Blood vessels growing into the wart give the wart black dots, which some people call "seed" warts, explains WebMD. These dots show the introduction of blood into the wart.
Warts spread from one person to another, either indirectly or directly. For example, a person with a plantar wart on the bottom of his foot might use a shared shower in a locker room without wearing flip-flops. If another person uses the same shower before it has been cleaned, that second person could develop plantar warts as well. People with weakened immune systems are more likely to develop warts than those with healthy immune systems, but no one is completely immune from warts, notes WebMD.
Blood vessels can grow into both palmer and plantar warts, and so both types can develop the black dots that some refer to as "seeds." Palmer warts appear on the hand and can protrude above the skin more than plantar warts, which appear on the bottom of the feet. The average size of a wart is about that of a pencil eraser, but plantar warts sometimes grow in cluster formations that are called mosaic warts. Whether they develop black dots or not, most warts clear up without any treatment within a couple of years, although while they are in place, they can look unattractive and provide irritation, depending on the site where they grow, states WebMD.