Plant Kingdom and Life cycles

The Plant Kingdom

Kingdom Plantae- Plants have cells with a cellulose cell wall, plasmodesmata allowing for intercellular exchange and chloroplasts having a double membrane and chlorophyll a and b.

Plants are classified into 4 groups:

·     Mosses, hornworts and liverworts

·     Ferns and fern allies

·     Gymnosperm

·     Angiosperm

Phylum Bryophyta

Characteristics of Bryophyta:

·     Have multicellular sex organs, i.e. the gametes are enclosed by a sterile jacket of cells.

·     Are parenchymatous, not filamentous.

·     Retain the zygote within the female sex organ and allow it to develop into an embryo there.

·     Have cutin (a cuticle but not a waxy layer) on the plant and spores.

·     Have no lignin usually.

·     Are small, low-lying, (generally) moisture-loving plants.

·     Have no roots, only filamentous rhizoids.

Mosses- have minute "leaves" and stalks bearing a terminal capsule (sporangium) containing spores; moss sex organs (male antheridia and female archegonia) are typically produced on the leafy gametophytes of separate male and female plants

Life cycle of a moss

·     The dominant phase of the moss life cycle is the gametophyte. 

·     The plant is called a thallus as it has no differentiated leaves, stems or roots

·     It has "leaves" and "stems" (no xylem/phloem) which are green and photosynthetic.

·     It is small enough that minerals can be distributed by diffusion.

·     The leaf-like structures – phyllid - are arranged spirally around the ‘stem’. They consist of a single layer of photosynthetic cells with a midline vein for mechanical support

·     Anchorage is via rhizoids

·     Gametangia are antheridia and archegonia. The archegonia differ externally from antheridia in having a longer neck and a longer stalk. The neck encircles a tube which leads to the venter (cavity) containing the female gamete.

·     Gametangia have a sterile jacket of cells.

·     Gametes are sperm and egg.

·     Sperm are biflagellate and swim in free water to arrive at egg.

·     Usually grow in clusters to support each other and facilitate transfer of gametes.

The sporophyte is dependent on the gametophyte

·     The sporophyte produces spores by meiosis.

·     Zygote (still in archegonium) forms the embryo (hence embryophytes) which gets nutrition and protection from dessication from the gametophyte.

·     Sporophyte growing from the zygote is nutritionally dependent (heterotrophic) on gametophyte.

·     Sporophyte consists of foot, stalk, and capsule (sporangium).

·     Sporangium has sterile jacket surrounding sporocytes

·     Sporocytes divide meiotically to produce spores.

·     Spores are shed by wind.

·     Moss spores do not have many adaptations that allow them to remain dormant and survive for long.

·     Spores are the only means for dispersal of the species.

·     Spore Germination produces a photosynthetic filament of cells called the Protonema.

·     Leafy Buds are produced by the Protonema as it Matures.

·     The Buds form the Leafy Gametophores which produce the Gametangia (Antheridia or Archegonia)

Example with Funaria:

The gametophyte reaches a height of 3 cm. The rhizoids are branched and multicellular. The main ‘stem’ is also branched. The ‘leaves’ are simple, small and spirally arranged. The medulla of the ‘stem’ and the midrib of the ‘leaves’ is made up of small elongated cells that help in conduction (not hollow). Reproduction is both sexual and asexual. Asexual reproduction is by fragmentation or secondary protonema. Funaria is monoecious with male and female branches on the same plant. The mature antheridium of Funaria is an elongated, club-shaped, orange-coloured body raised on a short, multicellular stalk. The body has a jacket layer of polyhedral, flattened cells. Within is a dense mass of small cells known as androcytes. Each androcyte produces a single, biflagellated sperm called antherozoid. The archegonial branch springs from the base of the male shoot. The archegonia are aggregated into a terminal cluster, stand erect and project from the surface of the female receptacle. The ‘leaves’ surrounding the archegonial cluster are called perichaetial leaves. Fertilization takes place with the aid of rain or dew. The transfer of antherozoids from the antheridial head to the archegonial head is brought about by water accumulated in the reproductive tips. Entry of antherozoids into the archegonium is due to chemotactic influence of the mucilagenous substances present in the neck. The diploid zygote formed by fusion of an egg cell with an antherozoid developes into the sporophyte generation which consists of a foot, seta and capsule. As the sporophyte matures, the water supply of the capsule is cut off. As a result all tissues of the capsule except the spores dry up. The thin walled cells of the annulus brake and the operculum is thrown away. The outer peristomial teeth are hygroscopic and in dry atmosphere they bend outwards with jerky movements, but the inner pristomial teeth remain in their position. Due to outward movements of the outer peristomial teeth, slits between the inner thin walled peristomial teeth become wider and spores escape through these slits gradually.

