Angiosperms
(Page by ZXU)
Angiosperms (Greek: angion, container; sperma, seed) are vascular, seed-bearing plants that produce flowers and seeds as their reproductive structures. There are roughly 250,000 known species of angiosperms.



  1. Diagnostic Characteristics of Angiosperms
  2. Habitats
  3. Major Types of Angiosperms
  4. Basic Anatomy of Angiosperms
  5. Transport Materials
  6. Reproduction & Life Cycles of Angiosperms
  7. Environmental Adaptations



Sunflower.jpg
Above is a picture of one of the most familiar angiosperms, a sunflower. (CC) (13)

1. Diagnostic Characteristics of Angiosperms


Angiosperm are distinguished by several key characteristics: they are seed-bearing plants, use flowers and fruits to increase their efficiency of reproduction, and contain refined vascular tissue, making them vascular plants. Angiosperm are further broken down into three main categories (monocots, eudicots, and “other early angiosperm lines”) whose diagnostic characteristics are discussed further under major types.

  • Seed - Bearing:


    When plants evolved to live on land, they evolved to reproduce via seeds, which are much more efficient in a terrestrial ecosystem. Two main branches of seed-bearing plants emerged from this divergence: gymnosperm and angiosperm.
  • Flowers and Fruits:


    Angiosperm differ from gymnosperm in their use of flowers and fruits to protect their seeds and increase their reproductive efficiency. The general structure of flowers and fruits can be found under Basic Anatomy.
  • Vascular Tissue:


    Vascular tissue is comprised of special tube-shaped cells whose function is to transport water (xylem) and nutrients (phloem) to the body of the plant. Most angiosperms have adapted highly refined vascular tissue. The breakdown of the three types of vascular tissue is discussed under the heading Transport Materials
  • Heterosporous:


    All angiosperms are heterosporous, as are gymnosperms – it is a trait that all seed-bearing plant species share. This means that the sporophytes of angiosperms produce two kinds of spores – megaspores and microspores – which will develop into the plant’s female and male reproductive systems respectively.

  • Sexual Reproduction:


    Angiosperms produce two kinds of spores: Microspores, which develop in the microsporangium, will germinate and develop into the male gametophyte generation, and megaspores, which develop in the megasporangium, will develop into gametophyte generation.
    In most angiosperms the flowers are perfect, each has both microsporangium and megasporangium, however some angiosperms are imperfect, having either microsporangium or megasporangium but not both. Different plants with different flowers have different names: Monoecious plants have both types of imperfect flower on the same plant, and Dioecious plants have imperfect flowers on spate plants; that is, some plants are males and some are female. Some examples include willows, poplars, and the date palm.(6)(ZS)

  • Double-Fertilization:


    This phenomenon is characteristic of all angiosperm species, but also evolved independently in some gymnosperms. It is a reproductive phenomenon that will be explained in further detail in the section Reproduction & Life Cycles of Angiosperms.




2. Habitats


Angiosperms are incredibly diverse and versatile plants and can be found in almost any habitat, including aquatic. Because they are flowering and fruit-bearing, however, they are unable to thrive in excessively dry, cold, or dark environments. Recent studies did however show that Angiosperms are best suited on unstable, and darker (shaded) soil, and can even survive on tundra.(GR)(8)

Due to their importance to any ecological community, angiosperms are found in almost any major land habitat. Relatively few angiosperms live in underwater in the oceans, these are often referred to as seagrass, which is roughly 50 species and 12 genera. (MLK)15


3. Major Types of Angiosperms


All angiosperm are members of the phylum Anthophyta (Greek: antho, flower). Formerly, angiosperm were divided into two categories: monocots and dicots. Recent studies of angiosperm DNA, however, have shown that dicots are not monophyletic, as are monocots, and they have thus been divided further into the two groups: eudicots and “other early angiosperm lines”. Monophyletic means that groups of organisms share a common descendant. These three groups differ from one another in several morphological and anatomical ways, described below.

