ANGIOSPERMS : The Flowering Plants

Hanna Zhu
external image Generalised_Flower_Diagram.jpg
http://andromeda.cavehill.uwi.edu/Plant%20Propagation%20Practical%20Photos/Generalised_Flower_Diagram.jpg




You know what an angiosperm is … it signals the start of spring, it sits in our vases decorating our houses, and it makes a nice gift. Angiosperms, the flowering plants, are the widest-spread plants. They can be found just about anywhere plants can grow. They are also the most diverse group of plants, with 250,000 known species. Some examples of angiosperms are maple trees, tulips, and oaks. What distinguishes an angiosperm from other plants, in particular the gymnosperms, is that angiosperms have flowers as the reproductive structure. The seeds of angiosperms are encased in a fruit, while the seeds of gymnosperms are not (1). Angiosperms, specifically, are vascular seed plants, meaning they come from seeds and have xylem cells (which transport water to leaves) and phloem cells (which bring food back from leaves to other parts of the plant) (9 BL).

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An edelweiss flower, which is capable of surviving at certain climatic extremes, such as altitudes ranging from 1700 meters to 2700 meters. (8)(SS)


Types of Angiosperms

Angiosperms belong in the phylum Anthophyta (phylum is the second largest grouping of organisms, below “kingdom”). Up until the 1990’s, angiosperms were divided into two groups: the monocots and dicots. One difference between monocots and dicots are that in monocots, the veins on a leaf run parallel to each other, such as the veins of a daffodil leaf, while in a dicot, the veins spread out in a net-like pattern, such as the veins in the leaves of a maple tree. Another difference is that in monocots, the number of petals in the flowers occur in multiples of three, while in a dicot, the number of petals in a flower occurs in multiples of four or five (Campbell and Reece 721). Sometimes, the number of petals on a dicot can also occur in multipls of two. Another difference is that in monocots, the stem vascular system, the tubes that transport nutrients and water, has bundles scattered throughout the stem while in dicots, the stem vascular system has bundles arranged in rings (2) (DP). Additionally, monocots and dicots contrast in a couple other ways. Monocots originate from an embryo with single cotyledon while dicots originate from an embryo with two cotyledon. According to research, the number of cotyledons (seed leaves) within the embryo is essential in distinguishing between the two classes. Another difference between monocots and dicots is their pollen structure. Monocots have pollen with a single furrow (pore) while dicots have pollen with three furrows (pores). (Jesse Carmen) {7}
monocots_vs_dicots.jpg
(This image represents the differences between monocots and dicots. YS)
By comparing the DNA of different dicot species, scientists have discovered that not all dicots are part of the same monophyletic group, which is an evolutionary grouping that includes an ancestral species and all of the species descended from it. In other words, not all of the species of dicots were descended from the same common ancestor. This led scientists to further divide the dicot group into more categories. One group is the endicots, which includes most of the dicot species. Examples of eudicots are roses, maples, and buttercups. Other dicot groups evolved before either the eudicots or the monocots. One group is the water lilies. Another, which only has one species, the Amborella, is the earliest species of angiosperms to evolve. To summarize the order in which different species of angiosperms evolved, Amborella evolved first, then water lilies, then a group consisting of dicots that evolved before both the monocots and the eudicots, and finally the monocots, followed by the eudicots (Campbell and Reece 606).


Basic Anatomy

To colonize land, plants had to obtain nutrients from the soil as well as carbon dioxide for photosynthesis and oxygen for respiration (the process by which the body turns the food eaten into usable energy). To perform these two functions, angiosperms (as well as gymnosperms) have developed roots and stems (Campbell and Reece 722).


The roots absorb water and minerals from the soil. Two types of roots are fibrous roots and taproots. Monocots usually have fibrous roots, which is a network of thin roots. This allows the roots to have more surface area so that more water and minerals can be drawn into the plant. Dicots have taproots, which is made up of one large central root with smaller, thinner roots branching off of the main root. A carrot is an example of a taproot. Taproots also store food for the plant (Campbell and Reece 722).

