Pteridophytes are primitive vascular plants, commonly known as ferns. Unlike other vascular plants, the life cycle of ferns is split between free-living gametophytes and sporophytes phases. The gametophyte is generally simple in structure, containing egg-producing archegonium and sperm-producing antheridium. The sporophyte phase is structurally more complex in that it has highly specialized roots, stems and vascular systems as well as sporangia in which spores are produced. Ferns are among the first vascular plants that successfully invaded terrestrial habitats. They disperse mainly by spores and are regarded more primitive than seed plants.
The Life Cycle of Pteridophytes
The life cycle of pteridophytes shows apparent alternation of generations but is dominated by the sporophyte phase. They have been evolving towards having a more reduced gametophyte and a more developed sporophyte. The asexual generation of pteridophytes is the diploid(2n) sporophyte. Sporophytes are conspicuous and long-lived. They produce many sporangia, which contain a sporocyte. Sporocytes form haploid spores via meiosis and this is when the sexual generation starts. Ferns are either homosporous or heterosporous. As spores mature, they leave the ferns, germinate in appropriate environments and develop into a gametophyte. Gametophytes are small-sized, simple in structure and short-lived. Gametophytes of homosporous ferns are bisexual and enclose both archegonium and antheridium. Gametophytes of heterosporous ferns are unisexual and enclose archegonium and antheridium respectively. Archegonia contain eggs, while antheridia contain sperms. Sperms use their flagella to swim to the egg in the archegonia via watery media and under attraction of chemicals (mainly malic acid and salts). The fertilized diploid(2n) zygote will grow into an embryo and continue to develop in the gametophyte. Then the gametophyte will perish very quickly while the embryo will develop into a free-living sporophyte, which is the green fern we usually see.
Gymnosperms are seed plants that have archegonia, so they are a group of vascular plants between pteridophytes and angiosperms. Their gametophytes often stay within the sporophytes and a vast majority of gametophytes contain archegonia. Microspores germinate and form pollen tubes. At then, an embryo, female gametophyte and integument grow into a seed without forming an ovary and fruit. Gymnosperms have exposed ovules and seeds so they are named gymnosperm, Latin for “naked seed.”
The evolution of the pollen tube and seed marks a significant turning point in the history of plant phylogeny. The evolution of the pollen tube eliminates the water reguinement in fertilization, making them independent of water. It is a crucial adaptation of plants to terrestrial life. The evolution of the seed creates a more favorable condition for the reproduction and spread of plants. Therefore, in the phylogenic process, gymnosperms were able to replace ferns and occupy a certain dominant position in terrestrial vegetation.
Gymnosperms are mainly characterized by well-developed sporophytes. They are perennial, woody plants, and most of them are monopodial, tall trees. They have two types of branches--long shoots and short shoots. Long shoots are long and slender, extensively developing, with leaves spirally attached to the branches. Short shoots are thick and short, slowly growing, with leaves borne in tufts at apex of branches. Gymnosperms have a dictyostele, collateral bundles, cambium and secondary growth. The xylem of most gymnosperms is composed exclusively of tracheids, with only few having vessels. The phloem has only sieve cells and no companion cells. Leaves are mostly acicular, linear or squamiform, with extremely few flattened, fan-shaped and elliptical. Stomas on linear leaf surface form longitudinal single lines, called stomatal lines. Several closely located stomatal lines form light-colored stomatal bands on the underside of leaves. The protruding green midrib area between stomatal bands is called a midrib band. The green area arising between the stomatal band and leaf margin is called the marginal band.
Most sporophylls grow in groups and form a strobiliform body, called a strobile. Strobile are solitary or grow in clusters. They are usually unisexual and geitonogamous or xenogamous. Microsporophylls (stamens) grow into microstrobile (staminate strobile). On the ventral side of each microsporophyll are microsporangia (pollen sacs) full of microspores (pollens). Megasporophylls (carpels) grow in clusters or form macrostrobile (female cones). Megasporophylls of some species often transform into ovuliferous scales or leaves. There are exposed ovules on each megasporophyll or along its rim. The ovule consists of an embryo sac (female gametophytes), nucleus (megasporophyll) and integuments (appendage surrounding the megasporophyll). Each ovule has a micropyle, a small hole on the top end of the ovule. There is usually a pollen chamber on the top of the micropyle and with water stored inside. Sometimes the integuments near the micropyle elongate and form a micropyle tube. A gymnosperm gametophyte is extremely tiny and simply structured. It is parasitic completely on the sporophyte. Female gametophytes are developed from megaspores. The prothallium at the lower end of the female gametophyte is endosperm, which contains abundant nutrients. There are two or more archegonia on the top end of the female gametophyte. Only extremely few female gametophyte have no archegonia at all. Male gametophytes are pollen tubes developed from microspores. They contain two motile or immotile sperms. Microspores germinate in microsporangia and spread out by wind most of the time. Except the extremely few that form embryos directly, most fertilized eggs first develop into preembryos (proembryos) that have four or more cells or free nuclei, some of which then develop into a very long suspensor. Germ cells at the end of the suspensor continue to split to form embryos. Therefore, a gymnosperm embryo can penetrate deeply into the endosperm. Polyembryony, a phenomenon commonly seen on most gymnosperms, occurs because several or more eggs in the archegonia of the female gametophytes are fertilized at the same time or because of the splitting of proembryos into several embryos, a phenomenon known as cleavage polyembryony.
