Angiosperms


 * Angiosperms **

By: Divya Bhatia



** Diagnostic Characteristics of Angiosperms: ** ** Angiosperms ** are land plants with vascular tissue, seeds, and are characterized by the presence of reproductive structures called flowers and fruits. Most present-day plant species are angiosperms, or flowering plants. Angiosperms contain the largest number of species of all plants (about 250,000) and they are the most widespread of all plants. Angiosperms differ from gymnosperms, which do not use flowers to bear their seeds. Hypothesis based on the fossil record and molecular analyses indicate the angiosperms are the youngest lineage of plants. There are three major types of angiosperms: monocots, eudicots, and other older lineages. Angiosperms are **heterosporous**, which means the sporophytes of angiosperms produce megaspores, which develop into the female reproductive system, and microspores, which develop into the male reproductive system. The nucleus of the megaspore undergoes mitosis three times. From this, 8 nuclei are formed and then are partioned by cell walls and form an embyro sac. This is the **female gametophyte**. (AC) (7)

The **flower**, which characterizes the angiosperm, evolved about 130 million years ago and it is an adaptation that is specialized for angiosperms to reproduce on land. The **fruit** develops from the ovary as seeds develop in the angiosperm after fertilization. Also, angiosperms are characterized by a process known as double fertilization (explained under the “Reproduction” section). The phylogeny of Angiosperms.

The 250,000 or so species of angiosperms are scattered all around the Earth in both terrestrial and aquatic habitats. Many angiosperms are designed to disperse their seeds far from where they originated via mechanisms of seed dispersal. Thus, habitat of angiosperms is very widespread. Although angiosperms have adapted to live in some saline environments such as salt marshes, not many angiosperms live in marine environments. Those that do, mostly sea-grasses, are often located in tropical areas. [5][6] (LJ)
 * Habitats: **

Angiosperms are all part of the phylum **Anthophyta.** The two major classes of angiosperms are monocots and dicots, which are characterized by distinct structural and functional details. ** Monocots ** have leaves with viens that are parallel to each other, like that of a blade of grass. Other examples of monocots are orchids, bamboos, grains, and palms. Monocots have one embryonic seed leaf, or **cotyledon.** In monocot stems, there are complexly arranged vascular bundles in the ground tissue, and they have a fibrous root system. Also, monocots contain flowers with petals that are usually multiples of three. Most ** dicots ** contain leaves with net-like veins, like that of an oak tree leaf. Other examples of dicots are roses, sunflowers, and beans. Dicots have two embryonic seed leaves, or ** cotyledons. ** In dicots, there are vascular bindles that are arranged in a ring, and their root system contains a taproot. Also, dicots contain flowers with petals that are usually in multiples of four or five. Most dicots can increase their diameter through a process called secondary growth. This process produces the wood and bark that is not present in monocots.(KL)([|12]) However, DNA analysis has shows that not all dicots are from the same lineage. Thus, two subgroups of dicots are now eudicots and those of earlier lineages. **Eudicots** include most of the dicots and are the largest class of angiosperms.
 * Major Types ** :

(MLK)

Angiosperms, like all plants, have three basic organs: roots, stems, and leaves. ** Monocots ** have a **fibrous root system** that consists of a layer of thin roots that spread out below the surface of the soil, extending the plant’s exposure to water and minerals in the soil. Most **dicots** have a **taproot** **system** that consists of one large vertical root (the taproot) that produces many smaller, branching roots. The taproot functions in firmly anchoring the plant in the soil and storing food. ** Root hairs ** are found near the root tips of both types of angiosperms. They increase the surface area of the root, increasing the amount of water that can be absorbed.
 * Basic Anatomy: **

The leaves of angiosperms are found in many different varieties. They can be simple (one solid leaf) or compound (several small leaflets) in form, and they can be arranged in either an alternate, opposite, or whorled (more than two leaves branch off the same spot) pattern. (NI) [|1] "Stems come in various types such as stolon, an above-ground “runner”, rhizome, an underground “runner”, or bulb, a fleshy stem modified for nutrient storage." (AK) (2)

** The Flower ** A flower is a specialized **shoot,** or the part of the flower consisting of stems, leaves, and flowers, with four types of modified leaves: sepals, petals, stamens, and carpels. The **sepals** (usually green) are at the bottom of the flower and they are modified leaves that enclose the flower before it opens up. The **petals**, located above the sepals, are brightly colored to attract insects and other pollinators. The sepals and petals compose the sterile part of the flower and are not involved directly in reproduction. The non-sterile parts of the flower are the stamens and carpels. The **stamens** are the male reproductive organs that produce male gametophytes (haploid cells), and the **carpels** are the female reproductive organs that produce female gametophytes. These enclosed gametophytes and the existence of carpels in the angiosperm are what distinguish them from a gymnosperm, in which the seed is not protected by a fruit or flower, but exposed to the environment. (KS)5 The stamens and carpels are both **sporophylls,** which produce haploid spores. The stamen consists of a **filament,** or stalk, and the **anther,** which is a terminal sac where pollen is produced. The carpel has a sticky **stigma** at its tip to receive pollen and a **style** that leads to the **ovary,** which is the structure at the base of the carpel that contains the **ovules,** which develop into seeds after fertilization.

