The adaxial surface of a leaf is the upper side. For example, fibrous adventitious roots grow from the trunk above the ground instead of starting underground. An aerial root is a plant's root that is produced above the ground. An age is shorter than epoch, usually lasting from a few millions of years to about a hundred million years. Agrostology is a branch of botany that studies grasses .
Some algae are unicellular while others are multicellular .
This theory is that a large asteroid , meteor, or comet hit the Earth 65 million years ago, causing huge atmospheric and geologic disruptions, leading to a mass extinction which killed the dinosaurs and many other plant and animal species. Their flowers are used in reproduction. Angiosperms evolved about 140 million years ago , during the late Jurassic period , and were eaten by dinosaurs. An annual is a plant that goes through its entire life cycle within a year. It grows from a seed , matures, produces seed, and dies within a year.