The Evolution of Land Plants The first land plants likely evolved in estuaries from green algae (“sea weeds”). They had no true vasculature (xylem), no seeds, and no true leaves or roots. The earliest plant fossils are Ordovician in age and resemble modern liverworts. More modern groups of plants were well established by the Devonian, but differed considerably from today’s flowering and fruiting plants.

Major Plant Groups (There is considerable current disagreement about plant taxonomy, but these are the basics) Bryophytes: Mosses and liverworts. Low growing non-vascular seedless spore-bearing plants. Many have symbiotic relationships with internal fungal cells. Ferns, horsetails, lycopods, etc. A polyphyletic set of seedless vascular plants that arose in Silurian and Devonian times.

Mosses and Liverworts “Bryophytes”

Ferns, Horsetails, Lycopods

Major Plant Groups Continued Gymnosperms: Large spore-bearing and seed-bearing vascular plants and trees. Includes modern conifers, ginkgos, and cycads. Primitive forms first seen in Devonian. Angiosperms: Flowering and fruiting plants that arose in the Cretaceous. These constitute the majority of modern plant species, and include grasses, trees, etc.

Gymnosperms

Angiosperms

Plant Vasculature A major development in plant evolution was xylem and (later) phloem tubes that conduct fluids and nutrients in the plant. Xylem: tubes of dead conducting cells that carry water from the ground to the leaves. Older xylem layers (secondary xylem) is reinforced with lignin, and helps support much larger plants – this is wood. Phloem: specialized tubes of living cells that transport carbohydrates in all directions around inside the plant. Plant regions not producing nutrients by photosynthesis are fed by phloem.

Early Devonian Fossil Plants By the Devonian, plants like Cooksonia and Rhynia have xylem. Some reached 1m. Still localized to very wet areas (sperm problem). Among these were the first small lycopod plants.

Silurian Plant Fossils Aglaophyton is a common Late Silurian Plant fossil, and is typical of the period. It has a ‘conducting strand’ of cells instead of xylem. In place of leaves, it has green stems. In place of seeds, spores on a sporangium. Asexual reproduction by rhizomes.

Devonian Trees Secondary xylem (wood), true roots, and the first leaves allowed for the first ‘trees’ to appear in the later Devonian. Also, one of the first trees, Archaeopteris produced the first seeds (thus, the first gymnosperm).

Devonian Floras As you can see, there was considerable plant diversity by the late Devonian. Ecosystems were dominated by lycopods, ferns of several heights, and the first seed trees. Thus, a ‘modern-looking’ forest with canopy and understory plants was present. The first coal beds are Devonian.