The differing rule with Physical and Plant mutations
Accumulated Resistance is due to how they were named in Gamma World. The Physical Accumulated Resistance equivalent was called "Absorption", while the Plant equivalent was called "Adaptation." Where Absorption creates a hit point reserve for one non-changing form of energy-based attack, Adaptation grants instant immunity to any form of attack that did not kill it out right, save for mental attacks and
"having its limbs or stalks chopped off by swords or axes." As plants were not playable in early Gamma World, the idea was to have particularly tough plant-based NPCs and monsters.
Oh yes, the
Mutation Upgrades was my doing (as the primary user over there, any mishaps, misspellings, misunderstandings and misdemeanors are usually my doing
). You can find the rule
here. Folks are more than welcome to add what they like over there.
I left the rule deliberately vague and open-ended for Mutant Lord interpretation. How this rule works is that a Mutant Lord can "upgrade" a single mutation to be better in some minor way (more range, longer duration, more damage, etc.) or to have some useful side-effect. Such "side-effects" could be something like giving a mutant character with
Mental Phantasm and
Neural Telekinesis the ability to use them together seamlessly to create illusory forms capable of physical contact, or allowing someone with
Vampiric Field to redirect siphoned hit points to someone else, and so on.
If not handled right, it could be game-braking. (Mind you, "game-balance" was never a forethought with a game like this game, or the original game this was based off of.) If anything, it puts a lot of burden on the Ref to come-up with creative ways to improve a mutant character in creative ways. Although, asking the player "How would you improve your character with a small change to one of his/her mutations?" could be useful if one is running dry on creativity. (On the other hand, players tend to get a little... too creative, when granted that level of control.
)