Early vascular plants

The transition from aquatic to terrestrial habitats gradually took place over tens of millions of years. Plants steadily adapted to the environmental challenges of the land and became less dependent on aquatic habitats.

The first true vascular plants are Middle Silurian, although resistant, cutinized spores are known from Upper Ordovician sediments, suggesting that they may have evolved earlier. Known from the mid-Silurian, early leafless and/or rootless

Sporangia borne on tips on terminal branches

Leafless branches: probably photosynthetic and therefore green

Fig. 12.5 Rhynia (height 17 cm)

Sporangia borne on tips on terminal branches

Short, possibly deciduous, lateral branches

Stem covered with spiny projections

Fig. 12.6 Sawdonia (height 20 cm), a Devonian zosterophyll.

— Sporangia positioned between the spines on the stem, often forming fertile zones

Dichotomously branched stem

Stem covered with spiny projections forms do not have all the features associated with the vascular system. The oldest, simple vascular tissues are late Silurian, and stomata are first recorded in the early Devonian.

The earliest vascular plants were rhyniophytes. Rhyniophytes are slender, dichotomously branched plants without leaves or roots (Fig. 12.5). Cooksonia is a member of this group. Coexisting with the rhyniophytes in the early and mid-Devonian were the zosterophylls. Similar to rhyniopyhtes, some members of this group had small spine-like projections (Fig. 12.6). Most of these primitive plants were very short (only a few centimeters) and were anchored by rhizomes, a horizontal underground stem. Sediments show that early vascular plants lived in wet, marshy environments and were still associated with aquatic habitats.

Rhynie Chert

Complete rhyniophytes preserved in exquisite detail, as a result of siliceous perimineralization, are known from the Lower Devonian Rhynie Chert of northeast Scotland, UK. Material from this locality shows that the vascular system of these early land plants was very simple, constructed only of water-conducting xylem. Stomata preserved in the epidermis provide evidence that these plants photosynthesized and it is presumed that the stems were green. All the plants are without roots. Rhynia was anchored by tuft-like rhizomes, whilst Horneophy-ton had bulb-like structures with small thread-like rhizomes (Fig. 12.7).

Sporangia on branch tips. Some spores trilete

Sporangia on branch tips. Some spores trilete

Horneophyton

Dichotomizing aerial shoots

Bulb-like structures with thread-like rhizomes

Dichotomizing aerial shoots

Bulb-like structures with thread-like rhizomes

Fig. 12.6 Sawdonia (height 20 cm), a Devonian zosterophyll.

Fig. 12.7 Horneophyton.

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Responses

  • Rosanna
    What are cutinized spores?
    8 years ago
  • rahel
    What to check on a fossil classified as a vascular plant?
    1 month ago

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