Stems hold the leaves of a plant up toward the sunlight for photosynthesis. Angiosperms therefore do not need water to support themselves to an upright position. A stem consists of nodes, the points at which the leaves are attached to the stem, and internodes, the section of the stem between the nodes. In the angle between the leaf and the stem is the auxiliary bud, which is capable of growing into another branching of stems. At the top tip of the plant is the terminal bud. The terminal bud is one of the factors that inhibit the growth of the auxiliary buds. By giving most of the resources for growth to just the tip of the plant, that plant can grow taller and be more successful at reaching sunlight. This is another evolutionary adaptation that allows angiosperms to live on land successfully. If an animal eats the terminal bud, or there is more sunlight at another angle, then the auxiliary buds will start growing. This way, the plant will always be able to reach sunlight (Campbell and Reece 723).
The leaves are where photosynthesis occurs. They are coated by a waxy layer called the cuticle. This prevents the angiosperm from excessive water loss, another adaptation that allows angiosperms to live on land. The leaves also have stomata, openings through which oxygen and carbon dioxide enter or leave the leaf. The plant also loses water through the stomata, called transpiration. Each stoma is made of guard cells that open and close the stoma. When there a lot of water in the guard cells, the guard cells bend outward, opening the stoma and allowing the water to excess water to transpire. After losing water, the cells become loose and close up the stoma, preventing any more water to be lost. This keeps the plant from drying out, another adaptation to living on land (Campbell and Reece 760). Leaves can simple or compound, meaning they can be made up of several leaf units (compound), or just one blade (simple) (2, DJ). In addition, leaves can line up opposite each other on a stem, or alternating (2, DJ). Leaves can also be whorled, meaning that more than one leaf originates from the same place on a stem (2, DJ).


Reproduction

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Angiosperms reproduction cycles involve the alternation of generations (explained below).
(3 SES)


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Reproductive structures of an Angiosperm (6,NK)


Like other plants, the reproductive cycle of angiosperms also includes the alternation of generations. In the alternation of generations, the sporophyte produces spores by meiosis. The sporophyte is a diploid plant, meaning that it has two sets of chromosomes, with one coming from the male parent and the other from the female parent. The spores are produced through meiosis. In this process, the cells divide without duplicating each chromosome first. As a result, each resulting cell has only one set of chromosomes, making it haploid. The spores grow to be haploid plants, called gametophytes, which produce gametes, either the egg (the gamete produced by female organisms) or the sperm (the gamete produced by male organisms). The egg and the sperm then fuse to produce, once again, a sporophyte. In this way, the organism alternates between being diploid and being haploid (Campbell 783-784).


In angiosperms, the sporophyte is the dominant generation – all flowering plants are diploid. Angiosperms produce the smallest gametophytes compared with other plants as plants evolved to become more and more independent of a water environment. Angiosperms can be found in such a wide geographic range because of the reproductive structure that they evolved – the flower. As shown in the diagram above, the four parts to a flower are the sepals, the petals, the stamens, and the carpels. The sepals are leaf-like structures around the bud that protect the bud. The petals’ bright colors attract pollinators, such as insects or birds, to pollinate the flowers. The carpel is the female reproductive structure. It is made up of the stigma, which is the sticky top that catches pollen, the ovary, which contains eggs that will be fertilized, and the style, the stalk connecting the stigma to the ovary. The stamen is the male reproductive part of the flower (you can remember that it is the male part because there is the word “men” in stamen). It is made up of the filament, a thin stalk, to which is attached the anther, where the pollen is produced (Campbell and Reece 784-785)


The sporangia (singular: sporangium. This is also called a pollen sac in an angiosperm) on the anther, where haploid spores develop, has many diploid cells called microsporocytes. Each microsporocyte undergoes meiosis to form four haploid microspores. Each microspore then divides by mitosis, the process by which cells duplicate themselves, to form a generative cell and a tube cell. The generative cell will divide into two sperms, and the tube cell will grow a pollen tube when the pollen lands on the stigma. These two cells are encased in a hard wall, and together, the cells and the wall make up the pollen grain. This is the male gametophyte (Campbell and Reece 786).


