Gymnosperms
Gymnosperms, derived from the Greek words gymnos, meaning naked, and sperma, meaning seeds, refers to a group of plants whose seeds are not enclosed within an ovary wall. Unlike flowering plants (angiosperms), the seeds of gymnosperms develop in an exposed manner, typically on the surface of cones or strobili.
Characteristics
- Growth Forms: Gymnosperms include medium-sized to tall trees, like the giant redwood (Sequoia), and shrubs with tap roots often associated with mycorrhizal fungi or nitrogen-fixing cyanobacteria in some genera.
- Leaf Adaptations: The leaves are well-adapted to survive extreme environmental conditions, with many exhibiting needle-like shapes (as seen in conifers), which help minimize water loss via transpiration due to their smaller surface area, thick cuticles, and sunken stomata.
Reproductive Structures
- Gymnosperms are heterosporous, producing two types of spores: microspores (male) and megaspores (female). These spores are produced in specialized structures known as microsporangia and megasporangia, respectively, located on sporophylls arranged either in compact or lax cones.
- Male Gametophytes: Microspores develop into pollen grains, representing a highly reduced male gametophyte confined within the microsporangium.
- Female Gametophytes: The megaspore mother cell, found within the ovule, undergoes meiosis to produce megaspores, one of which develops into a multicellular female gametophyte that contains archegonia for sexual reproduction.
Fertilization Process
- Pollen grains are released from the microsporangium and transported by air currents to the ovules, where they germinate to form pollen tubes that carry male gametes to the female gametophyte's archegonia, leading to fertilization.
- After fertilization, the zygote develops into an embryo, while the ovules mature into naked seeds, hence the name gymnosperms.