Relationships between non-vascular, seedless vascular, gymnosperms and angiosperms plant groups
When the first aquatic plants went on to colonize the totally different terrestial habitat, they had faced countless challenges and difficulties in the process of surviving in the new habitat. As such, they had learned to adapt to the various harsh conditions of the enviroment and gradually evolved morphologically and physiologically over the course of time to overcome those problems. Today, we can observe two major plants groups in existing plants which are the non-vascularised and vascularised plants. Non-vascular plants include mosses, liverworts, and hornworts. Vascularised plants can be further divided into seedless vascular (ferns and their allies), gymnosperms (cones) and angiosperms (flowering plants).
As the name of this group implies, plants in the non-vascular lineage do not have vascular tissue (or if present, it is very reduced). Because they lack substantial vasculature, plants in this lineage are generally small in size, lack significant structural support, grow close to the ground in moist areas, and lack significant water-conducting cells (xylem). Non-vascular plants typically grow in moist environments such as on rocks and trees. Their lack of vascular tissue requires them to maintain close contact with water to prevent desiccation. They do not have true roots, true stems, or true leaves (which are distinguished by the presence of vascular tissue). Rhizoids are the root-like structures that function to anchor them to the surface they are growing on, however, they are not capable of water uptake. Water is absorbed throughout the “leafy” plant body of the gametophyte called thallus. They also require a moist environment for successful fertilization. Unlike gymnosperms and angiosperms, they do not produce pollen grains and have retained the primitive condition of a flagellated sperm. The male gametes are motile in water and must be released into a moist environment so that the sperm can swim to the female gametangium (where the egg cells are located).
The plant body that is most obvious is the gametophyte generation, which is haploid (n). There are separate male and female gametophytes; however, the gametophyte can also be bisexual (male and female gametangia are located on the same plant body). The gametophyte is the generation that produces gametes; sperms are produced in the male gametangium, the antheridium, and eggs are produced in the female gametangium, the archegonium. The motile sperm will swim and reach the egg, which is then retained in the archegonium and fertilization takes place. The diploid (2n) zygote undergoes mitosis and begins to develop into the embryo (also 2n). The embryo matures into the sporophyte, the diploid (2n) plant body. Meiosis takes place in the sporangium of the mature sporophyte and haploid spores are produced. The haploid spores are dispersed and each spore undergoes mitotic cell division to create a haploid multicellular gametophyte. The prominent haploid gametophyte is then ready to produce gametes and initiate the entire cycle again.
An important feature of the moss life cycle is that the developing embryo is retained on the gametophyte plant body. This is an adaptation to the terrestrial environment because the embryo is protected from desiccation throughout its development into the sporophyte. In addition, mosses are capable of asexual reproduction during unfavourable conditions through structures called gemmae cups, a feature absent in the other plant groups. Gemmae cups can be detached from the parent moss, usually by the force of rainwater, and germinate via mitosis once suitable conditions are satisfied.
Seedless vascular plants have a waxy cuticle, stomata, and well-developed vascular tissue but lack the ability to produce seeds. Their vasculature allows them to grow to larger sizes than the non-vascular plants, but they still largely occupy moist habitats. While this lineage is more well adapted to drier habitats than are the non-vascular plants, they still require moisture for reproduction. Although the developing diploid embryo is dependent on the haploid gametophyte for survival (like mosses), the diploid sporophyte is more conspicuous and is the prominent generation of seedless vascular plants. Phylogenetically, seedless vascular plants are basal to the seed plants. The seedless vascular plants include species such as ferns (Pteridophyte) and horsetails (Sphenophyta).
Vascular tissue is the characteristic that distinguishes the seedless and seeded vascular plants from those plants that preceded them. While protected gametes allowed plants to move onto land, it was vascular tissue that allowed plants to dominate the landscape. Vascular tissue provide means for transport of important substances and structural support for the plant body. Vascular tissue consists of xylem and phloem. Xylem is primarily the vasculature through which water and minerals travel from the roots up the stems to the various parts of the plant. Phloem transports the sugars made during photosynthesis down to the roots and around the plant body to provide energy. Thus, there is a net movement of water up and nutrients down. These two types of vessels differ structurally and functionally. The xylem is made of non-living cells that tend to be more fortified due to presence of lignin for plant strength. Lignin is a layer of substance embedded in plant cell walls between the cellulose matrix, and it is a very stable molecule that does not break down easily. The phloem consists of living cells (ample mitichondria) to aid in the flow of soluble organic nutrients. Vascular tissue gives additional support to plants.
