Flowering Plants
Useful Reading
Campbell, Biology 6th Ed - Chapters 29 & 30, pgs 575-585, 597-600, 606-615
Campbell, Biology 7th Ed - Chapters 29 & 30, pgs 573-574, 591-593, 596-606
Vocabulary
Cuticle – waxy covering around the outside of a plant that prevents water passage.
Sporophyte – diploid stage in the life cycle of plants that produces haploid spores through meiosis.
Gametophyte – haploid stage in the life cycle of plants that produces haploid gametes through mitosis.
Stomata – openings at the leaf surface of plants that allow gas exchange.
Vascular system – a system of tubes running through the body that transports nutrients, minerals, and water between cells; also often involve in structural support.
Carpel/ovary – vessel which encloses the ovules in angiosperms.
Fruit – The development of the ovary; functions in seed dispersal.
Endosperm – gametophyte tissue formed by fusion of a sperm nuclei with two megagametophyte polar nuclei. Provides nutrition to the sporophyte embryo.
Also see flower morphology and terminology below.
Taxonomy and Diversity – Kingdom Plantae
In the previous lab, we learned about the diversity of non-flowering plants. Here, we will look at flowering plants. As review, there are 10 Divisions within Kingdom Plantae. All plants go through an alternation of generations involving a diploid sporophyte and a haploid gametophyte. However, plants differ in the presence or absence of three important structures: a vascular system, seeds, and flowers/fruits. These adaptations aided plants in the colonization of land from aquatic ancestors, and in surviving in the changing terrestrial habitat. The description below lists the different evolutionary steps in plant phylogeny:
1. No vascular system : Non-vascular plants
Division Bryophyta (mosses)
Division Hepatophyta (liverworts)
Division Anthocerophyta (hornworts)
2. Vascular system : Vascular plants
A. No seeds :
Division Lycophyta (club mosses)
Division Pterophyta (ferns, horsetails, whisk ferns)
B. Production of seeds :
1) No flowers : Gymnosperms
Division Coniferophyta (conifers)
Division Cycadophyta (cycads)
Division Ginkgophyta (ginkgo)
Division Gnetophyta (gnetae)
2) Flowers : Angiosperms
Division Anthophyta (flowering plants)
Flowering plants (Angiosperms)
Division Anthophyta
Division Anthophyta contains the flowering plants, or Angiosperms. Flowering plants are similar to non-flowering seed plants (Gymnosperms) in having advanced vascular tissue, a dominant sporophyte stage, a markedly reduced gametophyte stage, and production of seeds. We saw in the previous labs that seeds are composed of a parent (sporophyte) cover, gametophyte nutrients, and an offspring sporophyte. Seeds are important adaptations for terrestrial life because they protect the sporophyte embryo from the environment, they provide nutrients to the embryo, and they can remain dormant during stressful conditions.
Angiosperms differ from gymnosperms by the production of a derived organ, the fruit. Angiosperm ovules are enclosed in a carpel. After fertilization, the ovule develops into the seed and the carpel develops into the fruit. Angiosperms also produce flowers, which are structures containing the reproductive organs of the sporophyte. The flower functions to protect the gamete and to aid in dispersal of male gametes (pollination) and fertilization of eggs. Familiarize yourself with the flower morphology below. In the diagram below, the term pistil is used for the carpel.
The reproductive organs are surrounded by the perianth, which are leaflike structures, and include the sepals and petals. All of the sepals collectively are called the calyx, and all of the petals together are called the corolla.
The female reproductive organ is the carpel, or pistil. At the top is the stigma, where pollen lands. The style connects the stigma to the ovary, where the ovules are. When pollen lands on the stigma, it grows a pollen tube down the style to conduct the sperm nucleus to the ovules. Once the eggs are fertilized, the ovary and remaining carpel grow to become the fruit.
The male reproductive organ is the stamen. At the top of the stamen is the anther, which produces pollen. The anther is supported by the filament.
Dicots and Monocots
Angiosperms fall into two different groups based on flower and body morphology, and on embryo development. These two groups are the dicotyledons (dicots) and the monocotyledons (monocots). The dicots are not a monophyletic group – they are actually paraphyletic. See the phylogeny below.
Dicots and monocots differ in germination and plant morphology. See the table below for the major differences in these two types of angiosperms. In addition to these traits, it is also the case that monocots tend to be herbaceous (e.g. grasses and grains) while dicots are ~50% herbaceous and 50% woody species.