The ovary of the carpel contains many ovules, which each have a sporangium in which the egg cell develops. A cell in the sporangium, the megasporocyte, divides by meiosis to produce four megaspores. Only one of the megaspores survives, and its nucleus divides into eight haploid nuclei. The megaspore then develops into an embryo sac, the female gametophyte. One of the nuclei develops into the egg cell, and two other nuclei develop into two polar nuclei, which are part of the same cell (Campbell and Reece 786-787).

When pollen lands on the stigma, the tube cell grows a pollen tube into the style to one of the ovules in the ovary. The generative cell divides by mitosis into two sperm cells that travel down the pollen tube to the ovule. One sperm fertilizes the egg cell, which will grow into a sporophyte. The other sperm fertilizes the polar nuclei, creating the endosperm, which is triploid (two sets of chromosomes from the polar nuclei plus the set from the sperm). The endosperm stores food for the seed. This process is called double fertilization. Afterwards, the ovule develops into the seed, and the ovary develops into the fruit (Campbell and Reece 789-790).

The method of using pollination as a way to bring male and female gametes together played a large part in liberating plants from dependence on water for fertilization. Insects and birds, as well as the wind, deliver the sperm straight to the female reproductive part of the flower so that the sperm does not have to swim through water to get to the egg, which is the case for mosses and ferns. Evolution of pollination was a large step in allowing land plants to evolve. In addition, pollination allows sperm to travel farther distances, and this would allow for more genetic diversity since fertilization is not just restricted to nearby plants, making angiosperms even more reproductively successful (Campbell and Reece 600).

The evolution of the flower and fruit has made angiosperms adapt even more successfully to a variety of environments than gymnosperms. The seed can now obtain nutrients from the endosperm. The fruit also protects the seeds. It may also attract animals, which will carry the seed to new locations after eating the fruit. This gives the seed room to grow in a new area away from its parents so that it does not compete with them for resources, increasing the chances of survival for the seed (Campbell and Reece 792).



The seed coat, the outer layer of the seed, protects the seed. When the seed matures, it enters into a stage of dormancy, during which the embryo, the developing plant inside the seed, stops growing. When conditions are favorable, the seed will germinate. Different factors cause seeds of different species to germinate. For example, in a place with extremely cold winters, the seeds may need to be exposed to the cold for a while before germinating. The seeds would then germinate in spring after winter, which would ensure them a lengthy period of growth in the coming warm weather. If a plant inhabits a dry environment, its seeds may germinate only after an ample amount of rainfall so that the soil would enough moisture for the seed to grow in. Adaptations such as these further aid angiosperms in colonizing a wide geography (Campbell and Reece 793).



Transport
Transport system of Angiosperms (HS 4)
Transport system of Angiosperms (HS 4)

Angiosperms are vascular plants, meaning they have vascular tissue, which transport water and nutrients throughout a plant. The two types of vascular tissues are xylem and phloem. Xylem tissue, which is dead, transports water and minerals from the roots to the stems and leaves. Phloem tissue transport food, such as glucose made during photosynthesis in the leaves, to other parts of the plant. The refinements and improvements in angiosperm's vascular tissue led to their success in inhabiting terrestrial habitats. Even in the category of angiosperms, improvements were made to the xylem tubes. The special speces Amborella lacked vessel elements, so vessel elements evolved after the Amborella separated from the majority of angiosperms. (SV)

The small diameter of these microtubules enables cohesion, which in turn facilitates the transport of water. (ER) (10)
The small diameter of these microtubules enables cohesion, which in turn facilitates the transport of water. (ER) (10)

The xylem tubes can be made of tracheids or vessel elements. Tracheids are long and thin, with sharp, tapered ends. Water or minerals flow from one tracheid to another through pits, which are thin parts of the cell walls of the tracheids. Vessel elements are wider and shorter. They are placed end to end, and the walls that they are joined by have holes through which water can flow.