Phylum Tracheophyte

Characteristics of Tracheophyte:

·   protective walls surrounding spores to prevent desiccation,

·   A waxy cuticle to reduce evaporation,

·   stomata for gas exchange,

·   xylem and phloem to conduct water and food throughout the plant,

·   incorporation of lignin in cells for support,

·   roots for anchorage and absorption of water and minerals from the soil and

·   the sporophyte branches (larger to produce more spores)

Club mosses- found in temperate forests

Horsetails- taller stems with whorls of slender green branches.

Life cycle of a Fern- Class Polypodiophyta 

Description of sporophyte- The fronds of the sporophyte may reach a metre or more in height and grow from a thick horizontal stem, or rhizome. This bears adventitious roots. Brances from the main stem may eventually break away and give rise to separate plants (vegetative reproduction). The bases of the fronds are covered with dry brown scales called ramenta that protect the young leaves from frost and drought. The young leaves show a characteristic tightly rolled structure. The ramenta gradually become smaller and less dense up the main axis of the frond. This axis is called the rachis, and the leaflets either side are called the pinnae. The smaller rounded subdivisions of the pinnae are called the pinnules. Ferns follow a similar pattern of development to that of mosses, although most ferns are homosporous. That is, the sporophyte produces only one type of spore within a structure called the sporangium. One gametophyte can produce both male and female sex organs i.e. antheridia and archegonia. The biggest contrast between the mosses and the ferns is that both the gametophyte and the sporophyte of the fern photosynthesize and are thus autotrophic; the shift to a dominant sporophyte generation is taking place.

The gametophyte generation of ferns are small heart or kidney shaped structures called prothallus. They are less than an inch (1 - 2 cm) in diameter and one cell thick. Male and female sex organs are located on the underside of the prothallus and, when conditions are right, the flagellated sperm swims from the male antheridium to fertilize the egg in the archegonium. The prothallus also has root-like structures called rhizoids, that consist of single greatly elongated cells for anchorage into the soil. Absorption takes place from all over the structure. Once Fertilization occurs the Embryo develops within the Archegonium. The embryo develops directly into the sporophyte (as in Bryophytes) - survival depends on the environmental conditions – it is relatively unprotected.  The sporophyte, grows to maturity and then produces spores by meiosis on the undersurface of its leaves or fronds in structures called sporangia. Inside each sporangium diploid spore mother cells divide by meiosis to produce haploid spores. When mature the indusium shrivels and drops off, and te exposed sporangium walls begin to dry out. Eventually the wall raptures and spores are catapulted from the sporangium. As in the Bryophytes the spores are the means of dispersal of the species. The spores of some species can remain dormant and viable for long periods of time. The sporangia are sometimes protected by a layer of cells called the indusium This entire structure is called a sorus. The developing sporophyte has vascular tissue and roots; the gametophyte does not. Fern sporophytes can grow as tall as trees. They can reproduce vegetatively from root cuttings. They never have flowers. Ferns prefer to live in environments having low light and relatively high levels of moisture and humidity. For reason they flourish in tropical forests.

