I was thinking about this while the dog and I were out for a walk. It is actually much easier to bring a leaf, fruit, etc. to the book than the other way around. The problem is that that can be hard on the plant, unless you are really isolated.

Better would be copious digital photos of the plant, which are even lighter. With current cameras and a decent chip, you have essentially unlimited photo capacity.

My current point and shoot, a fairly cheap Canon model, is the most capable camera I have ever handled, and this includes film cameras from subminiature to 35mm to 4x5s. It easily handles macrophotography, which can be quite useful in this application.

Thought # 2 - make good friends with a botanist....