|Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; email@example.com; http://www.employees.csbsju.edu/ssaupe/|
In an abstract sense, reproduction serves as the mechanism by which an individual passes genetic information through time. According to Harold Morowitz, one of my favorite science writers (hes a biochemist), "an individual is the transient caretaker of the genetic instructions that must be conveyed down the generations." So, how do plants convey instructions from one generation to the next?
II. Plants can produce offspring without sex (asexual
"A" means without; therefore asexual literally translates into reproduction without sex. But, exactly what is "sex"? - biologically "sex" is defined as the process that involves meiosis and fertilization. Thus, during asexual reproduction there is no production of gametes, no fertilization and hence, no genetic recombination. Obviously the offspring will be genetically identical to the parents (=clones). Some points to ponder:
III. Plants have sex, too.
IV. Plants have "kinky" sex
V. Flowers are the sex organs of angiosperms (flowering
The flower is the key feature of angiosperms. It is a complex organ, that produces the male and female gametes. Evolutionarily, the flower is a short shoot. Thus, a flower is evolved from modified leaves and stem. The "leafy" nature of the flower can be readily seen in many of the floral structures. The parts of an "idealized" flower are:
You are probably familiar with the term "pistil" that has traditionally been used to refer to the structure made of stigma, style and ovary. Botanists prefer to refer this structure as the carpel(s) for technical reasons, one of which is that carpel highlights the evolutionary origin of these structures. If you use the term pistil, make sure you spell it correctly - it's not a weapon. Check out the proposed evolutionary development in your text.
V. Plants like sexual variety - or, there is no such thing
as a "typical" flower.
Flowers can be highly modified from the basic floral plan described above. In fact, finding a flower that looks like the "textbook diagrams" is somewhat difficult. Most flowers are modified, to a greater or lesser extent, from the basic pattern.
The fact that all flowers share the same basic floral plan is good evidence for evolution. If the flowering plants didn't share a common ancestry, then there is no reason to expect that all flowers would have the same plan. If different flowering plants were created separately, by individual acts of creation, then we would expect many different floral designs.
Stephen J. Gould describes the petals of some orchid flowers as imperfections of nature, meaning that they are not the "best" or most "perfect" design for solving the problem of pollination. However, orchids and all organisms are constrained by their evolutionary history. Thus, these imperfections show the tracks of evolution. Check out Gould's essay, "The Panda's Thumb."
One common floral modification is to
lack one or more sets of floral organs. If a flower possesses all four sets of floral
parts it is said to be complete. If it lacks one or more it is incomplete. A flower with
both androecium and gynoecium is termed perfect; if missing male or female parts it is
called imperfect and the flowers are considered to be unisexual. Note that by definition a
complete flower must be perfect; but a perfect flower may be incomplete. The terms
monoecious (unisexual male and female flowers on the same individual such as in oak and
birch) and dioecious (unisexual male and female flowers on different individuals such as
in willow and poplars) refer to how flowers are distributed on different plants.
VI. Meiosis occurs in the anther (stamen) and ovule (carpel).
Spore mother cells, also called sporocytes (which are diploid), divide meiotically to produce haploid spores (microspores - male; megaspore - female). In turn, the spores divide mitotically and develop into the male and female gametophytes, respectively. The microspores undergo one division to produce a male gametophyte with two nuclei (tube and generative nuclei). The megaspore undergoes three mitotic divisions resulting in the female gametophye (also called the embryo sac) that has eight nuclei (egg, 3 antipodals, 2 synergids, 2 polar nuclei).
VII. Pollen is the male
gametophyte; actually the immature gametophyte.
The germinated pollen grain, with its pollen tube and two sperm nuclei represents the mature male gametophyte. Two sperms develop in the germinating pollen grain. The pollen tube follows chemical signals on its trip through the stigma and style to the ovule.
VIII. The female gametophyte is housed in the ovule
There may be from one (i.e., cherry) to many (i.e., watermelon) ovules per flower, depending on the species. In one type of female gametophyte, there are several cells, with a total of 8 nuclei. One nucleus, near the micropyle (opening into ovule), serves as the egg, and two others in the middle of the gametophyte (also called embryo sac) are called polar nuclei.
IX. Pollination - the plant version of copulation.
Refers to the fusion of sperm and egg, occurs in ovule. Pollen germinates on the stigma, the pollen tube grows out and grows through the style toward ovule. The pollen tube grows into ovule (through a gap in ovule covering called micropyle) and releases two sperm. One sperm fuses with egg to produce the zygote. The other sperm fuses with other nuclei to produce a triploid (or pentaploid, depending on species) cell that produces endosperm. The endosperm is a nutritive tissue that will be saved primarily for the seed to be used when it is germinating.