The blooms of flowers can be classified according to the role the flowers play in reproduction as well as the number and kinds of parts they have. Complete flowers are flowers that have four parts: sepals, petals, stamen and carpels, while incomplete flowers are flowers that are missing one or more of the parts that a complete flower possesses. Monoecious angiosperms produces male and female flowers on the same plant, while dioecious angiosperms produce flowers that are either all male or all female. However, both monoecious angiosperms and dioecious angiosperms still produce incomplete flowers. (CC) ([|4]) This figure points out the various parts of a flower.

The wall of the ovary thickens to become the **pericarp**, which is the wall of the fruit.

A **simple fruit** is derived from a single ovary, such as a cherry. An **aggregate fruit** is from one flower with several carpels, such as a blackberry. A **multiple fruit**, such as a pineapple, develops from a group of clustered flowers whose ovaries fuse and thicken to form one fruit. Fruits have three layers: the exocarp, the mesocarp, and the endocarp. The exocarp in the outermost layer, the mesocarp is the juicy, middle layer, and the endocarp is the layer that contains the seeds. (10)(SM)

** Transport of Materials ** : Since this plant is vascular, it is equipped with two types of vascular tissue known as **phloem,** which transports nutrients made in leaves to the roots and throughout the plant, and **xylem**, which transports water and dissolved minerals throughout the plant.

** Xylem ** cells are called **tracheids,** long cells that help support the plant structure and transport water. The xylem of angiosperms also contains **fiber cells,** which help support the structure of the plant, and **vessel elements,** which are shorter than tracheids and are arranged into tubes. They efficiently transport water around the plant because they are perforated.

** Phloem ** cells transport sucrose, organic compounds, and some mineral ions through the tubes. These tubes are formed by chains of **sieve-tube members** cells. In angiosperms, there are walls between sieve-tube members, called **sieve plates,** that are porous and allow fluid to flow from cell to cell. A **companion cell** is connected to the sieve-tube members by channels called **plasmodesmata,** which allow the flow of water between cells.

** Ground tissue ** is present in dicots and aids in transferring nutrients, photosynthesis, storage, and support.

There are two pathways that water and minerals can travel through the root to the stele. The symplastic pathway which is through cells and vacuoles, and the elements travel from cell to cell. (most cations use this pathway)The second method is the apoplastic pathway where the minerals and water travel through the cell walls and intercellular spaces, until the material reaches the endodermis. (most anions follow this pathway). (CW)(3)

Flowers are the reproductive system of angiosperms. Reproduction is basically when pollen from the stamen of one flower to the stigma of another flower. Pollen is released from the anther and transferred to the sticky stigma of another flower. Most angiosperms employ mechanisms that ensure **cross-pollination**, the transfer of pollen from flowers of one plant to those of another plant of the same species. Self-pollination is not likely. After attaching to the stigma, the pollen grain germinates and now contains one mature male gametophyte. The pollen grain extends a tube that grows towards the ovary through the style and discharges two sperm cells into the female gametophyte.
 * Reproduction: **

Meiosis produces two kinds of spores: microspores and megaspores. **Microspores** develop in the microsporangium and then germinate and develop into the male gametophyte generation. **Megaspores** develop in the megasporangium and then develop into the gemale gametophyte generation. Both of these sporangia are formed in flowers. Flowers that are referred to as **perfect flowers** have both microsporangia and megasporangia. ** Imperfect flowers ** have either microsporangia or megasporangia. (SI) (11)

** Double Fertilization ** : Double fertilization is a mechanism in angiosperms in which two sperm cells unite with two cells in the embryo sac to form the **zygote**, or fertilized cell, and **endosperm**, or nutrient-rich tissue of the seed. One sperm unites with an egg, forming a diploid zygote. The other sperm fuses with the two nuclei in the large center cell of the female gamtophyte, forming a triploid (3n) nucleus. The ovule matures into a seed, the zygote matures into an embryo, and the ovary develops into a fruit. Then, **cotyledons**, the seed leaves of a sporophyte embryo with a rudimentary root, form. Monocots cotyledons contain one seed leaf, while dicot cotyledons contain two seed leaves.The triploid nucleus divides and gives rise to the **endosperm**, which is a reserve for starch and other food. Monocot seeds store most of their food in the endosperm while dicot seeds transfer most of their food from the endosperms to the cotyledons. Double fertilization ensures that the endosperm will only develop in ovules where the egg has been fertilized, preventing wastage of nutrients.



