Gymnosperms: The term gymnosperm was used by Theophrastus in his book Enquiry into Plants. Robert Brown separated gymnosperms from angiosperms and placed them under a separate group. This is a class of naked seeded vascular plants, phanerogams without ovaries, and tracheophytes without a womb. It is the smallest group in Kingdom Plantae. It is mainly found in temperate regions except for Cycads, which are found in warmer ones. It is the smallest group of the plant kingdom with 70 genera and 900 living species. Gymnosperms arose in the Devonian period of the late Paleozoic era about 200 million years ago. They were dominant in the Triassic and Jurassic period of the Mesozoic era.
Angiosperms: These are also known as flowering plants. Angiosperms are classified by plants that germinate from seeds, seeds that are derived from inside fruits, and sporophylls are organized into flowers. They are 130 million years old and a dominant plant of today’s world.
The majority of gymnosperms are perennial woody plants forming either shrubs or trees. Ephedra is a climber. Some plants under this classification can grow very large and live for thousands of years. Flowering plants or angiosperms are the most recent and highly evolved plants. Their sporophylls are aggregated to form flowers therefore, they are also called flowering plants.
Unlike bryophytes and pteridophytes, in gymnosperms, the male and female gametophytes do not have an independent free-living existence. There are two types of sporophylls - microsporophylls and megasporophylls. The two distinct types of sporophylls are then combined to form cones or strobili. The cones take roles as pollen or male cones and seed or female cones. In angiosperms, both the microsporophylls and megasporophylls have distinct functionalities. The microsporophyll, which is also known as stamen, contains a filament and anther. A megasporophyll, which is also known as carpel contains a stigma, style, and ovary within which it contains ovules.
In gymnosperms, the ovules lie exposed on the megasporophyll and are unitegmic. The female gametophyte is represented by haploid endosperm and it contains archegonia. Whereas, the male gametophyte is highly reduced and is called a pollen grain. It produces only two male gametes or sperms, out of which only one remains functional.
In angiosperms, before fertilization, the female gametophyte or the embryo sac develops with eight nucleated stages. It has three-celled egg apparatus- one egg cell and two synergids; besides this, there are three antipodal cells and two polar nuclei. However, there is an absence of archegonia and is replaced with one oosphere which is surrounded by two distinct synergid cells which attract the pollen tube.
In gymnosperms, the pollination takes place directly as the stigma is absent. The pollen grains can inherently reach the micropylar ends of the ovules. The water from the environment is not essential for the transportation of male gametes. The alternative to this is the formation of a pollen tube by the male gametophyte for effecting siphonogamy or fertilization. Pollination is usually anemophily i.e., accomplished by the wind. In angiosperms, pollen grains or microspores reach the stigmatic surface by various biotic and abiotic agencies.
After fertilization, the zygote develops into an embryo, and the ovule develops into seeds. Seeds do not occur inside a fruit (ovary absent). They are naked or lie exposed on the surface of megasporophylls. The seeds comprise a food-laden tissue that functions for the growth of the embryo into a seedling and is represented as a female gametophyte or endosperm. In angiosperms, one male gamete fuses with an egg to form a zygote. The other male gamete merges with the aid of the diploid secondary nucleus and forms the triploid primary endosperm nucleus or PEN. Since two fusions occur; therefore, this event is called double fertilization, which is unique to angiosperms. The endosperm develops from PEN and is triploid. Thus, the endosperm is a post-fertilization tissue. The ovules after fertilization ripen into seeds and are covered with fruit. Fruits protect the seed and aid in their dispersal.
Vascular bundles are present in gymnosperms. Xylem has tracheids but vessels are absent. In phloem, sieve cells are present but sieve tubes and companion cells are absent. In angiosperms, the xylem contains vessels. Phloem contains companion cells and sieve tubes. The gymnospermic plant body is sporophytic, differentiated into tap root, stem, and leaves. Leaves show xerophytic features like thick cuticles, sunken stomata, and needle shape to withstand extremes of temperature, wind, and humidity. The leaves may be simple (Pinus) or compound (Conus). A tap root system is also present. Cycas also have special ageotropic N2 fixing coralloid roots having an association with Anabaena cicadae. Pinus root shows a symbiotic relation with ectomycorrhiza. Angiosperms have pentamerous, trimerous flowers, a primary root system, and broad leaves with chlorophyll. The secondary growth takes place with the roots and stem of the angiosperms that are classified as dicots.