BIOL 1020H Lecture Notes - Lecture 2: Gastrulation, Bilateria, Paleozoic
Document Summary
For much of earth"s history, the terrestrial surface was lifeless. Cyanobacteria and protists likely existed on land 1. 2 billion years ago. Around 500 million years ago, small plants, fungi, and animals emerged on land. In charophytes a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out. Sporopollenin is also found in plant spore walls. Charophytes protect the egg cell instead of releasing in for external fertilization. Five key traits appear in nearly all lad plants but are absent in the charophytes: alternation of generations, multicellular, dependant embryos, walled spores produced in sporangia, multicellular gametangia, apical meristems. Plants alternate between two multicellular stages, a reproductive cycle called alternation of generations. The gametophyte is haploid and produces haploid gametes by mitosis. Fusion of the gametes gives rise to the diploid sporophyte, which produces haploid spores by meiosis. Fossil evidence indicated that plants were on land at least 470 million years ago.