  • Monocots:


    Most monocots have leaves with veins running parallel to one and other. (Examples: lilies, yuccas, orchids, palms, grass, sugar cane, and grain crops.) Monocots all have a single cotyledon, the part of the plant embryo that upon germination will become the first "seed leaves" of the plant. These "seed leaves" will help absorb the nutrients stored in the seed before the plant grows its own true leaves and starts photosynthesis. (KL)(1)
This picture show the structure of Monocot angiosperms. (2. NG)
This picture show the structure of Monocot angiosperms. (2. NG)



  • Eudicots:
There are approximately 319 eudicot families, making eudicots the largest group of angiosperms. In fact, eudicots account for more plants than all other groups of living plants combined. Eudicots are also known as tricolpates. This name comes from the morphological characteristic that defines eudicots: there are three grooved openings in eudicot pollen grains. Eudicots are also unique because many have plastids containing starch grains in the sap-conducting cells (sieve elements). Buttercups, poppies, sycamore trees, tobacco, roses, broccoli and legumes are some of the well-known examples of eudicots. Eudicots include the true dicots, but not all dicots(4) (CSR).
  • Dicots:


    Dicotyledons or dicots are defined as having two cotyledons in their embryos. This, however, is not the only feature that separates them from monocots. Most dicots have three pores in their pollen, while monocots have one. In Dicots, mostly the flower parts come in multiples of four or five. The vascular bundles of dicots are arranged in a ring in the stem. These vascular bundles are stands of vascular tissue. Secondary growth, the growth of wood and bark, is possible in dicots, although some have lost the ability.(ORS)(3) Veins in dicots are usually net-like and webbed. (Examples: roses, peas, buttercups, sunflowers, oak trees, and maple trees.)

This image shows a monocot on the left with three petals, while the flower on the right belongs to a dicot plant; it has five petals. (SD) (11)
This image shows a monocot on the left with three petals, while the flower on the right belongs to a dicot plant; it has five petals. (SD) (11)

  • "Other Early Angiosperm Lines":


    This group is found to have diverged from earlier than either eudicots or monocots. The oldest known angiosperm at this time is a single species of small flowering plant known as Amborella. (Examples: star anise, water lilies, amborella.)

amborella3.gif
This is a picture of an Amborella plant. It is one of the oldest known angiosperm. [MS] 17







4. Basic Anatomy of Angiosperms


  • Flowers:


    Flowers are structures specialized for reproduction and are unique in plants to angiosperm. Flowers are essentially shoots with four circles of modified leaves that are known as sepals, petals, stamens, and carpels.
    • Sepals:


      Leaves that enclose the flower before it opens, sepals are usually green. They are found below the petals once the flower has bloomed. In addition, stamens are sterile floral parts, meaning that they are not directly used in reproduction.
    • Petals:


      Petals are usually brightly colored. They are found above the sepal, and are used to attract pollinators, such as birds and insects. In wind-pollinated plants (plants whose seed are spread by the wind and not by pollinators), petals are usually less brightly colored or non-existent. Petals, too, are sterile floral parts. Within the petal ring are the two fertile sporophylls, which are derived from leaves and used to create spores. The two types of sporophylls are stamens and carpels.
    • Stamens:


      Stamens are the flower’s male reproductive organs. These sporophylls produce male microspores, which then create male gametophytes. The stamen consists of two parts, the filament, which is a stalk that has at its end the second part of the stamen, which is called the anther. This is where pollen is initially produced.
    • Carpels:


      Carpels are the flower’s female reproductive organs. These give rise to megaspores, which in turn produce female gametophytes. At the tip of the carpel is the stigma, which is a sticky area that receives pollen when the flower is pollinated. Below the stigma is the style, which leads to the ovary at the base of the carpel. Ovules are protected within the ovaries, and, once the flower is pollinated, these turn into seeds. 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)

Flower_Anatomy.jpg

(YA)(5) Flowers which are the reproductive structures of angiosperms, have multiple different structures and characteristics:

Type of flower
Picture
A complete flower has all four layers and all four parts: sepals, petals, pistil, and stamen), which are attached to the floral stalk by a receptacle
complete_flower.jpg

An incomplete flower lacks one or more of the four layers( sepals,petals,pistil,and stamen). These knotweed flowers lack petals (corolla).
incomplete_flower.jpg
A perfect flower had both sexes, both stamens, and both pistils, meaning that both the female and male flowers are present on the plant. (for example, Easter lily, pea, dandelion, and rose)
perfect_flower.jpg
An imperfect flower is lacking either the pistil or stamens. A Dioecious plant has imperfect flowers on separate male and female plants (for example, marijuana, , persimmon, and boxelder).
boxelder_flowers.jpg
A regular flower is radially symmetrical. all of the members of a single whorl, such as the petals, are similar in shape and size. Lilies and the apple tree, for example, bear regular flowers.
regular_flower.jpg
An irregular flower has , and is also known as a Zygomorphic. A flower in which one or more members of a whorl, or of several floral whorls, differ in form from other members, is also considered to be an irregular flower.
JohnnyJumpUp.jpg



