Supplemental Lecture (97/05/11 update) by Stephen T. Abedon (firstname.lastname@example.org)
- Chapter title: Reproduction by Seed
- A list of vocabulary words is found toward the end of this document
- In the lecture titled "Evolution of Plants" the evolution of plants, from the nonvasculars through the angiosperms, was outlined. Though infused with information on reproductive strategies, many details of reproduction were avoided. An understanding of the details of reproductive strategies is important in understanding plant evolution, however. This is because plant reproductive strategies represent sophisticated adaptations to terrestrial life, exemplifying interesting evolutionary trends. They are employed in the classification of otherwise similar plant species.
- The seed-bearing gymnosperms clearly display excellent adaptations to growth under relatively dry conditions. These drier conditions are those which exist away from the swamps and high humidity landscapes more or less required for reproduction by the non-seed-bearing plants. The seeds of gymnosperms are quite capable of efficient propagation in these drier environments. Flowering plants (the angiosperms) are more dominant today than the gymnosperms, however. This is probably, at least in part, because the flowers and fruits of the angiosperms are adaptations which allow increased efficiency of dispersal relative to the gymnosperms.
- In this lecture we take up reproductive strategies in more detail, concentrating on those associated with the conifers and angiosperms (two seed-bearing plant divisions). Angiosperm reproduction, particularly, represents the pinnacle of plant reproductive adaptation to terrestrial living. Angiosperms, additionally, supply most of the food we eat. Therefore, angiosperm reproduction deserves especially careful scrutiny and will be considered it in some detail here. That is, in this lecture we will consider such familiar themes as flower and fruit, discussing what these terms mean, and how they function.
- Life cycle of the pine
- Typical seed-bearing plant:
- The pines are not angiosperms (they are gymnosperms/conifers) but nonetheless typify seed-bearing reproduction though in the absence of flowers and fruit (pines instead employ cones, wind-born pollen, and winged seeds).
- Many of the features of pine reproduction nevertheless are carried over by the angiosperms. Pine reproduction occurs as follows:
- The pine life cycle is typical of that seen among gymnosperms though it is unusual in that it spans two growing seasons.
- Pines produce both male and female cones, both at the beginning of the growing season.
- Pollen is produced by and released from male cones.
- This pollen is carried on the wind to female cones.
- Fertilization proceeds as follows:
- the pollen grains adhere to a sticky substance secreted by the female cone
- the pollen grows into the female cone forming what is known as a pollen tube which reaches from outside of the cone, through the cone (sporophytic) tissue, and finally reaching the female egg found within the female cone
- the formation of the pollen tube takes 15 months thus separating, in pines, the point of acquisition of pollen and the point of fertilization by over a year
- The embryo:
- Upon fertilization the embryo begins to form, encased within the sporophytic tissue, which together are matured into a seed.
- By now the female cone has matured, from the greenish immature cones observed the year before, into hardened, brown cones which bear seeds.
- The winged seeds are released and, with luck, sprout and then grow into a mature tree.
- Angiosperm reproduction differs from that of the gymnosperms in that it typically involves the use of flowers instead of cones.
- In addition, seeds are encased in maternal tissue referred to, in part, as fruit.
- The angiosperms, or flowering plants, may be subdivided into two major types:
- the monocots
- the dicots
Other characteristics of monocots which distinguish them particularly from dicots include:
- Single first leaf:
- The monocots are grass and "grass-like" angiosperms (flowering plants).
- Particularly, the embryos of monocots have only a single (mono-) first leaf (a.k.a., seed leaves or cotyledon).
- Derived trait:
- This single first leaf is a derived trait.
- The monocots are actually derived from primitive dicots with which they share various characters.
- Monocot leaves have parallel veins
- Few monocots display woody growth
- Few monocots are annuals
- monocots tend to have swollen underground storage organs
- Two first leaves:
- The dicots include all of the non-monocot angiosperms.
- The embryos of dicots have paired (di-) first leaves (a.k.a., seed leaves or cotyledon).
- Other distinguishing characteristics:
- Other characteristics of dicots which distinguish them particularly from monocots include:
- Dicot leaves have net-like leaf vein structures called reticulate veins
- About one-half of all dicots display woody growth
- About one-sixth of all dicots are annuals
- With only a little knowledge and experience it is possible to fairly easily distinguish monocots from dicots. It is a skill which almost gives the impression that one has a reasonably expansive knowledge of plant biology. In fact, possessing only a little additional knowledge (as outlined above and below), and some experience, it is actually possible to fairly consistently distinguish fungi, lichens, bryophytes, ferns, conifers, monocots, and dicots!
