The evolutionary significance is that coal does not change the properties of the plant from that time even though the plant might have evolved from that time. That's why you can study coal to find out how the type of plant was at that time period and how it has evolved until now.
Sleet is pellets of ice that form when snow falls into a warm layer and melts into rain. The rain then falls into a freezing layer of air that is deep enough to refreeze the raindrops into pellets. Sometimes the snow does not completely melt and the partially melted snowflakes refreeze into snow pellets.:
