Onions have large cells visible
under low
magnification, so onion tissue is often used in high school science laboratories for learning about microscope use and cell structure, as shown in this lesson from Rice University (http://teachertech.rice.edu/Participants/dawsonm/cells/microlab4.htm) and this
video of onion cells from YouTube: http://www.youtube.com/watch?v=Tdch3mxQ4oU.