Plant of the week (eon?) - Cooksonia

This picture of Cooksonia is pretty remarkable. Not so much for displaying this plant's simple beauty, not even because the picture is actually in focus, but because this plant has been extinct since the Devonian some 380 million years ago.

This fossil is looking particularly good because it is a model on display in the Darwin House at the Royal Botanical Garden at Kew.

Cooksonia is significant because this is the first land plant sporophyte to show apical branching. As the axes grew, the apex divides into two equal axes, a type of branching called dichotomous. Fossils show axes of Cooksonia branching thusly 1 to 4 times. Each axis then terminating with a sporangium. This appears to be a way of getting the maximum number of dispersable offspring from a single fertilization event.

Cooksonia is also significant because at the time this was the tallest plant of its day. These axes towered some 5 centimeters (nearly 2 inches) above the substrate. It is also the earliest appearing plant to have vascular tissue, xylem, although the oldest specimens found in Silurian sediments may not have possessed true xylem.

This plant is the first step on the way to trees, and it did not really take too long for the first trees and forests to appear (near the end of the Devonian).