but without drawing very much blood. They grant the host an automatic boost to grace (muscle memory, precision of movement), as well as the ability to manifest polearms like spears, long-handled axes, or glaives; their primary magical focus is healing others.
- Black/white roses have black petals and white stems, with black-tipped white thorns. They unite using insubstantial stems that cause damageless pain in the host - think Cruciatus Curse, and yes, I do mean to imply that this kind hurts way worse than the rest. They grant the host automatic magical flight capabilities, as well as the ability to manifest wands and staves; their primary magical focus is conjuring objects. Secondary focuses include enchanting objects and nullifying magic.
- After only one or two uses, or in someone who uses roses at most once or twice a month, there will be no fragment buildup. Each time the host uses a rose will be as though it is that person's first time, in terms of how the rose's magic reacts - the host will still be able to learn how to use its magic more skillfully, but they won't get any fragment buildup effects.
- After more than two uses in the same week, the rose will start leaving fragments behind. The fragments are cumulative. It's theoretically possible to remove them using invasive surgery, but they're small and hard to find; the host can sense their presence and get a rough idea of numbers and locations if they think about it, but that doesn't make it much more practical to cut them open and fish the little buggers out. Anyway. In the early stages of fragment accumulation, the host starts to gain greater control over the placement of stems while uniting with the rose, and can alter the design of their petal-bodysuit to include things like sleeves, leggings, or a skirt. They'll also find that they don't get sick anymore - just a few fragments are enough to make the host immune to all infectious disease.
- From that point forward, fragments continue accumulating. After 20-25 uses in total, the host will find that they contain enough fragments to facilitate transformation even without using an external rose - they just have to focus on it and the fragments will start assembling and growing into full stems. Transforming this way creates no flower heads, so it's possible to tell hosts in this phase from hosts in other phases by the presence or absence of a flower head on the wrist/back of hand. Likewise, they can detransform by focusing; the stems will retract and wither back down to the fragments that made them, instead of exiting outward. People with this level of fragment accumulation will find that they age more slowly, never succumb to age-related infirmities like brittle bones, heart trouble, or dementia, and can't die of old age. Also, they have much greater control over their stem/petal configurations when transforming, and can pretty much design their own outfits to arbitrary specifications (as long as everything they design is made of stems and petals) and alter them at will. Black/white rose hosts sometimes like to give themselves wings.
- After about a hundred uses in total, the host will find that they are now capable of producing new flower heads. This can be done at any time while transformed, and activates all stems of the relevant type, making them writhe around painfully while the new flower is growing. The flower can be cut off immediately, or if not, it will fall away with a length of stem attached when the host detransforms. Hosts with this level of fragment accumulation will find that they can do some minor magic without fully transforming, and retain their automatic healing even while not transformed, as well as the automatic boosts of the relevant kinds of rose.
Fragment accumulation for each of the eight colour variations is separate - accumulating blue/purple fragments won't add to your purple/blue fragment totals. Many of the fragment accumulation level effects are separate per flower type - for example, someone who has enough blue/purple fragments to transform at will but only a few fragments of red/black will be able to transform blue/purple at will, but can only transform red/black using flowers, until they have accumulated enough red/black fragments; similarly, someone who has enough black/white fragments to produce new flowers but has only a few fragments each of yellow/pink and white/blue will be able to produce black/white flowers, but not flowers of the other colours, and they will retain flight while detransformed but not the boosts to grace or senses. On the other hand, things like immunity to illness and old age work as long as you achieve that level in at least one flower, and outfit control works at the highest level you have no matter what - if you have a high accumulation of red/black fragments and no fragments of other colours, and you transform using a combination of pink/yellow and black/white flowers without using your red/black fragments directly at all, you'll still be able to control your outfit design just as easily as if you were transforming red/black.
I haven't worked out the details of weapon manifestation yet, but I think it involves sprouting stems and petals that shape themselves into the form of the weapon and then transmute into hardier materials, and then un-transmute and wither away when you detransform.
I want each colour variation to have at least five magical focuses, and the reciprocal pairs (blue/purple - purple/blue, yellow/pink - pink/yellow) should have at least one or two of those not in common, even though they'll probably be identical for most of them. But I haven't made up very many focuses yet.
I want black/white roses to have lots of really cool and widely applicable magic, but I haven't thought of it all yet. Other kinds can have some cool and widely applicable magic too, but black/white should have lots.
And whew, I think that might actually be it!
edit: Updated/expanded info on flower powers and fragment accumulation levels