Life cycle of a flowering plant- Class Magnoliophyta

Here, two types of spores are produced – microspores and megaspores. This is called heterospory. Only the microspores (develop into pollen grains) are released, megaspores are retained by the parent sporophyte. The microspores are formed within microsporangia (pollen sacs) within anthers. Megaspores are formed singly within megasporangia (ovules) found in the ovary.

The anther produces haploid spores, which develop into pollen grains, the male gametophytes. The megaspore is formed within the megasporangium inside the ovary. The megaspore is not released but remains enclosed within the sporophyte tissue. The megasporocyte undergoes meiosis to form four haploid megaspores. Three will eventually degenerate leaving only one functional megaspore. The female gametophyte develops endosporically within the megaspore. The megaspore nucleus divides mitotically three times to produce 8 haploid nuclei. The large eight – nucleated cell within the ovule consititutes the female gametophyte. The cell walls form around the nuclei to form a seven celled structure, where the central cell has two nuclei. One of the nuclei function as the female gamete (egg). It is usually the bottom one flanked by two synergid nuclei. At the other end, three antipodal nuclei are non-functional. The central two nuclei sometimes fuse together. During pollination, a pollen grain lands on the stigma (unlike in gymnosperms, in which the pollen lands directly in the ovule), and grows a pollen tube down the stem of the carpel and into an ovule, through which sperm are released from the pollen grain. The sperm fertilize the egg in the ovule, creating a diploid zygote that will then develop into an embryonic plant. Thereby completing the life cycle. After fertilization, each ovule grows into a seed with a hard coat, containing a developing embryo and a food supply. The ovary grows fleshy tissue and becomes a fruit, which aids in seed dispersal. After germination, each seed can grow into a new plant.

Two other reproductive adaptations enhanced the success of the angiosperms.

·       The rapid rate of seed production; (unlike in gymnosperms, which often have periods of over a year between pollination and fertilization, most angiosperms cut that period to a few weeks. This enables them to take advantage of short growing seasons in many inhospitable habitats, such as deserts.)

·       Seed dispersal. The evolution of the fruit allowed angiosperms to cultivate more relationships with animals: many animals like to eat the fruits, but the hard seed coats pass through animal digestive tracts and exit the animal, with some ready-made fertilizer, a good distance from the original plant.

Monocots- are a groups of flowering plants distinguished by having one seed leaf – codyledon, within the seed. They generally have parallel leaf veins, scattered vascular bundles within the stems and flower parts in threes or multiples of threes. To break through the soil for the first time, a protective sheath, the coleoptile, first develops around the embryonic shoot (plumule) and pushes up through the soil. 26 Then, the shoot can grow up into the air without first breaking in the hard, abrasive soil.

Dicots- are distinguished by having two seed leaves within the seed. The dicotyledons usually have leaf vains in the form of a net, a ring of vascular bundles in the stem and flower parts in fours or fives or multiples of these. The class Dicotylodonae is large and diverse. Dicot plants range from tiny plants to tremendous trees; fleshy succulents to delicate herbs that dry out almost as soon as they're picked; large and complex flower heads to tiny flowers; annuals and perenials; deciduous (shed leaves at end of growing period, usually Autumn) and evergreen. The life cycles of dicots are similar to those of monocots, the other class of angiosperms, though there are some differences that occur between the germination of the seed and the growth of the plant. In dicots, the first organ to develop is the embryonic root, which is soon followed by the embryonic shoot, the beginning of the above-ground plant.

 Alteration of generation

Alteration of generation- there are two forms of plants, the spore-producing sporophyte is diploid and the gamete-producing gametophyte is haploid.

The production of haploid spores by plants occurs as a result of meiosis. Spores undergo mitosis to produce the plant gametophyte. The zygote formed by fertilization also undergoes mitosis to form the sporophyte.

Dominant generation- the most long-lived from that we recognize as the plant. For non-vascular plants, the gametophyte is the dominant generation. For vascular plants, the sporophyte is the dominat generation.