  • Fruits:


    Fruits are essentially mature ovaries. They develop when a flower is fertilized, causing the walls of the ovaries to thicken. Fruits aid in the dispersal and protection of dormant seeds. Though there are several types of fruits (simple fruits, aggregate fruits, and multiple fruits), all are created from the growing ovary of a fertilized flower.
flower diagramm distinguishing flowers producing aggregate, simple and multiple fruit
flower diagramm distinguishing flowers producing aggregate, simple and multiple fruit
(MC) (10)



slide14.jpg
slide14.jpg
LJ (16)




slide13.jpg
slide13.jpg
LJ (16)


  • Pericarp:


    The thickened wall of the ovary, which then becomes the wall of the fruit. In most plants, as the ovary grows, it causes the flower to gradually wither and die.
  • Simple Fruits:


    These are fruits which are grown from a single ovary. They vary in size, shape taste, and texture. (Examples: cherry, soybean pods.)
  • Aggregate Fruits:


    These are fruits which are the result of a single flower with several carpels. (Example: blackberries.)
  • Multiple Fruits:


    These develop from inflorescences, which are a tightly clustered groups of flowers. When fertilized, the ovarian walls of these clustered flowers begin to thicken, and eventually fuse together to form one larger fruit. (Example: pineapples.)

Fruit_Anatomy.jpg



5. Transport Materials


Angiosperms have evolved highly refined vascular tissues that allow them to thrive by increasing the efficiency of water and nutrient transport.
http://universe-review.ca/I10-22a-root2.jpg
http://universe-review.ca/I10-22a-root2.jpg
(9)(MF)

  • Xylem:


    Angiosperm xylem is especially refined in its ability to transport water and dissolved nutrients to the body of the plant. Over time they have evolved three types of specialized xylem tissue, each with its own specific function.
    • Tracheids: Long, tapered cells known as tracheids aid in both water transport and the actual structural support of the plant. Tracheid cells are dead at functional maturity. The walls of both tracheids and vessel elements are scoured with pits, which are thinner regions where only the primary wall is present, meaning that there is no lignin, which makes it possible for water and dissolved nutrients to flow from cell to cell.
    • Vessel Elements: Vessel elements are found in most, but not all, angiosperm species. Vessel elements are both shorter and wider than tracheids, and are arranged in continuous tubes that connect to one and other end-to-end to create xylem vessels, which are more efficient than tracheids alone. Similar in structure to tracheids, however, vessel elements are also dead cells when at and age of functional maturity.
    • Fiber Cells: These cells are specialized for providing structural support to the plant.
  • Phloem:


    Phloem is the second major category of vascular tissue, and specializes in the transport of food (sucrose and other organic compounds) that has been made in the photosynthetic leaves of a plant to its roots and other non-photosynthetic parts. Phloem cells are alive at functional maturity, unlike xylem cells, and are made up of sieve-tube members.
    • Sieve-Tube Members:


      While sieve-tube members are alive at functional maturity, they lack such basic organelles as a central vacuole, ribosomes, and a nucleus. They link together for form a chain of cells with perforated walls, allowing nutrients to pass from cell to cell.
    • Sieve Plates:


      external image art0002.jpg (MT)

In angiosperms, the end walls between sieve-tube members are called sieve-plates, which have pores to increase the efficiency of fluid transfer between cells.
    • Companion Cells:


      Companion cells are connected to sieve-tube members by numerous channels called plasmodesmata, and run alongside sieve-tube member chains, though they are nonconducting. The nuclei and ribosomes of these cells serve the sieve-tube members they are connected to, as these cells have no nuclei or ribosomes of their own. Companion cells in "source" tissues (ex. leaves) take in sugars and amino acids from the cells that make them by active transport. Water also comes in by osmosis, and the pressure created by it moves materials through the sieve tubes. (SM) (7)
  • Ground Tissue:


    Ground tissue is neither dermal nor vascular, yet is present in dicots and is a part of the nutrient transfer process. It is divided in the roots of dicots into two categories – the pith, which is internal to the vascular tissue, and the cortex, which is external to it. Among the functions of ground tissue are photosynthesis, storage, and support.



