Populus tremuloides Michaux, 1803 - quaking aspen in Colorado, USA.

Plants are multicellular, photosynthesizing eucaryotes. Most species occupy terrestrial environments, but they also occur in freshwater and saltwater aquatic environments. The oldest known land plants in the fossil record are Ordovician to Silurian. Land plant body fossils are known in Silurian sedimentary rocks - they are small and simple plants (e.g., Cooksonia). Fossil root traces in paleosol horizons are known in the Ordovician. During the Devonian, the first trees and forests appeared. Earth's initial forestation event occurred during the Middle to Late Paleozoic. Earth's continents have been partly to mostly covered with forests ever since the Late Devonian. Occasional mass extinction events temporarily removed much of Earth's plant ecosystems - this occurred at the Permian-Triassic boundary (251 million years ago) and the Cretaceous-Tertiary boundary (65 million years ago).

The most conspicuous group of living plants is the angiosperms, the flowering plants. They first unambiguously appeared in the fossil record during the Cretaceous. They quickly dominated Earth's terrestrial ecosystems, and have dominated ever since. This domination was due to the evolutionary success of flowers, which are structures that greatly aid angiosperm reproduction.

Quaking aspens are subalpine trees in the mountainous terrains of western North America. The common name “quaking” refers to the shaking or fluttering of the tree’s leaves in breezes. In this species, the petiole (= “stem” leading to a leaf) is flattened. The flattened surface catches even slight air movements, resulting in the leaves “trembling” or “quaking”. This plant principally propagates by spreading out lateral root systems. Entire groves of quaking aspens can have identical DNA fingerprints - they're all “twins”, or “clones” of each other.

Classification: Plantae, Angiospermophyta, Malpighiales, Salicaceae

Locality: Many Parks Curve, Rocky Mountains National Park, northern Colorado, USA

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