In phloem, sucrose and other organic compounds are transported through chains of cells called sieve-tube members, which do not have organelles, such as the nucleus. The sieve-tube members are joined by sieve plates, which have holes to allow materials to flow more easily through the phloem. Besides the sieve-tube members are companion cells, which may help in moving the sugar produced in the leaf to the phloem (Campbell and Reece 725-726).

Cohesion and adhesion of water molecules play a large role in transporting water throughout a plant. Water is made of two hydrogen molecules joined to an oxygen molecule. Because the oxygen molecule attracts more electrons toward itself than do the hydrogen molecules, the oxygen molecule is slightly negatively charged, and the hydrogen side of the water molecule is slightly positively charged. This makes water polar means each side of the molocule has an oppiste charge(like a magnet)(1SJB) The slightly positive hydrogen side of a water molecule attracts the slightly negative part of a neighboring oxygen. This attraction is called hydrogen bonding. Cohesion is when water molecules are attracted to each other because of hydrogen bonding, and adhesion is when water molecules are attracted to other slightly charged molecules, such as those along the walls of xylem tissue. When water leaves stomata, the water molecules pull on the water molecules below them. This helps pull the water up through the xylem. In addition, because the water “sticks” to the sides of the xylem tissue, it helps prevent the water from sliding back down (Campbell and Reece 757-758).



Review Questions
1. What is the purpose of stomata? [OZ]
2. What distinguishes angiosperms from other types of plants? What is different about their reproductive structures? (RK)
3. What are xylem and phloem tissue? What purpose do they serve for the plant? (JAC)
4. What are the differences in transport methods between the xylem and phloem? (AR)
5. Explain the basic anatomy of an angriosperm, and go into detail about what gives angiosperms the ability to thrive on land. (GR)
6. What is the alternation of generations and how do the structures of Angiosperms change as generations alternate?(RJS)
7. Explain the difference between monocots and dicots. (Nangia)

Edited by:
Sarah Vlach
Sarah Schwarzschild
Daisy Joo
Hilary Stepansky
Rachel Kornetsky

Josh Czik
NK

Jesse Carmen
SS
Brittany Marcus-Blank

Becca Levenson
Sam Blatchford

Ethan Richman
Meru Nangia

References:

Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston: Benjamin-Cummings Company, 2002.
2.) Carter, J.S. "Angiosperms." 30 November 2008. <http://biology.clc.uc.edu/courses/bio106/angio.htm>.
3.) 7 Dec. 2008. <http://kvhs.nbed.nb.ca/gallant/biology/angiosperm_reproduction.html>
4) "Introduction." Angiosperm Anatomy. 9 Dec. 2008 <http://www.botany.uwc.ac.za/sci_ed/grade10/anatomy/>.
5) http://hawaii.hawaii.edu/laurab/generalbotany/images/monocots%20vs%20dicots.jpg
6)http://www.vidyavahini.ernet.in/shishya/products/AcademicContent/CBSE/XI/Botany/Kingdom%20Plantae/images/image013.gif
7) "Monocots versus Dicots: The two Classes of Flowers." 16 December 2008. <http://www.ucmp.berkeley.edu/glossary/gloss8/monocotdicot.html>.
8) Mahendra, Raj. "Edelweiss - The Plant and The Song." Rajmahendra.com. 17 Dec. 2008 <http://www.rajmahendra.com/2006/10/25/edelweiss-the-pland-and-the-song/>.
9) "vascular system." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 17 Dec. 2008 <http://www.britannica.com/EBchecked/topic/623731/vascular-system>.
10) http://www.woodmagic.vt.edu/Images/activities/fibers.jpg