Since I found out we all share plant DNA in our makeup, and that some static plant-like life is actually animal, I would not be too concerned about that.
Regarding the high impedance. Do not confuse that with "having to fight its way through a resistance". You cannot get any information from anything without also changing it a bit, but if you can take so little that you do not substantially load the thing to collapse the voltage significantly, then your sample is of a representative accuracy.
Nerve cells are not power stations! The voltage they have can only source a tiny current without collapse.
Connecting into an amplifier that will accept and amplify that tiny current without the amplifier input itself being an unacceptable load, means that the input appears as if it presents a very high impedance to the plant. The voltage available divided by the current gives a value in Giga-Ohms.
I would not be too eagar to use "as high as possible" as a design philsopy. I would go for "as high as needed, and not more". The advantages of finding the effective tradeoff are many. 500Meg-Ohm or 1 Giga-Ohm may still be high enough for the plant sample, and much easier to design with and find devices for, and cope with noise. at 10Giga-Ohm, you have to build it on glass or PTFE, and worry about surface contamination leakage. It gets harder to tell what is circuit and what is substrate.