Two concepts, one simple, one complex...
The simple one first:
The plants are real plants. Let's put them under the caster's control (his will), but since they are invading the hosts nervous system, the host gets a will save to try and control them to resist the intrusion. After the first round, the plants are in, no longer being controlled, and it's down to a fort save to resist the pain they are causing. Fort Save for 1/2 damage, and no Con Damage. Every round. Remember, a 1 fails a saving throw
But even with a 1 - can the plant infest clothing and grow from there??
Do you have to target the fighter, or can you create a plant at his feet?
For the complex concept:
First round, that wizard is attempting a ranged touch attack against the fighter... He's actually 'summoning' a 'carnivorous vine' (CV) plant... So let's instead think about that 'summoning' aspect instead of 'aiming'....
Let the wizard create up to 1 per 3 levels of caster CV plants in a 60' cone shaped area. Small green tendrils sprout from the caster fingers and hit the targeted locations, instantly erupting into fully mature CV plants that are, temporarily, under the control of the caster. The caster makes a ranged touch attack against the fighter, who has a will save to avoid taking damage if hit as it is an opposed contest of wills to control the plants this first round. If the fighter should happen to somehow win that contest of wills, the plants are held at bay this round. After the first round, the plants autonomously attack their current target (the fighter) with their own melee touch attacks, against which the fighter can only make fortitude saves to 'resist' the pain or 'tough' it out.
That fort save, fyi, should only be for half damage - slowing the intrusion, but not stopping it.
The original plants dissappear after the duration of the spell, but anyone infected with the plant remains in pain, and the new plant continues to grow within them, causing will saves to avoid the internal pain effects - justified by the simple fact that internal pain is traditionally resisted by will... saves vs disease are sometimes will based, as are some of the more pathogenic poisons (can't think of any from the DMG, but I remember seeing some)
Since the spell creates several plants for several rounds and since each plant can apparently cause 1-3 tendrils to attack per round...
Perhaps that'd up the level of the spell too much, as well as creating too much 'flavor' that might be hard to transfer to a new setting...