This section is from the book "A Guide To The Poisonous Plants And Weed Seeds Of Canada And The Northern United States", by Robert Boyd Thomson, H. B. Sifton. Also available from Amazon: A guide to the poisonous plants and weed seeds of Canada and the northern United States.
A botanical key, based on the flower structure and morphological features of plants, has proved invaluable to botanists. The value of such a key depends on the ease and definiteness with which the name of a plant can be ascertained by its use. Such definiteness cannot be claimed for the work which follows. It is, in the first place, quite conceivable that any of the symptoms named might be caused by something other than the eating of a poisonous plant. Even if a plant has caused the trouble, it is often impossible, judging by symptoms alone, to decide definitely what particular species is responsible. It therefore follows that the worker with poisonous species must know his plants, as the botanist knows them, by a knowledge of and familiarity with their distinctive characteristics.
This key is included, however, so that when an animal exhibits symptoms of poisoning and a plant is suspected, an idea may be rapidly gained of the plant that should be looked for in pasture or feed. To make a complete diagnosis the plant must always be found and carefully identified.