6. Reproduction & Life Cycles in Angiosperms


angiosperm_life_cycle.jpg


Angiosperms are heterosporous, meaning they produce both male and female gametophytes. The components and processes of the angiosperm life cycle are described below.

Various Angiosperms have different timing mechanisms for sexual reproduction. The sex organs may mature at different times. When the anthers mature first, this is called protandry. When the stigma is receptive and mature before the anthers, this is called protogyny.
(LW) (14)
  • Pollen Grains:


    Pollen grains contain immature male gametophytes. These develop within the anthers of the stamens. Each pollen grain contains two haploid cells.
  • Ovules:


    Ovules develop into the ovaries of the plant, and contain the female gametophyte, which is also known as the embryo sac. This structure consists of only a few cells, one of which is the egg. Pollen is then released from the anther and carried to the sticky tip of the carpel, or the stigma.
  • Cross-Pollination:


    While most angiosperm species cross-pollinate, it remains the trend that some self-pollinate, meaning that they reproduce with themselves and thus do not require the aid of a pollinator species. Cross-pollination, however, is the transfer of pollen from one flower to another individual of the same species. The pollen grain then germinates, or begins growing, after adhering to the stigma of a carpel. At this time the male gametophyte is fully mature, and the pollen grain extends a tube which grows down into the style of the carpel until it reaches the ovary, at which point the tube punctures the micropyle, which is a small pore into the integument of the ovule.
  • Double-Fertilization:


    Once the micropyle has been punctured, the pollen grain’s tube releases two sperm cells into the female embryo sac. One of these sperm cells will unite with the egg to create a diploid zygote, whereas the other will fuse with the two nuclei of the large center cell of the female gametophyte to create a central cell with a triploid nucleus.
    One hypothesis that explains the evolution of a process as strange as double-fertilization reasons that double-fertilization synchronizes the development of the embryo with the development of the food stores in the endosperm/cotyledons, which would keep plants from wasting nutrients on infertile ovules.
  • Cotyledon:


    After double-fertilization, the ovule develops into a seed, while the zygote develops a sporophyte embryo with a root and one or two seed leaves, known as cotyledons. Monocots have one cotyledon, whereas dicots have two.
  • Endosperm:


    The triploid nucleus of the center cell undergoes its own transformations, developing into a triploid tissue called the endosperm. This tissue is rich in food reserves, such as starch, which are vital to the new plant’s early growth. Though most monocot seeds store their food reserves in the endosperm, most dicots transfer these stores to the cotyledons.
  • Seed:


    The seed ultimately consists of the endosperm, the embryo, the sporangium, and a seed coat derived from the integument, or the outer layers of the ovule.
  • Fruit:


    As their ovules develop into seeds, the ovary grows into a fruit which is distributed through one of various methods. If conditions are favorable, the seed contained within the fruit will germinate. In germinating seeds the seed coat ruptures, allowing the embryo to emerge as a seedling and use its food stores to begin its growth.




7. Environmental Adaptations


The oldest plant fossils that are still considered part of the genus Angiosperm date from approximately 130 million years ago during the Mesozoic era of the Cretaceous period. At this stratum, however, they represent a scarce minority among the much more common ferns and gymnosperms (non-flowering, seed-bearing plants). About 65 million years ago, angiosperms became the dominant plant species on Earth, probably following some major environmental disturbance. Because of the significance of the change in the fossil record around this time, this sudden increase in angiosperm abundance is used as the boundary between the Mesozoic and the Cenozoic eras.

Coevolution is believed to have played a large part in the evolution of angiosperm species. This means that there is evidence leading scientists to believe that land animals and angiosperm species evolved in tandem, with the changes in one prompting adaptations in the other.

