Most plants and some animals spend huge amounts of time basking in the sun. Very little is known about what protections against UV-light are involved. Let's look at the possibility that a sunscreen is present.     While it has long been known that leaves and skin are strong UV-absorbers, that doesn't answer the question. DNA is also a strong UV-absorber, and that's the problem: the DNA absorbs the UV and mutates. So where should a sunscreen be located? Yes, before the UV can reach the DNA. Thus, on the outside of the organism. TRY THIS:     Using a fine sewing needle and thread dipped in food coloring containing some highly UV-sensitive bacteria, draw the thread lengthwise into and completely through a section of a thick leaf so that the dye leaves tracks as shown. Do this at various depths to see if and where they are protected after exposure to strong UV-light. You may need some hints on the the bacteriology of this.* * For more details: www.science-projects.com/Sunscreen.htm