This figure illustrates the process of double fertilization.

**Life Cycle** Angiosperms are **heterosporous** like all seed plants. The flower of the sporophyte, or diploid cell, that produces male and female gametophytes, the haploid form. The male gametophyte are in the **pollen grains**, which develop in the anthers of the stamen. **Ovules** develop in the ovary and contain the female gametophyte, or **embry sac.** The evolution of angiosperms reflects the trend to reduce the gametophyte generation in vascular plants. The pollen grain is made up of two haploid micro-spores. These micro-spores each undergo a mitotic division, producing one generative and one tube cell. when these cells combine they make up the pollen grain which then develop into the Stamen as stated above. (GR)[|13]

Terrestrial adaptations that made it possible for plants to colonize land include: - ** Vascular tissue ** - has cells that are joined into tubes that transport water (xylem) and nutrients (phloem) throughout the plant. The presence of the three types of xylem cells: tracheids, fiber cells, and vessel elements evolved as adaptations that allow angiosperms to survive in various habitats. - Scientific research has shown that a certain species of angiosperm, called //Amborella Trichopoda//, is the oldest branch of angiosperms. This species contains tracheids, but not vessels, which suggests that true vessel elements evolved in angiosperms when this species branched off as a distinct lineage. - ** Seed ** – evolved in angiosperms as an adaptation that facilitated reproduction on land. A **seed** consists of a plant embryo with a food supply (nutrients) surrounded by a protective coat. The embryo within the seed of an angiosperm is a sporophyte, or diploid, cell. - The seed protects the delicate female gametes and the embryos they produce from environmental stresses, such as drought and UV radiation, and provides them with enough moisture and nutrients from their parents. - Some **seeds** are enclosed in fruits that are like propellers that enhance dispersal by wind, such as dandelions. Other fruits are like burrs and attach themselves onto animal fur. Many angiosperms produce edible fruits that animals eat; the fruit itself is digested, but the seeds usually pass undamaged through the digestive tract. This way, animals deposit seeds far from where the fruit was eaten.
 * Environmental Adaptations ** :

Adaptations in Flowers: - ** Flowers ** are complex reproductive structures that bears seeds within protected **ovary.** - ** Carpels, ** or the female reproductive system of angiosperms, evolved from seed-bearing leaves that over time rolled into tubes. This led to better protection of the seeds and higher reproductive success. - ** Pollinator-plant relationships ** caused the diversity of flowers in angiosperms due to **coevolution**, or mutual evolutionary influence between two species, because of the flowers’ ability to attract pollinators. This has led to environmental adaptations such as flower color, fragrance, and structure, which illustrate the specialization of a particular taxonomic group or pollinators. - ** Reproduction ** of angiosperms consists of pollen transferred from one flower to another by insects and birds. This is less random and more reliable than the pollination of most gymnosperms, which are dependent on the wind.

Adaptations in Fruits: - The **fruit** is a mature ovary that encases the seeds, protecting them and assisting in their dispersal. - Interactions with the animals that transport the pollen and seeds of angiosperms are the reason for angiosperms’ success as plants. - The fruit of angiosperms vary, depending on the way in which they are dispersed. The fruits dispersed by animals are "fleshy fruits" like berries, while the fruits dispersed by wind tend to be drier and have adaptations for flying. (RG) [9]

1. Angiosperms are generally characterized by the flower that has four distinct characteristics. What are these four characteristics and how do they aid angiosperms? (MP) 2. Describe the flower of an angiosperm. Exlain how cross pollination works. [MS] 3. Why are flowers and fruits advantageous for angiosperms to produce? How have these adaptations allowed angiosperms to become more successful on land? (SD) 4. Describe how angiosperms transport materials. Define Xylem/Phloem. (JS) 5. Explain the process of double fertilization, and state one difference between double fertilization and fertilization in humans. 6. Compare and contrast monocots and dicots. (SI)
 * Review Questions: **

Sources Used: 1. Reece, Jane B., and Neil A. Campbell. // Campbell Biology. // San Francisco: Pearson Benjamin Cummings, 2009. Print. 2. [] 3. [] 4. http://www.ehow.com/facts_5366271_types-angiosperm-flowers.html 5. [] 6.[| http://cgi.unk.edu/hoback/marineinsects/FloweringPlants.htm] 7. [] 8. [] 9. [] 10. @http://www.inhs.uiuc.edu/~kenr/grocery/fleshyfruits.html 11. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Angiosperm.html 12. [] 13.http://www.cartage.org.lb/en/themes/sciences/botanicalsciences/plantreproduction/angiosperms/angiosperms.htm