Take another look at the moss and fern life cycles. Both have flagellated sperm. This means that they are both dependent on water for fertilization. Mosses are already very small and low to the moist ground, but ferns have vascular tissue and are much taller. This could be another reason for the dominance of the sporophyte. Also, another major difference is that the sporophyte and gametophyte live independently for part of the life cycle as opposed to mosses where sporophytes are dependent on gametophytes. In the case of the fern, the gametophyte is photosynthetic, much smaller and lower to the ground, where moisture is more available for fertilization.
The other similarity between mosses and ferns is that both have antheridia (male reproductive organ that produce sperms) and archegonia (female reproductive organ that produce eggs). These structures are the specialized gametophytic tissue where gametes are produced. The archegonium is also the site of fertilization to form the zygote once the sperm from the antheridium swims through water to reach it.
Gymnosperms, meaning ‘naked seeds,’ are a diverse group of seed plants. Their characteristics include naked seeds, separate female and male gametes, pollination by wind, and existence of tracheids (which transport water and solutes in the vascular system).
Unlike bryophyte and fern spores (which are haploid cells dependent on moisture for rapid development of gametophytes), gymnosperms and angiosperm have seeds to replace spores and within the seed contains a diploid embryo that will germinate into a sporophyte. There also are the storage tissues to sustain growth (called endosperm in angiosperms) and a protective coat give seeds their superior evolutionary advantage. Several layers of hardened tissue prevent desiccation and omittance of water for reproduction further increases their evolutionary superiority. Furthermore, seeds remain in a state of dormancy until conditions for growth become favourable. Whether blown by the wind, floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus avoiding competition for resources with the parent plant.
Pollen grains are male gametophytes which contain the sperm (gametes) of the plant and are carried by wind, water, or a pollinator such as insects. The whole structure is protected from desiccation and can reach the female organs without dependence on water. The small haploid (n) cells are encased in a protective coat that prevents desiccation (drying out) and mechanical damage. Male gametes reach female gametophyte and the egg cell gamete though a pollen tube: an extension of a cell within the pollen grain.
The life cycle of a gymnosperm involves alternation of generations, with a dominant sporophyte in which the female gametophyte resides, and have reduced gametophytes. All gymnosperms are heterosporous (produces two different kinds of spores, known as microspores and megaspores). Male and female sporangia are produced either on the same plant, described as monoecious (‘one home’ or bisexual), or on separate plants, referred to as dioecious (‘two homes’ or unisexual) plants.
The success of angiosperms is due to two novel reproductive structures: flowers and fruit. The function of the flower is to ensure pollination by possessing various attractive features such pleasant fagrance, myriad colours, distinctive shapes etc. Flowers also provide protection for the ovule and developing embryo inside a receptacle. Angiosperms produce their gametes in separate organs (anther and ovary), which are usually housed in a flower. Both fertilization and embryo development take place inside an anatomical structure that provides a stable system of sexual reproduction largely sheltered from environmental fluctuations.
Following fertilization of the egg, the ovule grows into a seed. The surrounding tissues of the ovary thicken, developing into a fruit that will protect the seed and often ensure its dispersal over a wide geographic range. Not all fruits develop from an ovary; such structures are ‘false fruits’. Like flowers, fruit can vary tremendously in appearance, size, smell, and taste. Tomatoes, walnut shells and avocados are all examples of fruit.
The function of the fruit is seed dispersal. They also protect the developing seed. Different fruit structures or tissues on fruit, such as sweet flesh, wings, parachutes, or spines that grab, reflect the dispersal strategies that help spread seeds. Many fruits attract animals that will then eat them and pass the indigestible seeds through their digestive systems, then deposit the seeds in another location by either throwing them away at random places or through random defaecation. For example, monkeys will dispose the tough and hard seeds of fruits by simply tossing them anywhere while faeces of birds bears the indigestible seeds from the fruits they consumed.
