The earliest land plants

Plausible fossil green algae, dating back to 850 Ma, have been described from the Bitter Springs Chert, a siliceous deposit from Australia. Green algae may have colonized shallow water and shoreline habitats subject to periods of exposure. In this way algae living in marginal environments would have become adapted to periods of exposure and therefore life on land. Such plants would have had a selective advantage over plants living continually submerged.

The first land plants were probably non-vascular, that is they did not have a specialized system for transporting water and nutrients throughout the plant body. Non-vascular plants form three separate groups: hornworts (anthocerophytes), liverworts (hepatophytes), and mosses (bryophytes) (see Table 12.1). Due to their low preservation potential, their fossil record is fragmentary. Non-vascular plants probably orginated in the Silurian. The oldest fossilized non-vascular plants are liverworts described from the Lower Devonian of Belgium (Fig. 12.3). The reconstruction of this plant shows numerous, erect, slender stems with terminal sporangia. Liverworts are also known from the Devonian of New York State, USA.

Simple vascular plants are known from the Silurian. The plant bodies of these early forms were slender, leafless, and without roots. Sporangia were simple, swollen branch tips. The oldest true vascular plant is Cooksonia (Fig. 12.4), flourishing from the mid-Silurian to early Devonian times. Silurian forms are only a few millimeters tall, while incomplete Devonian forms have been found up to 6.5 cm in height.

Devonian Plant

Sporangium: contains the spores

Sporophyte: spore-bearing plant stage

Gametophyte: produces gametes (sex cells)

Fig. 12.3 Sporogonites - this fossil liverwort is the oldest fossilized non-vascular plant (2 cm tall).

Sporangium: contains the spores

Sporophyte: spore-bearing plant stage

Gametophyte: produces gametes (sex cells)

Fig. 12.3 Sporogonites - this fossil liverwort is the oldest fossilized non-vascular plant (2 cm tall).

Dichotomously branched stems: simple Y-shaped branching

Sporangia

Sporangia

Unequal dichotomous branching (monopodial)

Fig. 12.4 Two species of Cooksonia; the taller specimen is 6.5 cm high.

Unequal dichotomous branching (monopodial)

Dichotomously branched stems: simple Y-shaped branching

Fig. 12.4 Two species of Cooksonia; the taller specimen is 6.5 cm high.

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