MONOCOTS DICOTS Embryo with single cotyledon Embryo with two cotyledons Pollen with single furrow or pore Pollen with three furrows or pores Flower parts in multiples of three Flower parts in multiples of four or five Major leaf veins parallel Major leaf veins reticulated Stem vascular bundles scattered Stem vascular bundles in a ring Roots are adventitious Roots develop from radicle Secondary growth absent Secondary growth often present
It is the difference in seed structure that defines these two groups: dicots have 2 seeds leaves (cotyledons), while monocots have only 1 seed leaf. Seed germination also differs between these two groups.
The following images provide more detail on the differences in stem structure between monocots and dicots. Monocots have vascular tissue in bundles throughout the stem. In contrast, dicot vascular bundles are arranged in a ring around the outside of the stem.
Monocot stem (left) and dicot stem (right).
Detail of monocot vascular bundle.
Detail of dicot vascular bundle.
Pollination and flower structure
Although many angiosperms have the potential to self-fertilize, most plants cross-fertilize. To transfer pollen from flower to flower, angiosperms make use of wind dispersal and animal pollinators. Animals which pollinate flowers include insects, birds, and mammals. One of the important functions of flowers is to attract animal pollinators. Because of this, flower structure, color, and odor vary dramatically in correlation with the primary pollinators. For example, flowers which are pollinated by flies smell like rotting meat, while some flowers pollinated by wasps mimic the appearance of a female wasp (potential mate). The benefits to pollinators include nutrition from nectar and pollen.
It is often possible to determine what type of pollination/pollinator a plant uses by the structure, color, shape, and smell of the flower. You should be familiar with this table:
Flower Trait
Pollinator
Reduced sepals and petals
Wind
Large sepals or petals
Animal
Petals white
Moth or bat
Petals colored
Flower tubular
Sweet odor
Butterfly
No odor
Hummingbird
Flower not tubular
Bee or beetle
Angiosperm Life Cycle
Review the general plant life cycle below, which shows the universal plant alternation of generations. You should remind yourself of the ploidy of the different life stages, the production of spores and gametes through meiosis and mitosis, and the variations on this life cycle in the other plant divisions.
The following life cycle is specific for angiosperms. Compare it to the general cycle above. Also compare it to the life cycles of non-vascular plants, and seedless and non-flowering seeding vascular plants. What are the major differences?
Inside the anther, microspores are produced through meiosis. These microspores give rise to the microgametophyte – the pollen grain.
Anther with pollen Pollen grains with two nuclei
The female reproductive organ, the ovary, contains developing ovules. Each ovule produces megaspores (haploid) through meiosis. One megaspore survives in each ovule and develops into a multi-nucleated megagametophyte.
Lily ovary with multiple ovules Close-up of a mature ovule
When pollen reaches the stigma, it grows a pollen tube to extend down the style. Two sperm nuclei enter the ovule: one fuses with the egg to produce the zygote, the other fuses with two polar nuclei to make the 3n endosperm. The endosperm is part of the seed and provides nutrition for the sporophyte embryo. The fusion of two sperm nuclei with megagametophyte nuclei is called double fertilization.
Germinating pollen grains
Following fertilization, the zygote develops into an embryo, the endosperm grows, the ovule forms a seed, and the ovary develops into a fruit.
For more on angiosperm reproduction: click here or here
Fruits and Seed Dispersal
The primary function of the fruit (produced by the ovary) is to aid in dispersal of the seed. A seed must leave the parent plant in order to grow into a new plant, and often disperses fairly large distances from the parent plant. A seed benefits from dispersal by 1) avoiding competition with its parent, and 2) being deposited in locations good for germination and plant growth. Fruits can be dispersed by gravity, wind, water, ingestion by animals, or adhesion to animals.
Seeds and fruits have diverse and impressive adaptations to increase dispersal. For example, wind-dispersed seeds have “wings”. Seeds dispersed via ingestion and defecation by animals have edible fruits.
Fruits vary from hard and woody to soft. They can have one to multiple seeds, and hard or soft seeds. Some seeds are readily eaten (e.g. peas, nuts), while some are not (e.g. cherry pits, apple seeds).
Two peanut seeds in the hard ovary (left); apple seeds in the fleshy fruit, composed partly of flower petals and sepals (right).
Plant Adaptations
Adaptations of leaves
The plant leaf is well-adapted to generate energy via photosynthesis without losing more water than necessary. Traits are well-designed for a function:
1) Leaves are thin to maximize surface area exposed to sun.
2) The leaf epidermis is covered by the waxy cuticle to prevent desiccation.
3) Leaves have stomata with guard cells so they can control the amount of CO2 entering and water leaving the leaf.