- Sex organs:
- Flowers are angiosperm sex organs.
- Flowers give rise to and contain the not-mobile, female gametophyte generation.
- Flowers also give rise to pollen which is the mobile male gametophyte generation.
- Efficient pollination:
- Flowers play important roles in increasing the efficiency with which pollen is dispersed from plant to plant, thus assuring outcrossing.
- Particularly, flowers are well adapted, depending on the plant, to fertilization by wind blown pollen or via the actions of animals.
- Gymnosperms, which lack flowers, rely upon the wind to effect their pollen dispersal. On a per pollen grain basis, at least, this may be a less efficient mechanism of dispersal. However, it nevertheless clearly is sufficiently effective within the environments in which the gymnosperms dominate.
This efficiency is often achieved via the attraction of insects and other small animals to the flower where they receive a nectar reward.
In turn, these small animals serve as flying penises in the transfer of pollen from one flower to the next (this pollen reversibly adheres to the animal's bodies allowing it to be picked up in one flower and deposited in the next).
It should be pointed out that not all animal pollen-disseminators fly, nor necessarily are small.
Not all flowers are optimized for the occurrence of outcrossing (i.e., some flowers appear, instead, to be optimized for transfer of pollen only within individual flowers and consequently strongly encourage inbreeding).
Sepal (see' pol)
- Layered parts of flowers:
- The term whorl is used to describe the layered parts of flowers.
- From the outside going in, these layers (i.e., whorls) are:
- the sepal
- the petal
- the stamen
- the carpel
- All of the four whorls are basically modified leaves. That is, they arise on the plant from tissue in the same manner (i.e., from the same cell types) that leaves arise.
- Typical number and multiples:
- While flowers consist of four whorls, each whorl consists of characteristic similar or identical components.
- The number of these components is typical (and different) for dicots and monocots:
- components of whorls in dicots are typically four or five in number
- components of whorls in monocots are typically in multiples of three
- Bud protection:
- The sepal represents the outermost ring of petal-like or more leaf-like whorl on a flower.
- Sepals protect the bud.
- When colorful, sepals serve additionally the same function as petals.
Stamen [anther, filament]
- Showy visual aids:
- The petal is the typically "showy" leaf-like, portion of a flower.
- Showy petals serve as visual aids employed in the attraction of flying penises.
- Male portion:
- The next whorl heading inward are the stamens.
- These are the male portion of the flower, i.e., those in which pollen is produced and stored.
- The stamen consist of sacs (anthers containing two pollen sacs) posted on stalks (filaments).
- The stamen are typically arrayed in a manner such that they project toward or into whatever it is which typically acts to disseminate pollen (such as the wind or flying penises).
Pollen first adheres to a sticky coating on the stigma as the process leading toward fertilization is commenced.
- Female portion:
- The carpel is the innermost whorl of flowers.
- The carpel makes up the structures which hold the plant female gametes, the eggs.
- Three components:
- The carpel itself consists of three structures:
- Additional or associated components include:
The style is a stalk-like structure which separates the stigma from the ovary.
Just as in the conifers, fertilization does not occur until the pollen tube grows down the style to the ovary.
The ovary contains the egg which is fertilized by the sperm deposited by the pollen tube.
The ovule is the part of the ovary which develops into the seed.
The base of the flower, below the ovary, consists of the flower receptacle.
This is the portion of the flower which forms into much of the fruit in such things as apples and pears.
Imperfect flowers [monoecious, diecious]
- Single gender:
- An imperfect flower is one which lacks either the stamen or the carpel (though not both).
- In other words, imperfect flowers contain the whorls associated with only one of the two possible genders, not both.
- Plants with imperfect can be divided into two types:
- Monoecious plants are those which display imperfect flowers but display both the male and female flowers on individual (the same) plants.
- Think one (mono) plant with two genders.
- Dioecious plants which display imperfect flowers but in which the male and female flowers do not appear on individual plants.
- That is, the flowers on such plants contain either no stamen or no carpel.
- Think two (di) plants in order to have two genders.
Pollination by wind:
- Pollen transfer:
- Pollination is the act of transfer of pollen from the stamen of flowers to the carpel of flowers.
- Pollination between two flowers requires that the pollen be transferred in some manner.
- Self pollination:
- Pollination may occur within the same flower, though not with high likelihood in all plants.
- Some flowers, such as those of peas, are optimized for self pollination (i.e., the pollen stays inside the flower).
Wind is one method employed by many angiosperms for pollen dispersion.
In such plants, flowers tend to be optimized for wind aided dispersal (efficient dispersal of pollen to the wind and recovery of pollen from the wind).
These flowers tend not to be showy: To the wind the beauty of a flower may be described solely in terms of fluid dynamics.
Pollination by flying penises:
Flowers which are pollinated during the day by flying penises tend to be very visually impressive.
These serve to guide flying penises by sight.
Note that not all of the visual allure is visible to humans. This is because bee sight tends to be optimized in the ultraviolet, a spectrum in which humans are blind.
Night pollinated flowers are often white, white being the most visible of shades under low light conditions.
Fragrance also plays important roles in guiding flying penises toward flowers, particularly in plants which are optimized toward nighttime pollination.
Many flowers display distinct adaptations which play specific, additional roles in attracting and utilizing specific pollinators.
Pollen [pollen grain]
- Male gametophyte:
- Pollen are essentially male gametophytes capable of movement through the air (depending on the plant, the pollen gametophyte may mitotically produce sperm either prior to or following this dispersal).
- Transportation through the air typically occurs either on the wind or via dissemination on the bodies of flying (and walking) penises.
- Upon reaching a flower stigma, a pollen must deliver its sperm cargo to the female egg.
- This delivery is the job of the pollen tube.
- A grain of pollen consists of four components:
- a haploid cell which gives rise to the pollen tube
- Two haploid sperm cells
- A tough outer covering around these three cells
- Descendants of single meiotic division:
- All three pollen cells are haploid descendants of a single product of a meiotic division followed by a total of two mitotic divisions.
- The first post-meiotic mitotic division gives rise to the pollen tube cells and the sperm cell progenitor.
- The second mitotic division of the sperm cell precursor gives rise to the two sperm cells
Without undergoing further divisions, the two sperm cells find their way down the pollen tube.
They make their way intracellularly, following the pollen tube cell nucleus.
One of these sperm cells fertilizes the awaiting egg.
The second sperm cell contributes to the development of the embryo by fertilizing the already quasi-diploid supporting cells (endosperm) which do not contribute cells to the embryo.
Note that since both sperm are genetically identical (as are both to the cells of the pollen tube), one would expect no reason that sperm cells coming from a single pollen grain would compete with each other. Instead, the cooperation seen between the three lineages (pollen tube and two sperm) likely occurs as a kind of kin selection: It is far better evolutionarily to give one's life to assure the survival and reproductive success of an identical twin (in this case an identical sperm) than to allow both of you to die (or, at least, to have an overall lower fitness).
Egg [egg sac]
- Endosperm is the triploid tissue found within seeds which supplies the developing plant embryo with nutrients.
- Female gamete:
- The female gamete, the egg, forms entirely within maternal tissue found in the carpel.
- The female gametophyte forms via meiosis within a mound of cells which constitute a plant's ovules and which eventually form into seeds.
- The egg itself is not formed directly by meiosis, of course, but instead is formed mitotically by the gametophyte:
- three rounds of mitosis occur following meiosis and prior to fertilization
- these mitotic events produce eight nuclei (1 x 2 x 2 x 2 = 8) which all remain within a single cell's cytoplasm
- Cytoplasmic division results in the production of seven cells which constitute the female gametophyte (together they are known as an egg sac)
- Ploidy of egg sack cells:
- Six of the cells which make up the egg sack are haploid and one is quasi-diploid.
- One of the six haploid cells is the egg.
- The quasi-diploid cell is fertilized by the second sperm supplied by the pollen and the resulting triploid cell forms the endosperm.
Illustration, a typical fruit
- Ovary wall and flower receptacle:
- The ovary wall and flower receptacle surrounding the embryo become the fruit associated with the seed.
