First Vascular Plants

• Lignin, a complex polymer found in plant cell walls, is made up of three monomers: coniferyl alcohol, sinapyl alcohol, and p-hydroxyphenyl alcohol

  
• The structure of lignin determines the properties of the plant cell wall, making it strong, rigid, and protective against pathogens
• Evolution of lignin in secondary cell walls of plants added rigidity, aiding in water transport and allowing taller growth for better light exposure
• Protracheophytes and early vascular plants evolved around 440 million years ago, with lignin evolution being a key development
• Simple lignins evolved in vascular plants, providing strength and resistance to degradation
• Plant secondary cell walls, strengthened by lignin, are difficult to break down and are formed in three layers inside the primary cell wall
• Rhynie chert in Scotland holds the oldest protracheophyte and vascular plant fossils, dating back to around 410 million years ago
• Rhynie chert fossils include Aglaophyton, Horneophyton, Rhynia, and Asteroxylon, all appearing to be sporophytes with unclear gametophytes
• Aglaophyton major, an early plant, had terminal sporangia and possibly isomorphic gametophyte generation
• The earliest vascular plants shared common characteristics like small size, dichotomously branching stems with cuticles, and lignified tracheids in protostele
• Rhyniophytes, Trimerophytes, and Zosterophyllophytes were the earliest vascular plants, with unique characteristics like endarch or exarch xylem differentiation
• Rhyniophytes had terminal sporangia, while Zosterophyllophytes had lateral sporangia on short stalks, all being homosporous with larger embryos
• Cooksonia and Rhynia are among the oldest vascular plant fossils, with unique features like dichotomous branching and stomata on lower stems
• Trimerophytophyta likely arose from Rhynia, with complex vascular systems and alternation of heteromorphic generations
• Zosterophyllophytes were small plants with dichotomous branching and some pseudomonopodial species, possibly growing in mud with enations