I do not know if this would apply to what you are trying to accomplish and I do not know much about chemistry, but 2 things come to mind if looking for a type of plant that would make a gelatin like product.
One would be agar and the other carrageenan, one is a culture product of algae used for growing mushrooms and the other is obtained from seaweed used as a common ingredient in milk products like ice cream.
They are both gelatin-like.
I have no idea how these would react to any type of photographic process or if they would have application at all and I also don't think these would be very permanent.
Could these products be used in some type of non-silver based photography?