As humans we take many things for granted. One is surely the ability to walk, crawl, or even, after a little too much to drink, drag ourselves over to a lovely member of the opposite sex. Plants have no such luxury. For much of the long history of green life on land, plants had to be near each other, touching almost, to mate. Moss lets its pale sperm into rainwater to float to nearby partners, as did other early plants, but this method requires moisture. Vegetation could only survive in those damp corners where beads of water connected, dependably, a male to a female. Most of the Earth was brown.