Trees and the first seed-bearing plants evolved before the end of the Devonian, 360 mya. Forests of primitive plants grew, covering the land and changing the way rivers sashayed across the landscape.
The earliest trees shed spores for reproduction. These plants had extensive roots and megaphyll leaves.
The evolution in plants of a branching vein system, and thin, laminate (megaphyll) leaves, did not occur for at least 25 million years after their arrival on land. Megaphyll leaves evolved as an extension of the branching patterns of the earliest vascular plants.
The coming of megaphyll leaves in all but the driest habitats was an adaptive response to the success of plants in altering the carbon cycle: pouring oxygen into the atmosphere, which developed the planet’s atmospheric water cycle, increasing cloud cover and rainfall. Megaphyll leaves were designed to take advantage of these new conditions, especially moister air.
The vein patterns of leaves reflect a plant’s condition. Vein density indicates the resources dedicated to the leaf network, while the distance between veins shows how well veins are supplying energy to the leaf. Veining patterns emerge to allow alternate pathways, to minimize damage to the whole leaf if a portion is ruined.
The earliest land plants reproduced by spores, first of 1 size (homosporous), progressing to heterosporous: a plant with 2 different spore sizes. The large spores evolved into seeds: essentially, a megaspore with a protective coating. Small spores were the precursor to pollen.
The Devonian ended in a major extinction event, primarily affecting the marine community, particularly warmwater reef builders. Land plants and freshwater species were relatively unaffected.
In the early Carboniferous, 340 mya, gymnosperms arose: the origination of seeds as a reproductive device, conferring greater robustness in progeny, and granting greater dormancy potential. Extant gymnosperms include conifers, cycads, and gingkoes. Other early seed-bearing plant groups have come and gone.