Angiosperm Floral Morphology


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I. Introduction to Angiosperms - the flowering plants

Latin base for "angio-" or vessel; so "angiosperm" means "vessel for the seed" or those seed plants with the seed encased in an ovary. Angisperms are the dominant group of landplants today. They arose about 140 million years ago during the late Jurassic and experienced rapid diversification during the Cretaceous such that by 90 million years ago they achieved world wide dominance. There are about 250,000 - 275,000 species of flowering plants and in terms of importance to the world's ecosystems and to human nutrition and medicine they surpass all other groups. Angiosperms are typically faster at reproducing then gymnosperms and have adapted in numerous ways to animal pollination, a factor that undoubtedly has promoted the incredible adaptive radiation of this group.

Four features delimit flowering plants:

1. Flowers - see below.

2. Further reduction of the gametophytic stage with respect to that already seen in gymnosperms.

3. Double fertilization: the sperm cell has two nuclei; one nuclei fertilizes the egg nuclei to form the zygote; the second sperm fertilizes the 2 polar nuclei which creates a triploid nutritive tissue called the endopsperm.

4. Vessel elements: Much of the evolutionary success of angiosperms can be attributed to the effecient water conducting tissue in xylem called vessels. Gymnosperms (except for Gnetophytes) only contain tracheids.


II. Classification of Angiosperms

The classification of angiosperms has been unclear for some time. However, with recent DNA sequencing studies, the relationships of flowering plants are now being uncovered and a new (APG - Angiosperm Phylogeny Group) classification system has been recently published. Many books (including most manuals and floras) use older classification systems, especially that of Arthur Cronquist from the New York Botanical Garden. Traditionally the flowering plants are called Magnoliophyta and were used to be divided into two classes - dicots and monocots. The following links will get you to the Plant Systematics Collection using the now outdated Cronquist classification system.

     Magnoliophyta - angiosperms or flowering plants

(1) Magnoliopsida - dicot class
(2) Liliopsida - monocot class


II. Floral structure of Angiosperms

The outstanding and most significant feature of the flowering plants (and that which sets them out from other vascular plants) is the flower. Understanding the flower structure and names of the parts is important as it is the most important set of characters for both recognizing and keying species, genera, families, etc.

Flower: highy specialized shoot = stem + leaves. In flowers the shoot is highly modified and determinate (ceased to grow) and houses the reproductive structures. All flowers have the same basic bauplan and the importance of learning the basic plan will be clear. As the semester goes on, you will hopefully be amazed, dazzled and appreciative all the variations on this theme.

 B. Floral parts (terms & illustrations)

1. Peduncle / pedicel - floral stalk

2. Receptacle - the modified shoot or floral axis

3. Sepals / calyx - the outer most whorl; collectively all sepals are called the calyx. Sepals are typically green and protect the inner floral parts in buds

4. Petals / corolla -the next whorl, collectively all petals are called the corolla. Petals are typically brightly colored and and assist in attracting pollinators. The sepals and petal combined are called the perianth. If the perianth parts cannot be differentiated into sepals and petals, that is, that look so much alike, then they are called tepals.

5. Stamens (androecium) - the next whorls, each stamen has two parts: filament and anthers. Androecium or "male house", the name for all the stamens. The anthers house the microsporangia which undergo meiosis and produce pollen grains.

6. Nectaries - are often associated with flowers, they are found at the receptacle and offer a reward to animal pollinators.

7. Carpels (gynoecium) - The innermost and final whorl is composed of all the carpels and is the site for pollination and fertilization. Collectively all carpels are called the gynoecium: "female house". Carpel has three parts: stigma which receives pollen, the style which is the a slender region specialized for pollen tube growth and the ovary which is an enlarged basal portion and surrounds and protects the ovules. The structure(s) in the center of the flower are often referred to as the pistil(s). Pistil is a layman's term for "flask-shape" structure, so anything with that structure is called a pistil. This is one of the more confusing parts of the flower; more on this below.

 C. Placentation


 D. Arrangement of floral parts

   1. Numerical plan

   2. Symmetry

actinomorphic / radial / regular
zygomorphic / bilateral / irregular

   3. Fusion of floral parts

connation or fusion of same parts

       petals - corolla tube
       stamens - staminal tube
       carpels - compound pistil

adnation or fusion of unlike parts

eg. position of ovary

hypogynous flower
perigynous flower
epigynous flower

   4. Floral formulas

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