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Definitions or discrete facts are more easily found in dictionaries and handbooks. Books often give good overviews and background on topics, and articles in journals are a good source of current research and trends.
This beautiful hand-drawn illustration of an Arkansas Black apple is from a collection of images at the National Agricultural Library (linked below).
Offers a worldwide collection of digitized images of plant specimens. The advanced search allows searching by scientific classification, geographic distribution, source collection, and more.