For observing plants in the field, I have found few better than this little hand-held Radio Shack 30x scope.
At 13.5cm tall x 4.5cm wide x 2cm thick its very portable. The scope is self illuminated and very easy to use. The optics are quite good and its comes with a built in 8x magnifier. The best part about the scope is that it costs around $10. Radio Shack also sells a 100x pocket scope but it is not good for plants. The field of view is too small and its too hard to focus. Besides, you don't need 100x magnification for looking at plants in the field.
If you REALLY want to look at plants, nothing beats a dissecting scope.
These things can get pricey.
I use a 40x Ken-O-Vision scope. It cost around $350, but these scopes can run well over $1000. Nothing beats a dissecting scope
for getting great detail on small flower parts. Ken-O-Vision is a great company based in Kansas City. When my scope wasn't quite right they fixed it fast. I can't say enough about their desire for quality.