Govt Exams
In early vascular plants like Rhyniophytes, the evolution of true leaves with expanded surface area was critical for maximizing photosynthesis in terrestrial environments. Though roots evolved to access underground water, the immediate advantage for terrestrial survival was increased photosynthetic capability. True leaves with organized vascular tissue provided both structural support and efficient light capture, giving competitive advantage over earlier bryophytes.
In gymnosperms like Pinus, the mature male gametophyte (pollen grain) at the time of pollination contains 2 vegetative cells (tube cell and prothallial cell) and 1 generative cell that later divides to form sperm cells.
The evolutionary trend shows progressive increase in sporophytic dominance, development of xylem-phloem in pteridophytes, transition to seed production in gymnosperms, and finally evolution of flowers in angiosperms.
The integration of female gametophyte (nucellus tissue) within the ovule in gymnosperms shows reduction of gametophytic independence and represents a major evolutionary transition from pteridophytes to seed plants.
Heterospory in Selaginella produces microspores (male) and megaspores (female), representing an evolutionary step toward seeds, and megaspores can produce female gametophytes that are partially protected, reducing water dependence for fertilization.
In angiosperms, the diploid sporophyte produces haploid spores through meiosis, which develop into haploid gametophytes that produce haploid gametes.
The cushion-like growth form in mosses helps in water retention, provides mechanical support without vascular tissue, and increases surface area for photosynthesis and water absorption.
As the endosperm develops and stores nutrients, the nucellus degenerates, allowing the endosperm to provide nutrition directly to the embryo through absorptive cells.
Polyembryony occurs through apomixis (asexual reproduction) and cleavage polyembryony where the embryo sac or embryo divides to form multiple embryos.
Rhynia and Cooksonia fossils are from the Silurian period, representing early vascular plants with primitive conducting tissues.