  • Possible Examples of Coevolution:


    General examples of coevolution can be seen in the size and shape of angiosperms in varying locations and their relation with the area’s fauna. Early on in angiosperm evolution, when terrestrial animals were also just appearing, it might have been beneficial for a tree to evolve, as its seeds are higher above the ground, and thus more difficult for land creatures to reach. This, in turn, may have prompted the proliferation of flying insects that were able to reach these elusive fruits and flowers. As time passed, however, symbiosis became a key factor in coevolution: plants were fertilized, and pollinators were fed. Not all angiosperm species, however, are fertilized by pollinators. Many types of fruits and flowers have adapted to help angiosperms reproduce, as can be seen in the following subsections.
  • Adaptations in Flowers:


    • Carpels:


      Angiosperms are separated from gymnosperms by a variety of morohplogical differences, chief among them the fact that angiosperm enclose their seeds in ovaries. It stands to reason that the carpel, or female reproductive organs of a flower, evolved from seed-bearing leaves (sorophyll) that over time rolled into tubes. Some angiosperm flowers have a single carpel, others have multiple carpels that are either separated or fused. In the latter case, ovaries with multiple ovule-containing chambers are usually formed.
    • Pollination:
      Many flowers are pollinated by insects, or birds, making their pollination patterns less random than that of wind-blown angiosperms or gymnosperms, which reproduce without flowers or fruits. Because flowering plants must attract pollinators, many of them have brightly colored or even specifically shaped petals to attract the most effective creatures. Wind-pollinated flowering plants, however, do not have these same constraints and so are often much less brightly colored than other angiosperm species, and sometimes lack petals all together.
  • Adaptations in Fruits:


    Several types of seeds have evolved in angiosperm species, each with its own adaptations that make it an effective method of reproduction in its specific environment.
    • Wind Dispersal:


      Some plants have seeds within fruits that are designed specifically to travel long distances when carried by the wind. These fruits are shaped to act as propellers or kites, as in dandelions or maple trees.
    • Carried Seeds:


      Some plants protect their seeds within fruits shaped like burrs that are meant to cling to the fur of animals, or, as plants and humans often interact, cling to the clothing of human passerby. This is an effective method allowing seeds to travel long distances, and can be seen in cockleburs, as well as many other species of angiosperms.
    • Eaten Seeds:


      Some seeds are contained within fruits that are appetizing to animals, and are thus meant to be eaten so they may then be released again some distance away from the parent plant in the animal’s feces. Berries, for example, are often eaten by small rodents in the wild.
  • Vascular Tissue:


    Vascular plants as a whole delineated into two types of plants: seedless and seed-bearing. The seed-bearing plants are further divided into two groups, gymnosperms and angiosperms. Vascular tissue, as explained in the section Transport Materials, is an evolutionary adaptation that helped angiosperms spread as widely as they have by increasing the efficiency of water and nutrient transfer, as well as acting to support the plant physically.




Review Questions

How is the structure of the flower related to its function? (NI)
Explain why some angiosperms cross-pollinate while others self-pollinate. What may have been the reasoning for the adaptation? How might each type of pollination effect the offspring?(CW)
Compare and contrast the characteristics of Angiosperms to Gymnosperms. (RL)



References


Main source: Campbell, N.C., Reece, J.R. (2002). Biology. (Sixth Edition). San Francisco: Benjamin Cummings.
http://www.ucmp.berkeley.edu/glossary/gloss8/monocotdicot.html (1)(KL)
http://www.biologycorner.com/bio2/notes_plants.html (2. NG)
http://www.ucmp.berkeley.edu/glossary/gloss8/monocotdicot.html (3)(ORS)
http://www.biologyreference.com/Ep-Fl/Eudicots.html (4, CSR)
http://biology.clc.uc.edu/Courses/bio106/angio.htm (5)(YA)
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Angiosperm.html (6)(ZS)
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/PlantTissues.html (7) (SM)
http://mygeologypage.ucdavis.edu/cowen/historyoflife/Feild.html(8)(GR)
http://universe-review.ca/I10-22a-root2.jpg (9)(MF)
http://www.backyardnature.net/frt_aggr.htm (10)(MC)
http://oak.cats.ohiou.edu/~braselto/readings/images/mon-dicot_flowers.jpg (11) (SD)
http://www.amborella.org/ (12) [MS]
http://wallpaper-s.org/45__Sunflower.htm (CC) (13)
http://www.cavehill.uwi.edu/FPAS/bcs/cape/capefl.html(LW) (14)
http://waynesword.palomar.edu/trmar98.htm (MLK) (15)
http://www.botany.hawaii.edu/nlc_biology/1411/lab/flfr/slide14.jpg (16)
http://www.amborella.org (17) [MS]