|A tree can be defined as a large perennial woody plant. Though there is no
set definition of minimum size, it is generally at least 6 m (20 ft) high at
maturity, and with branches supported on a single main stem. Compared with
most other forms of plants, trees are long-lived. A few species of trees
grow to 100 m tall, and some can live for several millennia.
Trees occur in many diverse orders and families of plants, and thus show a
wide variety of growth form, leaf types and shapes, bark, reproductive
organs, etc. The earliest trees were tree ferns and horsetails, which grew
in vast forests in the Carboniferous Period; tree ferns still survive, but
the only surviving horsetails are not of tree form. Later, in the Triassic
Period, conifers, ginkgos, cycads and other gymnosperms appeared, and
subsequently flowering plants in the Cretaceous Period. Most species of
trees today are flowering plants and conifers. The list below gives some
examples of well known trees and how they are typically classified.