Lucas and Sean,
There are two main components to plant identification. The first is to learn the major, common plant families, seven of which are introduced in my
online article, "Learning to Identify Plants by Families: It will forever change the way you look at plants."
http://www.wildflowers-and-weeds.com/Plant_Identification/Patterns_in_Plants.htm
Take the Mustard family, for example. Mustard flowers have four petals, plus six stamens (4 tall + 2 short). There are more than 3,000 different species in the world, and they are all more-or-less edible, if not always palatable. Therefore, with that much information, you could be wandering around Mongolia and encounter some plant you've never seen before, but identify it as a Mustard by the information above. You don't have to identify the species, just identify it to the Mustard family. If its a mustard, then you can try it. If it is palatable, then add it to your salad and enjoy! My kids book and card game teaches eight core families and their basic properties and uses:
http://www.hopspress.com/Books/Shanleya's_Quest.htm
If a plant specimen doesn't match any of the plant families you already know, then you can turn to Botany in a Day to key it out. Botany in a Day covers most of the plant families in the northern latitudes, and a great many from farther south. In regards to the question, "What's the first step in narrowing down the species of plant you're observing?" - The literal first step is to determine if your specimen belongs to the Plant Kingdon, Animal Kingdom, Fungus Kingdom, etc, and yes, there is actually a diagram in Botany in a Day guiding the reader along. The second step would be to determine if your specimen is a nonvascular or vascular spore plant, a naked seed, or a true flowering plant. Assuming you have a flowering plant, you move to monocots versus dicots, and then into the family keys.
In many cases, it is only necessary to identify the family. But you will often want to key out the species. There are some species examples included in Botany in a Day. However, I typically start with the family (Yes, I actually have to key some plants out in my own book to identify them), and then move to another book, such as Wildflowers of North America to narrow it down to the genus or sometimes the species:
http://www.amazon.com/Wildflowers-North-America-Identification-Martins/dp/B001GVJC0G
By itself, Wildflowers of North America is pretty useless, but it includes thousands of flowers grouped by families, so you can ID the family in Botany in a Day and then flip through the samples shown in Wildflowers of North America (or any
local book organized by families, such as the Lone Pine guides). To identify a plant down to the species, I sometimes turn to a botanical key and start at the proper genus. Or I do a Google image search for the genus and see if my specimen pops up. That works pretty well for me, since I take a lot of pictures in summer and key them out in winter.
I hope this helps!
Sincerely,
Thomas J. Elpel
http://www.GreenUniversity.com
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