Here are the answers to this question(s) one-by-one:
What does [Pix's] damage scale from?
Pix's damage scales with Lulu's level. The formula for Pix's damage is:
Pix fires a barrage of 3 bolts, dealing 3 + (2 × level) magic damage each
Does it scale with AD, AP, etc.?
As you can see from the formula above. it does not scale with Attack damage (AD), Ability Powers (AP) or any other champion stat (Mana, Health, etc.) and is based solely on Lulu's level.
Is Pix's damage Magical or Physical?
Pix deals magic damage exclusively. Therefore his damage is dependent on the users Magic Penetration and the targets Magic Resist stats. There is some speculation about whose Magic Penetration Pix uses when making his attacks from an ally (not Lulu) as noted in this link.
Does Pix attack when the ally uses spells, or just for autoattacks?
Pix's attack is triggered by autoattacks only. Casting spells that refresh the attack animation (Nasus's Siphoning Strike, Blitz's Power Fist, etc) may provide additional attacks from Pix but to maximize the damage you probably want to target a champion with a higher attack speed.
For more information about Lulu and Pix check out her entry on the LoL Wikia.
Tenacity is a mechanic that reduces the duration of crowd control
effects. It does not affect the potency/strength of the effect.
Tenacity works on: Blind, Forced Actions (Charm, Fear and Taunt), Pacify, Silence, Snare, Slow and Stun.
Tenacity does not work on: Airborne (Knockback, Knockup and Pull) and Suppression.
Contrary to Deotronic's answer, Tenacity does not reduce the amount of slow. It only reduces the duration of the any of the CC that is affected by Tenacity. The reduction is reduced by a percentage equal to the Tenacity value. So a champion with 35 Tenacity will have all affected CC reduced by 35% duration.
EDITED to reflect changes to Season 3 tenacity items:
Please note that Tenacity does not stack. There are currently 3 items which provide Tenacity, and they all give 35 Tenacity. If you buy two of these items, you will not get 70 tenacity. You will only benefit from 35.
- Mercury Treads
- Spirit of the Ancient Golem
- Zephyr
Source
Best Answer
All plants can killed by 3 AAs from Champs or one single target spell and one AA.
Minion Damage is treaten diffrently, but from a siege minion one shot equals one bar. Turrets instantly kill the plant (one shot).
EDIT:
Single-Target spells do 4 damage each (2 bars).
AOE abilites deal 1.5 damage each. (per tick if applicable, like Morgana's tormented soil)
DoTs deal .5 damage each tick.
smaller mobs (including neutrals) do 1 damage per auto attack (1/2 a bar) .
bigger creeps (big wraith, big golem etc) and champions do 2 damage (1 bar) per auto attack.
Towers kill the plants instantly (6 damage).
EDIT2:
From Reddit: Zyra's Plants Take Damage in Extremely Buggy Ways