XI. Pollination and fertilization are different.
This seems obvious but there is a common misconception that the two terms can be used synonymously
XII. Plant sex is twice as much fun.
In other words, there are two fertilization events (one to produce the embryo and the other to produce food storage tissue, the endosperm). This process, double fertilization, is unique to flowering plants.
XIII. Plant embryos are not miniaturized
Embryogenesis refers to the sequence of developmental events that produce the embryo.
XIV. Embryo development in plants is discontinuous.
In other words, after the embryo develops a while, it undergoes a dormant period. In animals, once fertilization occurs, development of the individual is continuous throughout the life cycle. Dormancy is built in to allow time for seed/fruit dispersal.
XV. After fertilization, the ovule develops into the seed.
A seed is essentially a sleeping baby in a suitcase with his/her lunch. The three major parts of a seed are:
To summarize, seeds contain: (1) an embryo (dicot or
monocot type); (2) with its lunch (in the form of endosperm which may be present or the
endosperm may be reabsorbed and stored in another form in cotyledons); (3) in a suitcase
XVI. Seeds are an interesting hybrid of maternal and
The seed coat is derived from the integuments of the ovule.
XVII. The ovary develops into the fruit.
A fruit is thus a ripened, mature ovary and its contents (seeds). Fruits are frequently associated with accessory structures (i.e., tissues other than those from just the ovary).
XVIII. Whats the difference between a fruit and a
Botanically, fruits are derived from the ovary or reproductive parts of the plants. Vegetables are derived from "vegetative" parts, such as leaves, roots and stems. Simple, right? So, is a zucchini a fruit or vegetable? How about cauliflower? or tomato? or squash? Many foods that we call vegetables are actually fruits, and vice versa. Some "fruits" are really vegetables (such as rhubarb). The problem arises because "fruit" is commonly used to describe a food that is usually sweet and eaten as dessert or perhaps salad; whereas vegetable is used to describe a food, usually green, eaten during the main course of a meal.
XIX. Seeds vs. Fruits.
These terms are also often incorrectly applied (from a strict botanical sense) by the non-botanist. For example, sunflower seeds and corn grains are actually fruits. Conversely, seeds like coconut, are often considered to be a fruit. Take-home-lesson: fruits have seeds, vegetables do not...the seeds are inside the fruit.
XX. The function of fruits (and seeds) is to send
"the kids off to college".
Fruits protect the seeds and function in dispersal. They can be disseminated by wind, water, animals, etc. Note: fruits do not provide nourishment for the germinating seedling - they provide nourishment and an entincement for the dispersal agent.
XXI. The mathematics of plant sex.
1 pollen grain/ovule; 2 sperms/ovule. How many ovules, sperms, eggs, ovaries, flowers were necessary to produce a watermelon? or a cherry?
XXII. Seed germination.
(not on exam)
The first stage is water uptake (imbibition). This is followed by rapid swelling of the seed and then initiation of germination events. The first real sign of germination is the appearance of the radicle. In grains, this process is mediated by the plant hormone gibberellic acid that is released by the embryo that stimulates the release of amylase from aleurone cells.
XXIII. Mechanisms for knowing when to germinate - light, cold treatment, scarification (not on exam)
Summary of plant reproduction:
- Plants have sex (meiosis & fertilization)
- Meiosis occurs in the stamen (anther) and pistil (ovule)
- The female gametophyte is in the ovule, protected from the environment
- Pollen is the male gametophyte
- Pollination is the transfer of pollen from anther to stigma
- Pollen only germinates on a compatible stigma
- Two sperms develop in the germinating pollen grain
- Fertilization occurs in the pistil (ovule)
- Fertilization and pollination are different events
- Plants have two fertilizations - one to produce the zygote (embryo) and the other to give rise to food storage tissue (endosperm)
- Plant embryos are not miniaturized adults (as in animals)
- Plant embryo development is discontinuous (to allow time for dispersal)
- Plants exhibit an alternation of generations having a distinct haploid (gametophyte) and diploid (sporophyte) stage
- The gametophyte phase of flowering plants is short-lived; the sporophyte is the dominant phase
- After fertilization, the ovule develops into a seed
- The ovary develops into a fruit
- For every seed there was one ovule in the ovary
- Each seed required one pollen grain for development
- Each seed requires 2 sperm (present inside the single pollen grain) for development
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Last updated: February 18, 2004 � Copyright by SG Saupe