A gymnosperm seed consists of an embryo, female gametophyte, testa and other parts. The embryo, developed from a fertilized egg, represents a new generation of sporophyte. The endosperm that comes from the female gametophyte and the testa from integuments represent the old generation of sporophyte. Therefore, a gymnosperm seed contains parts from three generations and the source of its endosperm is completely different from that of angiosperms. When mature, in addition to testa, which develop from integuments, the seed of some groups of gymnosperms is wrapped by the aril, which develops from the megasporophyll. The seed is also protected and supported by seed scales and cone-scales. The macrostrobile of some groups of gymnosperms transform into strobile.
Angiosperms are highly evolved and are the most diverse and widespread group of plants in the world. They have vegetative and reproductive organs more complex than gymnosperms. The internal structures of their roots, stems and leaves are more adaptive to the natural world. Their reproductive organs are also more successful. It is for these reasons that they have been dominant plants since the Cenozoic era. Angiosperms consist of over 10,000 genera and 200,000 species, accounting for more than half of the total number of species in the plant world. There are more than 2,700 genera and 30,000 species in China and more species are being discovered. In Taiwan, there are more than 4,000 species of angiosperms.
The Characteristics of angiosperms can be summarized as follows. 1. Angiosperms have real flowers. Flowering is one of their conspicuous characteristics so they are also called flowering plants. An angiosperm flower consists of perianths (sepals and corollas), stamens and pistils and other parts. The function of perianths is to enhance pollination efficiency in order to achieve cross-pollination. Stamens are transformed from microsporophylls and consist of a filament and anthers. Carpels (pistils) are developed from megasporophylls and consist of an ovary, style and stigma. 2. An angiosperm ovule is wrapped within a single carpel (megasporophyll) or located within the ovary, which is a chamber created by folded megasporophylls. Such a structure is much more complicated and complete than the open carpel and exposed ovule of a gymnosperm. 3. An angiosperm sporophyte is highly-developed and well-differentiated. It also has very efficient physiological functions. For instance, most of their xylems consist of vessels, thin-walled cells and tracheids. They allow water to move smoothly along the plant body and provide mechanical support, which in turn can support leaves of much larger surface areas and thus enhance the efficiency of photosynthesis. 4. An angiosperm gametophyte is further simplified. Pollens develop into male gametophytes, which contain only two cells: one tracheary element and one generative cell. Most embryo sacs (female gametophytes) consist of eight cells: two synergids, one egg cell, two polar nuclei and three antipodal cells. The archegoium no longer exists. Prothallial cells nearly do not exist either. 5. Double fertilization and endosperm are conspicuous characteristics of angiosperms. The source and nature of their endosperm are both different from that of gymnosperms. Their endosperm is not a haploid female gametophyte (n) but a triploid cell (3n) resulting from fertilization. A heterogeneous endosperm provides angiosperm with more viability. 6. After fertilized, an angiosperm ovary develops into a fruit. The evolution of fruits provides special protection and spreading ability to angiosperm seeds. In addition, angiosperms use various kinds of pollination methods. They mainly rely on entomophilous pollination and then anemophilous pollination. They also use ornithophilous and hyrophilous pollination. Therefore, angiosperms can adapt to various types of habitats. Their diverse and complex morphological structures help them to function much more efficiently than any other kind of plant. They can also adapt to terrestrial environment more effectively. Angiosperms are trees, shrubs, vines or herbs. They are annual, biennial or perennial.
A dicotyledon is a plant with two cotyledons. Decotyledons are dominated by wood plants. They tend to have tap root systems, vascular cambia and vascular bundles in eustele arrangement in the stems, reticulate-veined leaves, and floral parts in multiples of four or five.
A monocotyledon is a plant with only one cotyledon. Grasses are the most notable monocotyledons whereas woody monocotyledons are rarely seen. Monocotyledons tend to have fibrous root systems, inatactostele vascular bundles arrangement and no vascular cambia in the stems, and floral parts in multiples of three, rarely in fours or fives. Their leaves usually have parallel or compylodromous venation. Reticulate or pinnate venation is rarely seen in them.