Hi riverminty,
Here's a site that may interest you. It talks about Botany, fields of study and a lot of other interesting areas and topics relating to it. One of the best places to learn about plants are the forums here on GardenWeb. A lot of experienced people with a ton of information.
BTG

Here is a link that might be useful: nbii.gov

Look into university degree courses in botany or plant sciences.

Resin

riverminty, I cannot answer your questions specifically but in the broadest terms, you will find that there are two paths to learning:- Synthesis and Analysis. You can look back on your own learning and discover which works better for you. In the first method, one starts off with the cell and works up to tissues, organs, systems and ultimately the whole plant. In the second method, one starts off with the whole plant and proceeds to break it down to macro parts - leaf, stems, roots and working down through (in the case of the stem) xylem, phloem, cambium, cell. There is a third way, (and I am still working on it) the historical perspective. To give an example, I struggled with Archimedes Principle in high school but I did not get it right until I "studied" the life Of Archimedes a little. The legendary bath; the legendary golden crown of Hieron all helped but this was long after high school. All I understand now about microbes was learnt from the "Microbe Hunters" by Paul de Kruif which I first read two years after my formal education was over (I am still a slow learner). From the 'historical perspective' one starts from the point of ignorance (Archimedes did not always know the flotation principle)and follows the pioneer through the discovery process. "Microbe Hunters" does it particularly well. It is the only book I describe as enchanting; and I have read many, many fairy tales.

Hi riverminty,

All the suggestions so far are excellent. I started with a book called "Botany in a Day," can you believe it? A very useful book that gives you a jump start on identifying plant families. More books and other resources at the link.

Here is a link that might be useful: Books for Learning Plants