- "The next time you start to eat a pear or an apple, you can apply your knowledge of plant anatomy while examining the food you are eating. Opposite the stem of a fresh pear or apple at the base of the fruit you can probably see the sepals and sometimes the stamens still attached. As you bite into the flesh of the fruit, you are devouring the receptacle, the enlarged base of the flower. When you get to the core of the pear or apple, you will have reached the ovary wall. The oval seeds inside develop from the ovule, and the tough brown seed coat is the mature wall of the ovule." (pp. 803-804, Postlethwaite and Hobson, 1995)
- Enable dissemination:
- The fruit can take on many forms ranging from the edible portions surrounding the seeds of many trees and plants (ranging from squash to apples) to the "shells" of peanuts to various adaptations which impart mobility on the seed (including the attachment to animals, hitching a ride on a breeze, rolling along the ground, or floating on the water).
- Note that many fruits, like pollen, are adapted to propagation by animals, either as unwanted hitchhikers on fur, benign passengers through the gut, forgotten squirrel fodder, etc.
- Sprouting of seed:
- Germination occurs when a seed is found within a sufficiently warm and wet environment.
- Often germination cues are more complex, however. For example, in many cases a seed must over winter (i.e., sufficient exposure to cold) prior to the occurrence of germination. This allows the seed to avoid undesirable Autumn germination.
- Germination is accompanied by increases in metabolic rate, nutrient utilization, and, of course, growth. Growth leads to a cracking of the seed coat and the emergence of the root, followed by the emergence of the shoot. Cotyledon or endosperm material is used up in this process to fuel this early growth, especially prior to the initiation of photosynthesis by the young plant.
- "The zygote, surrounded by the maternal tissues of the ovule and ovary, divides into cells that suspend the embryo from the maternal tissues and cells that form most of the embryo proper. As embryonic cells continue to divide, they form a little globe-shaped mass, which then becomes heart-shaped. The lobes of the heart becomes the cotyledons (the first leaves) with the shoot apical meristem lying between them. The rest of the embryo becomes the embryonic stem and embryonic root. From that point on, the meristems take over and form a recognizable plant." (p. 804-805, Postlethwait and Hobson, 1995)
- "You can investigate how an embryo grows in a seed by buying a bag of peanuts. Carefully open one of the peanut shells and look closely at the various parts as you munch. The woody peanut shell is actually the fruit formed from the enlarged ovary wall, while the reddish papery coating around each peanut is the seed coat, the wall of ovule. The two oval halves of the peanut are the dried, salted remains of the cotyledons, the thick first leaves of the embryo. The embryos of peanuts and many other plants, including pears, have two cotyledons and are therefore called dicotyledons (dicots). Plants with just one cotyledon---lilies and palms, for example, as well as corn and other grasses, are called monocotyledons (monocots)." (p. 805, Postlethwait and Hobson, 1995)
- Cotyledon are the first, embryonic leaves (or leaf) of a plant.
- Note that in dicots the cotyledons are often involved in food storage whereas in the monocots the cotyledons gather food through absorption.
- Cells that are capable of dividing throughout the life of a plant are contained within tissue called meristem. The meristem tissue remains essentially embryonic and is capable of dividing and differentiating into all of the cell types found in a plant.
- Angiosperm reproduction
- Egg sac
- Flying penis
- Imperfect flowers
- Life cycle of the pine
- Pollen grain
- Pollen tube
- A typical fruit, illustration
Practice question answers
- Diagram or describe the cells found in a pollen grain and their relationships. [PEEK]
- You have before you a flower which has a total of six (and only six) carpels. This flower is the product of a _________. [PEEK]
- all of the above
- none of the above
- Another, general name for the shell of a peanut is ___________. [PEEK]
- Draw a cross section of an apple indicating where the flower receptacle is found. [PEEK]
- What is meant by double fertilization? [PEEK]
- A pollen, or male gametophyte (a.k.a., microgametophyte), consists of three cells, two sperm and a pollen tube cell. These three cells are derived from a single, meiotically derived progenitor. The first mitotic division of this progenitor results in the production of the pollen tube cell and the sperm cell progenitor. A second mitotic division of the sperm cell progenitor produces the two sperm cells.
- it's the part of the apple one eats
- in angiosperms, the first fertilization forms the zygote, the second the endosperm
- Postlethwaite, J.H., Hobson, J.L. (1995). The Nature of Life. Third Edition. McGraw-Hill, Inc., New York. pp. 488, 796-804.
- Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 675-682, 683-700.
- Raven, P. H., Johnson, G. B. (1996). Biology. Fourth Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 773-738, 741-744.