The Atlas of Plant Anatomy offers some good micrographs with very brief explinations.
A good site for general micrographs is Plant Anatomy, however there are no accompanying explanation of the images.
A Study of Plant Anatomy
This is a site set up and maintained by Frances M. Cardillo, O.S.F. and Tonya S. Samuels
at the Department of Biology, Manhattan College and the College of Mt. St. Vincent.
The Photographic Atlas of Plant Anatomy includes a broad range of high quality images.