Evolution of Paleozoic Land Plants
From Paleos
Green Algae
The Green Algae - the Chlorophyta and Charophyta - include a number of mostly aquatic forms, including some unicellauar and primitive colonial forms. and other multi-cellular types that however lack a true root system
They are very closely related to (and probably the ancestors of) the higher plants in the kingdom Plantae. Molecular and cellular similarities between green algae, particularly the charophytes, and land plants include the following:
(1) Both the green algae and plants have chlorophyll b and beta-carotene
(2) Green algae and plants both have special intracellular membranes (the thylakoid membranes) which contain the chlorophyll stacked into grana.
(3) Charophytes have a cellulose content of 20 to 25% of the cell wall, a composition similar to that of plants.
(4) Cell division in green algae is very similar to that of land plants. Both use microtubules to bring vesicles containing new material in to form the cell plate which will divide the cell into two.
(5) Nuclear genes and RNA are similar between charophytes and plants.
Plants Conquer the Land
The Early Devonian Rhynie Chert Flora - from Life Before Man by Zdenek V. Spinar, illustrated by Zdenek BurianIf the great evolutionary radiation of metazoa (multicellular animals) in the earliest Cambrian oceans was the first great dramatic even of the Phanerozoic era (indeed ushering in the Phanerozoic), the conquest of land by multicellular plants was the next, and of equal importance. Indeed, without the plants no animals would ever have been able to survive on land.
But whereas the Cambrian explosion was very rapid, in the order of perhaps 3 to 5 million years for the origin of all major phyla (and many others now extinct), the colonization of the land by vegetation was a much slower and more protracted. The reason for this is not hard to understand. Cambrian animals were moving into a favorable new environment with no competitors. Plants had to brave desiccation, extremes of temperature, and harsh ultra-violet radiation.
Enigmatic traces are known from the early and middle Ordovician, These are fossils of spores, cuticles, and tubes and don't reveal much about the structures or nature of these plants. All we can say is that these plants were probably of a bryophyte grade of evolution - small, non-vascular, and lacking morphological differentiation into roots, stems, and leaves, like modern mosses and liverworts.
The first unambiguous record of land plants is from the Silurian period. They were mostly small, primitive forms, dependent on the proximity of water, and with the most rudimentary stem and leaf structure.
A common middle Silurian to early Devonian plant is Cooksonia, which had dichotomous branching and terminal sporangia (spore cases) at the tips of its green leafless stems. It is not known whether Cooksonia was a proper vascular (tracheid-bearing) plant. True vascular plants evolved and began to diversify during the Latest Silurian and Early Devonian.
