I. Flower morphology
A What is a flower? Definition: A flower is a branch system terminated by a series of modified leaves that are specialized for reproduction.
B. The modified leaves are called:
1. SEPALS (collectively all are referred to as the calyx) - abbreviated CA
2. PETALS (collectively all are referred to as the corolla) - abbreviated CO
CA & CO collectively called the perianth
3. STAMENS (collectively all are referred to as the androecium) - abbreviated A
4. CARPELS (collectively all are referred to as the gynoecium) - abbreviated G
C. These floral parts are always positioned at the terminal (sometimes swollen) portion of the branch called the receptacle and always occur in the same order relative to each other. The position of each floral whorl can be visualized as occupying a location on a hemisphere. For example, the corolla is both further in or further up on the receptacle.
D. Generalized flower structure. Figure 4.16 from text showing flower with separate carpels.
1. Calyx - composed of sepals - lowest position on the receptacle. Usually green and leafy.
2. Corolla - composed of petals - interior to the sepals. Usually the showy part of the flower (but not always!).
3. Androecium - composed of stamens - interior to petals. Each stamen composed of a filament and an anther (anther has sacs called thecae).
4. Gynoecium - composed of carpels - highest position on the receptacle. Plants may have one or many carpels and the carpels may be separate or fused to each other. When fused, referred to as a compound gynoecium. The compound gynoecium may have three regions:
a. the stigma (receptive portion where pollen adheres and germinates)
b. the style (where pollen tubes grow)
c. the ovary (the swollen lower portion containing the ovules)
5. The term "pistil" is not very precise, for it can be a "simple pistil" (meaning a single carpel) or a "compound pistil" (meaning a compound gynoecium composed of fused carpels). We will avoid using the term pistil in this class.
E. How do the floral whorls of angiosperms compare to gymnosperms? The fertile whorls (androecium and gynoecium) are foliar organs that bear micro- or megasporangia (anthers and ovules). Diagram.
1. The stamen is really a microsporophyll bearing microsporangia (inside anther sac).
2. The carpel is really a megasporophyll bearing an integumented megasporangium (nucellus), otherwise known as an ovule.
II. Sexual Conditions
A. Flower sexual conditions
1. The generalized flower we just saw contained both anthers and carpels - called the essential floral parts. Since both sexes are present, the flower is called bisexual (also called hermophroditic or perfect).
2. If a flower lacks one or more of the four main floral whorls, it is referred to as incomplete. The missing floral whorls may be either essential (A & G) or non-essential (CA & CO). A complete flower must have all four floral whorls.
3. If a flowers lacks one of the essential floral whorls, it is called unisexual (or imperfect). Since it lacks at least one whorl (essential or non-essential), it is also imperfect. Unisexual flowers contain either stamens or carpels. The former is called a staminate flower whereas the latter a carpellate (or pistillate) flower.
B. Whole plant sexual conditions
1. Staminate and carpellate flowers occur on the same individual plant, the species is referred to as being monoecious.
2. If staminate and carpellate flowers occur on different plants, the species is referred to as being dioecious.
3. If both bisexual (perfect) and unisexual (imperfect) flowers occur on the same plant, the species is referred to as being polygamous.
There are various forms of polygamy. You are not required to learn all of these types, but this diagram and the explanation below is included simply to illustrate the complexity and variation in plant sexual conditions:
III. Angiosperm Life Cycle
A. Figure 4.17 from text.
B. Production of the male gametophyte. Each anther contains from one to four microsporangia (anther cross section). The young microsporangia contain microsporocytes (microspore mother cells) that undergo meiosis producing four tetrads (haploid microspores). Each of the four components of the tetrad differentiate into pollen grains. Mitotic divisions result in a mature microgametophyte (pollen) containing only two nuclei (tube and generative nuclei). When the pollen grain lands on the stigma, the generative nucleus divides to form two sperm nuclei.
C. Production of the female gametophyte. Takes place inside the ovules (cross section of lily ovary). Inside a young, integumented ovule, one cell differentiates into a megasporocyte (also called the megaspore mother cell). Through the process called megasporogensis, meiosis produces four megaspores, but usually only one of these survives. During megagametogenesis, this haploid cell divides mitotically three times giving eight nuclei (in seven cells). One cell at the micropyle end of the ovule becomes the egg and the two nuclei in the large central cell function as polar nuclei. The seven celled, eight nucleate structure is the female gametophyte or embryo sac. Diagram of megasporogenesis and megagametogenesis. Diagram of the angiosperm male and female gametophytes.
D. Fertilization (Diagram). The pollen tubes grow through the style, destroying the stylar tissue as they do (photo). When the pollen tube reaches the micropyle opening, it releases two sperm nuclei. One fuses with the egg forming a zygote, the other fuses with the already fused polar nuclei making a triploid fusion nucleus. This DOUBLE FERTILIZATION (photo) is a feature found only in angiosperms. The triploid fusion nucleus divides mitotically to form endosperm which is used as a nutritive tissue for the growing embryo.
E. Fruits and seeds. The mature ovule becomes the seed. The integuments become the seed coat that surround the endosperm and embryo. The mature carpel (and sometimes associated tissues) becomes the fruit.
| A useful educational aid is the concept map. The link below shows one possible concept map produced for the angiosperm life cycle. Typically, one starts with a group of terms and then relates various terms to one another by means of arrows and qualifying statements. For a group of people each presented with the same terms, all would likely produce a different concept map. But that's OK! The idea is to develop connections between the terms and learn their hierarchical relationships to one another. If you want to develop your own angiosperm life cycle concept map, use the list of flower phytographic terms given on THIS PAGE. Compare your map to the one given HERE. |