4) Air spaces in the leaves allow circulation of CO2 throughout the leaf.
Plants that live in arid habitats have xerophyte leaves, with adaptations to deal with the increased risk of desiccation. For example, they have a thicker epidermis, prominent cuticles, and stomata located in pits (see picture below). How do these traits help a plant deal with water loss in arid environments?
Xerophyte leaf (left) compared to mesophyte leaf (right)
Plant structure in the desert environment
Plants which live in deserts are exposed to high amounts of solar radiation and very dry air. The physical and behavioral adaptations of desert plants are as numerous and innovative as those of desert animals. Xerophytes, plants that have altered their physical structure to survive extreme heat and lack of water, are the largest group of such plants living in the deserts of the American Southwest. Adaptations to arid conditions include:
1) Leaf morphology mentioned above.
2) Vertically-oriented leaves to catch early- and late-day sun rather than mid-day sun.
3) Spines or pubescence to trap air close to the plant and keep it cooler and more humid.
4) Pubescence is colored to reflect light.
Cacti are among the most drought-resistant plants on the planet due to their absence of leaves, shallow root systems, ability to store water in their stems, spines for shade and waxy skin to seal in moisture. Cacti originated in the West Indies and migrated to many parts of the New World, populating the deserts of the Southwest with hundreds of varieties.
Barrel Cactus Opuntia Cactus Saguaro Cactus Cholla Cactus
Cacti depend on chlorophyll in the outer tissue of their skin and stems to
conduct photosynthesis for the manufacture of food. Spines protect the plant
from animals, shade the plant from the sun and also collect moisture. Extensive
shallow root systems are usually radial, allowing for the quick acquisition of
large quantities of water when it rains. Because they store water in the core of
both stems and roots, cacti are well-suited to dry climates and can survive
years of drought on the water collected from a single rainfall.
Many other desert trees and shrubs have also adapted by eliminating leaves --
replacing them with thorns, not spines -- or by greatly reducing leaf size to
eliminate transpiration. Such plants also usually have smooth, green bark on
stems and trunks serving to both produce food and seal in moisture, such as the
Paloverde. Some plants produce ephemeral leaves during the brief rainy season to
help increase transpiration and photosynthesis. Sometimes these leaves only last
for one day.
Anti-predator adaptations
Plants have many different types of adaptations to reduce the consumption of their body by animals, including physical and chemical adaptations. Physical defenses include spines, such as those found on cacti.
Chemical defenses of plants are extensive and are often the focus of a co-evolutionary arms race between plant and predator. These chemicals are called secondary plant metabolites. These bad tasting and sometimes toxic compounds have been one of plants most powerful means of defense. They can be divided into six easily identifiable classes based on plant material and extract. The table below lists each compound and how they affect vertebrates in general. Humans have long taken advantage of plant compounds for medicinal purposes.
SECONDARY COMPOUND
HUMAN PHARMACOLOGICAL EFFECTS
Alkaloids
Antibacterial, stimulants,
sedatives, vaso-constrictors &
dilators, diuretics,
expectorants, antidiarrheal
Cyanogenic glycosides
cough suppressants, treatment
of digestive disorders
Saponins
expectorant, diuretic; treatment
of skin diseases, anemia & diabetes
Cardiac glycosides
Regulation of heart activity
Tannins
Astringent used in treating
cuts & burns, antidiarrheal
Simple phenolics
Antihelmenthics, antiseptics
analgesics, diuretics
Plants can be tested for the presence of alkaloids, saponins, tannins, phenolics, and antimicrobial activity. You should familiarize yourself with these tests.
Review Questions
-Name two traits angiosperms share in common with gymnosperms and two traits which are derived in angiosperms.
-Is an angiosperm plant the sporophyte or gametophyte stage?
-What is the male reproductive organ of an angiosperm? The female reproductive organ?
-What is the function of the stigma?
-What part of the female reproductive organ becomes the fruit?
-How do monocots and dicots differ in the following traits?
1. Arrangement of vascular bundles in the stem
2. Root structure
3. Leaf venation
-What two types of pollinators visit white flowers?
-What is the shape of flowers pollinated by butterflies and hummingbirds? How does this shape correspond to the pollinator’s morphology?
-Where are the male and female gametophytes located in an angiosperm?
-What is the function of the endosperm? What is its ploidy? Is it composed of sporophyte or gametophyte tissue?
-Name three adaptations plants have for living in the desert.
-What is the function of a sweet, fleshy fruit, such as seen in a peach?