Flower Study Group 2020
This web page (under construction) is devoted to the study of flowers. Many people admire flowers and photograph them or dedicate their free time to gardening or flower composition. We believe that it is possible to move one step further by analyzing them in detail and finding information about their morphology and classification. A good place for collecting information on flowers is Wikipedia, the free on-line encyclopedia, while consulting books on botany and plant science is recommended for those wishing to learn more about the biology of flowers. An instructive and enjoyable activity is visiting botanical gardens where many different species of flowering plants (angiosperms) can be admired and carefully studied often with the help of professional gardeners and botanists.
Flowers adopt a variety of shapes and colors, they are important for the reproduction of the plant which takes place through the pollination process. This occurs with the aid of insect pollinators such as bees, wasps, and butterflies which, by moving from one flower to another, do transfer the pollen hence contributing to the reproduction of the plant (enthomophilous pollination process). Pollen can be also transferred by the wind (anemophilous pollination process) while self-pollination represents the third option. The latter process, however, does limit genetic variation in plants.
Do you recognize the names of the flowering plants shown above? The round-shaped blue flower is a Hydrangea (also known as Hortensia), the light-pink pendant flowers are those of Wisteria while the five-petals pink flower is that of an Azalea. The Hydrangea flowers are made of two kind of flowers, small ones at the center and large ones at the periphery that grow together to form a flowerhead. Wisteria, on the other hand, is a climbing plant that produces pendant flowers from the buds located on the shoot or branch. The Azalea is a shrub or perennial woody plant. Some technical terms are explained in the following section.
The external structure of plants (plant morphology)
On the left you can see a schematic drawing of a flowering plant with the names of its most important parts. The root of a plant is normally not visible being immersed into the soil. Above the root there is the stem which develops several branches at the corresponding nodal positions (here there are three nodes). The axil is the part that forms an angle between the stem and the branch. On the axils are located buds which are dormant until they develop a flower with its stalk. The upper part of the stem has a terminal bud. Each branch carries several leafs which are attached to it through petioles. These scientific terms are used to describe the external structure of plants, a field of study called plant morphology. On the other hand, the internal structure of a plant is part of the field of plant anatomy. This type of study is best achieved with the aid of electron microscopes which have the ability to reveal many details of the plant cells. The functioning of plants which comprises processes such as photosynthesis, germination, nutrition, etc., is part of the field of plant physiology. These fields along with plant biochemistry, plant genetics, plant ecology, and plant evolution constitute the science of botany. The Swedish botanist Carl Linnaeus (1707-1778) is considered the founder of modern botanical nomenclature which appeared in his book Species Plantarum published in 1753. The Linnaeus' Garden in Uppsala is open to the public. In the next section we will introduce some basic terms about the structure of flowers.
The structure of flowers
Let us now learn some basic terminology of the flower's structure. The non-reproductive, external part of a flower is called perianth. This is made of the sepal (calyx) and the petals (corolla), as shown on the left side of the figure. The reproductive, internal part of the flower which is surrounded by petals is shown on the right side of the figure. This is made of a style at the center with a stigma that captures the pollen which is next transferred to the ovary containing an egg. The stamen, on the other hand, is the part that produces pollen. This is an example of a so called perfect flower with both male and female reproductive structures; both tulips and roses are perfect flowers. Interestingly, a recent theoretical study on the ancestral flower of angiosperms suggested that this was a perfect flower with radial symmetry (for additional information see the paper by H. Sauquet et al., The ancestral flower of angiosperms and its early diversification, Nature Communications 8:16047, 2017).
The structure of flowers can be conveniently described using both floral diagrams and floral formulas. A floral diagram, introduced in the 19-th century by the German botanist August W. Eichler, is a graphical representation of the flower which resembles a cross section taken at a specific point of the flower. A floral formula, also introduced in the 19-th century, is made of a combination of letters, numbers and special symbols which describe the type and number of organs, floral symmetry, and other characteristics of a flower (for further details see the paper by G. Prenner et al., Floral formulae updated for routine inclusion in formal taxonomic descriptions, Taxon 59: 241-250, 2010).
The symmetry of flowers
Many flowers do possess radial symmetry. In other words, when a flower with N petals is rotated by 360/N degrees about an axis coaxial to the style (see the above figure), then the flower looks (almost) same. Hence, we say that the flower is radially symmetric or that it possesses rotational symmetry. The flower shown below on the left side is C3-symmetric (it possesses a rotational C3 symmetry axis) while the white daisy shown on the right side has (ideally and without considering the central part of the flower) a very high rotational symmetry that arises from having more than 60 petals. Another interesting aspect of this flower is the presence of a spiral pattern at its center (the yellow region) which is associated to the phenomenon of phyllotaxis (we will discuss it in detail in a separate section). Some flowers such as orchids do possess bilateral symmetry meaning that there is one mirror plane that bisects the style. The white flower with four petals, one of which is larger then the others, has bilateral symmetry owing to a mirror plane bisecting the large petal and the petal opposite to it. Other flowers, on the other hand, are asymmetric; an example of this group is the Canna Indica. Several studies have suggested that floral symmetry is important in the pollination process for pollinator insects do perceive the symmetry of a flower when they approach it along the main rotation axis. For further information on floral symmetry see the following papers:
P.K. Endress, Symmetry in flowers: diversity and evolution, Int. J. Plant Sci. 160: S3-S23, 1999;
Neal et al., Floral symmetry and its role in plant-pollinator systems: terminology, distribution, and hypotheses, Annu. Rev. Ecol. Syst. 29:345–373, 1998.
The term phyllotaxis (or phyllotaxy) refers to the spatial arrangement of leaves on the stem of a plant (see the above drawing). By looking the plant top down along the stem axis, spiral arrangements of the leaves are often observed. Spirals do also appear in the seedheads of flowers such as sunflowers (Helianthus annuus) and daisies (see the above picture). By counting the number of spirals in seedheads, integer numbers (called parastichy numbers) that belong to the Fibonacci sequence 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ... were discovered in the past. Fibonacci (aka Leonardo di Pisa, 1170-1250) discovered the sequence while studying the growth of rabbits' populations and published it in his book Liber Abbaci (1202). Besides botanists, both mathematicians and physicists are also involved in the study of phyllotaxis thus contributing to the development of a mathematical theory of phyllotaxis (see Jeans' book Phyllotaxis: A Systemic Study in Plant Morphogenesis).
A recent citizen science experiment where data from 657 sunflowers were collected and analyzed, seedheads with the parastichy numbers 21, 34, 55, and 89 were the most common. Normally such parastichy numbers do appear in pairs, for example the pair (89,55) indicates a sunflower's seedhead with 89 clockwise spirals and 55 anticlockwise spirals. Interestingly, sunflowers with the non-Fibonacci structures were also observed whereby the number of spirals that are present on their seedheads belong to the Lucas sequence (1, 3, 4, 7, 11, 18, 29, ...), the double Fibonacci sequence (2, 4, 6, 10, 16, 26, ...), and the F4 sequence (1, 4, 5, 9, 14, 23, ...). Another interesting result of this experimental study is that some regions of the seedheads are disordered, meaning that spiral patterns are not present. This might indicate that a phase transition from a disordered to an ordered structure or vice versa could take place during flower morphogenesis.
C. Kuhlemeier, Phyllotaxis, TRENDS in Plant Sci. 12: 143-150, 2007;
M.F. