Arcanis

Description:
Arcanis grows into small (approximately two inches tall and scant millimetres thick at maturity), smooth, brightly coloured little shoots - usually a deep mauve or violet, and sometimes interspersed with flecks of rich, navy blue. It grows in clumps, usually no larger than a few square inches, the roots of each clump adjoining into one web which weaves through the topsoil, much like the roots of a cactus. As such, it resembles a purplish moss, albeit somewhat shaggier than would be expected were this the case. It has a subtle but distinctive odour, which is only truly released when the shoots are crushed or burned. It tastes much like it smells.
Growth Cycle:
This plant is unusual in that it has no seeds of its own, per se. Rather, it grows out of hardy plant seeds wherever it can - grass is the normal candidate, sometimes ivy or kudzu, but in mountainous regions it can grow from various mosses and lichens. So how does a standard plant seed manage to grow into something so different to its intended form? Well, the answer is simple, if a little condescending:
Magic!
Yes, Arcanis is the result of spellcasting - not any deliberate attempts to transform grass into Mageweed, however (in fact, attempts of this nature have to date been fruitless), but general spell-slinging in the immediate vicinity. Perhaps it's the inherently chaotic nature of magic, perhaps there's something common to all plants that magic can tweak just so, but in the end such discussion is academic (Yes, academics discuss it all the time). The standard peasant's attitude to why magic works can be invoked here: It just does.
Of course, not every spell will change all the surrounding grass into Arcanis - Most major harvests of the stuff come from sites in which great mages have battled for dominance, or perhaps in the mythical glades wherein elves and unicorns frolic within a magical utopia. For the average mage, the best that can be expected is perhaps the odd clump which starts to infest his spellbook. Regardless, the chances of it sprouting in a given location can be summed up as both 'variable' and 'unlikely'.
And so, with a seed having been fertilised (or perhaps raped is a better word?) by the pervading magical essences, it begins to grow, and within a month those distinctive purple shoots poke through the ground. Within another month, they have grown to their full height and are ready for collection by the lucky wizard who stumbles over them. If left too long, however, they will flower, with vibrant petals (usually a primary colour). They only last a few days like this before turning brown-black and decaying away. They do not reseed.

Arcanis does not need water or sunlight to flourish. The original spark of magic is enough for it to subsist healthily. The roots are more to hold the clumps together and in place than to provide nutrients.
Uses:
Alchemists and mages the world over hunt for these prized little patches of colour, and not just to look at! It should be noted, however, that once Arcanis has flowered it becomes useless, the burst of the bud releasing not just the flower, but also all the stored magic within. When correctly prepared, the uses for the shoots are numerous:
Of primary concern to the standard student of the magical arts, when the shoots are dried and powdered, they can be turned into snuff powder or mixed in with tobacco to be inhaled or smoked, respectively. This is the most common use, and has the effect of infusing the imbiber with all the magic that went into the growth of the plant - a small amount, admittedly, but enough to perk up the tired mage and give him the energy for one or two extra spells. Arcanis burns with a distinct smoke, the same colour as the shoots were originally, and of course the burning releases the smell of the plant.
As is ever the case, however, it isn't all good. Smoking Arcanis causes light-headedness and minor hallucinations, usually taking the form of patches of colour floating through the smoker's field of vision. Some non-mages smoke it purely for this sensation, which is not too far from the feeling of having drunk a few too many glasses of good wine. It is very slightly addictive, although no more so than tobacco. Repeated use can stain teeth and any grey hairs a faint lilac colour, and ultimately it may be difficult to get the smell out of one's robes.
Arcanis can also be crushed, and the moisture from the shoots distilled and mixed with several other substances to create 'Mage Water'; This has several uses of its own, being the basis of a number of potions and (if sufficient amounts of Arcanis are used) acting as a strengthening reagent that can be mixed with other elixirs to increase their potency. Under the watchful eye of a skilled alchemist, these mixes can be purified to the extent that the narcotic effects of the plant are removed, leaving a safe brew.
And, naturally, someone somewhere managed to play up the drug aspect, and so it is not unknown for shady fellows at home in the back alleys of the cobbled cities to surreptitiously hand small vials of a vaguely mauve powder to those who would hand over certain sums of gold in return. And if one were to follow the jumpy new owners of this powder, they might find them scraping it into lines on their dressers at home and breathing it strongly in through their nose, before slumping into their chairs and enjoying what it must be like to be a powerful mage for the few hours until it wears off. And then the cravings start, and the money for the next vial has to come from somewhere... It's a familiar, if sad, story. This form of Arcanis goes by the name 'Purple' on the streets, and is both highly addictive and highly illegal. Not to mention dangerous if cut with the wrong substances.
Conceivable Plot Hooks and Other Interesting Tidbits:
A mage of some power has, foolishly, snorted some of the highly addictive and highly illegal 'Purple'. Instead of (as most are content to do) enjoying the feel of becoming a powerful mage, he is actively roaming the streets and cackling madly while burning townsfolk alive with raw magic!
A mage or merchant claims to have discovered how to force Arcanis to seed at his will - Maybe he's lying and just after a quick buck, or maybe he's actually succeeded - How will the face of magic change if Arcanis becomes plentiful?
What if there were variants of Arcanis based on what types of magic fertilised the original seed? What might the effects of Necromantic Arcanis be on the unwary wizard? Alternatively, what if there were a plant which outwardly looked exactly like Arcanis, but had some completely different/dangerous effect? Perhaps the good natured healer of a village stumbles upon a patch, and soon a plague is ravaging the poor farmers under his care.