Comparing the flower structure of different angiosperms
Class practical
The primary aim of this investigation is to study the structures involved in pollination. Having seen the common structures and how they vary in form from species to species, students should be able to make deductions about which pollinating agents (such as wind or insects) and perhaps to identify mechanisms that affect the likelihood of cross-pollination.
Lesson organisation
This will depend on how many flowers and suitable dissecting tools you have available.
Four steps in the investigation are described: a simple identification of the parts of the flower, a closer look at the anthers and stigma in the context of cross-pollination, a consideration of the gross structure of the flower to deduce the likely agent of pollination (for example, wind or insects), and a dissection of the flower which is then laminated for later inspection. You could select the parts of the investigation where you most want to focus.
For comparison of agents of pollination, we have provided a number of photographs of flowers. You could present these as a slideshow, or print in colour and laminate to use as cards. If printed as cards, each student could state their deduction about the flower picture they have been given and justify their deduction with observations of the flower.
Apparatus and Chemicals
For each group of students:
Stereo microscope, low power (optional)
Hand lens, 1 per student
Dissecting kit: mounted needles (2), scalpel (1), forceps (1)
White tile (1)
Flowers to dissect (Note 1)
White paper (1 sheet)
Clear sticky-backed plastic (around 20 cm square), optional
Rulers
For the class – set up by technician/ teacher:
Printed (and laminated if possible) copies of images of flowers (see file linked below), or a slideshow of flower pictures
Health & Safety and Technical notes
Take care with scalpels and needles. Always carry them held tightly to a tile, with the blade pointing away from the body.
1 In principle you could use any flower. Widely available and with a long growing season are flowers of Impatiens (busy lizzie), Pelargonium (pot geranium), nasturtium (Tropaeolum majus), and Viola (many varieties of pansy are available with some in flower in each season). In spring, Primula vulgaris (primrose) may be interesting as they occur in two distinct forms: pin-eyed (pin) and thrum-eyed (thrum).
2 If your sticky-backed plastic has acquired a static electric charge and is attracting the tiny parts of the flower to itself, ‘squirt’ it a couple of times with a Bunsen burner lighter. The charge from the Bunsen lighter will reduce the static charge on the plastic.
Ethical issues
Collect flowers from a florist, garden centre or from a domestic garden, not from the wild. Discourage students from picking wild flowers to study, unless they can see that they are very abundant (such as daisies in grassland or bluebells in a woodland carpet).
Procedure
SAFETY: Be aware that students may have previously unknown sensitivities to plant material – especially plant saps and hairy plant tissues. Be prepared for allergic reactions or skin irritation with appropriate first aid response and the possibility of thorough hand-washing.
Preparation
a Collect flowers.
b Print and laminate pictures of flowers, or set up as slideshow (for Investigation 3).
c (Optional for Investigation 4) Cut sticky-backed plastic to size – 20 cm x 20 cm.
Investigation 1 Flower anatomy – observation
d Set out flowering shoots of any species available.
e Examine several flowers of each type carefully, using a hand lens and a low power stereo binocular microscope.
f Describe common features of the flowers, such as the pistil (usually central, with the sticky stigma at the top, and the style leading down to the ovary), stamens (with anthers attached), petals, and sepals.
Investigation 2 Cross-pollination
g Make sketches to show the relative position of stigma and anthers.
h Decide whether the anthers in the flower have opened to release pollen and whether the surface of the stigma has matured.
i For each species, record whether the anthers or the stigma mature first, or if they appear to mature at the same time, or if it varies from flower to flower within the species.
j Measure the length of stamens and pistil in each flower. If you have several examples of any type of flower, collate the results to see if the lengths are common across the examples, or if there is a noticeable variation.
k (Optional) If you have access to pin-eyed and thrum-eyed Primula sp, measure the length of style and stamens in each, and sketch the different arrangements of style and stamen.
l Look at your sketches showing the relative position of stigma and anthers. Decide whether the arrangement favours or hinders cross-pollination.
m Draw up a table to compare, in the plant species studied, any mechanisms that favour either cross-pollination or self-pollination.
Investigation 3 Pollinating agents
n Study the general structure of the flower for any evidence that may help you to decide whether the plant is wind-pollinated or insect-pollinated.
o Study the photographs provided, or any other images you can find of different flowers, and for each one decide whether you think it is wind- or insect-pollinated.
p Prepare another table comparing the general characteristics of wind-pollinated flowers with those of insect-pollinated ones.
Investigation 4 Flower dissection
q Dissect the flowers to isolate the pistil (usually central, with the sticky stigma at the top, and the style leading down to the ovary), stamens (with anthers attached), petals, and sepals.
r Set out the different parts on a clean piece of white paper, either in a grid or in whorls to represent the concentric layers of the flower structure.
s Place sticky-backed plastic over the dissected flower to maintain the display (Note 2).
Teaching notes
Most flowering plants produce hermaphrodite flowers – flowers that produce both ovules and pollen. There are many mechanisms that prevent or reduce the likelihood of self-pollination, although some species self-pollinate as a matter of course. Cross-pollination requires pollen to be transported from one flower to another. This could be caused by an abiotic agent such as wind or water. However, for 80% of flowering plant species a biotic agent is responsible for carrying the pollen from one flower to another. These are usually insects, but sometimes birds (for example, humming birds) or mammals (for example bats).
