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Chapter 21: Vascular Plants Without Seeds (The Microphyll Line of…
Chapter 21: Vascular Plants Without Seeds
Early Vascular Plants
Zosterophyllophytes
Lateral;not terminal
Grew as small bunches, only approximately 15 cm high
Stems were naked (smooth)
Contained a small amount of xylem
Consisted of tracheids with annular and scalariform
Naked (smooth)
Branched dichotomously
Named after the principal genus Zosterophyllum
Sporangial walls were several layers thick, and all spores were the same size
Enations
Outgrowths
Ranged from quite small to long, thin scales
Rhyniophytes
They had equal dichotomous branching, both branches being of equal size and vigor
Rhynia and Aglaophyton were other rhyniophytes, early vascular plants similar to Cooksonia
The earliest fossils that definitely were vascular land plants belong to Cooksonia, a genus of extinct plants
Another fossil from the same time, Horneophyton, had sporophytes with naked axes that branched dichotomously and were up to 20 cm tall
Each sporangium had a short columella
Hornworts evolved from species like Horneophyton by means of elaboration of the gametophyte and a reduction of the sporophyte
They had stomata and terminal sporangia
Xylem Structure of Early Vascular Plants
Siphonostele
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One in which pith is present in the center
The xylem in many specimens of Rhynia and Aglaophyton is well preserved
Early vascular plants had two types of xylem organization. In both, the center is a solid mass of xylem with no pith; this is
Protostele
The Microphyll Line of Evolution: Lycophytes
Lycophytes represent a distinct line of evolution out of the early land plants that resembled zosterophyllophytes
Lycophytes have lateral sporangia and exarch protosteles, and thus, they may have come from a Zosterophyllum type of ancestor
Morphology, Heterospory, and Extant Genera
Microphylls
Enations in the division Lycophyta
Many extinct lycophytes such as Lepidodendron, Sigilaria, and Stigmaria had a vascular cambium and secondary growth
The earliest lycophytes were members of the genera Drepanophycus and Baragwanathia
Isoetes
The Megaphyll Line of Evolution: Euphyllophytes
Equisetophytes
Division Arthrophyta
Horsetails
Origin of Megaphylls (Euphylls)
Megaphylls are thought to have evolved from groups of branched stems that have become fused together.
Ferns
Eusporangia and Leptosporangia
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Psilotum and Tmesipteris
Trimerophytes
Overtopping
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Trimerophytes had an unequal branching in which one stem was more vigorous
A group of extinct land plants that flourished during the latter part of the Devonian period (380–360 million years ago)
Division Trimerophyta
Psilophyton
Perticula
Trimerophyton