Cross-pollination is just one stage in the formation of seeds. After pollination, the pollen grains must be able to develop pollen tubes (see related practical Observing the growth of pollen tubes) in order for the pollen grain nucleus to fertilise the ovary.
There are many mechanisms in flowers to reduce the likelihood or effectiveness of cross-pollination. However, even in self-fertilised plants the process of meiosis allows for some reassortment of genes and generates some diversity in the offspring from seed.
Maturation of pollen and stigma at different times ensures that individual flowers do not self-pollinate. In some plants with long flower spikes, such as foxgloves (Digitalis sp) and verbascum or mullein (Verbascum sp), the individual flowers on the inflorescence open and mature in turn. This reduces the chance of self-fertilisation. The variability exemplified by pin-eyed and thrum-eyed primula is called heterostyly – meaning a variability in the structure of the style and its length relative to the stamens. This mechanism also makes cross-pollination more likely.
The introduction and first chapter of Charles Darwin’s work Form of flowers – now available online – cover a general description of pollination mechanisms and describe Darwin’s own work on Primula veris (and other related species). This document could provide an interesting source for students. It is, perhaps surprisingly, readable and it shows the methodical approach with cautious conclusions that could be said to typify a good scientific approach. The student notes include some suggested questions related to the first chapter of this book. Skimming the rest of the book could give further insight into Darwin’s systematic approach to natural history/ biology. This approach characterised his work throughout his lifetime and put him in the position to make his presentation of the theory of evolution by natural selection so credible and with so much evidence to support it.
It is relatively easy to identify flowers as wind-pollinated structures – such as maize (sweetcorn, Zea mays) and hazel (Corylus avellana). In wind-pollinated plants the anthers and the pistils hang outside the flower – making the release of pollen on the wind possible and making it more likely that wind-borne pollen will be trapped on another flower. Each ‘silk’ on a sweetcorn flower is a separate pistil and each one, when pollinated and fertilised, makes a single kernel of corn.
Some insect-pollinated flowers have stamens and stigma exposed and outside the shelter of the petals – for example, winter honeysuckle (Lonicera fragrantissima). If a flower is highly fragranced, that is a good indicator of insect-pollination. Nectar also attracts insects, and other pollinating animals such as hummingbirds or bats. The best way to be sure how a plant is pollinated is to observe it for some time during the flowering season and see how often it is visited by which types of animals. Look also for guiding patterns on flowers that point out the route to nectar – and pollen – for visiting insects. These are sometimes invisible to human eyes but visible to insects which can see in the ultraviolet range of the spectrum. (See link below.) UV patterns may not be the same as the patterns visible to us.
Many insect-pollinated plants have complex shapes, ensuring close contact of the visiting insects’ bodies with particular parts of the flower. These morphologies, coupled with different timings of maturation of stigma and anthers, ensure cross-pollination. Perhaps the most remarkable examples are the orchids, and some of the flowers that mimic insect forms.
Pollination is both environmentally and economically significant to humans. Although some of our staple foods (wheat and corn) are pollinated by wind, many of our food plants are pollinated by insects (most fruit, tomatoes, peas, beans and many other vegetables). Currently there is enormous concern for the long-term effect on food crops of the collapse of bee populations across the globe. Some commercial crops are routinely pollinated by hand – sometimes to ensure particular crosses. However, if the populations of pollinating insects decline dramatically, there is no scope for humans taking over the activity.
Students often confuse pollination mechanisms with seed dispersal mechanisms. It’s important to make sure you are clear what is being presented. Pollination mechanisms do not affect how plants colonise new areas.
Health & Safety checked, October 2009
Downloads
Flower photos slides 9 MB
Flower photos separate jpg images in zip folder 9 MB
Student sheet with questions and answers 
Comparing the flower structure of different angiosperms (71 KB) .
Related experiments
Observing the growth of pollen tubes
This could be a useful follow-up to a session on flower structure as it makes the point that pollination is not sufficient for fertilisation. It also introduces the idea that the conditions on the top of the stigma could greatly influence the germination of pollen grains and so contribute to self-infertility in any species of plant.
Web links
http://www.saps.org.uk/library/plant-science-image-library
The SAPS Plant Science Images Database also contains images of flowers pollinated in different ways and notes on their structure.
www.darwinsbeagleplants.org/Darwin/Home.aspx
This is an online collection of plants from Darwin's herbarium. You could use this to show how plants were and are collected for later study.
www.darwin-online.org.uk
This, and other stores of Charles Darwin’s work, present his publication 'Forms of flowers: The different forms of flowers on plants of the same species' (1877: London: John Murray). In this book, Darwin describes his observations of the pistil and stamens of various flowers including the common cowslip, Primula veris (Chapter 1). See Form of flowers.
http://news.bbc.co.uk/1/hi/uk_politics/7951009.stm
A BBC news article from March 2009 on the declining bee population in the UK. It gives a quoted statement that 39 commercial crops rely on insect pollination, and bees were estimated to be worth about £200m to the British food economy.
www.naturfotograf.com/UV_flowers_list.html
A site with a series of photographs of flowers in normal light and showing up their UV reflectance, arranged by plant family. UV patterns are often shown in bright purple, but as this site explains, we can show UV any way we want to – it doesn’t really have a colour of its own in any way that we can understand. These are very clear photographs and show a range of different patterns. A selection of these photographs appear side by side in the Daily Mail article in this link: /www.dailymail.co.uk/sciencetech/article-473897/A-bees-eye-view-How-insects-flowers-differently-us.html
(Websites accessed October 2011)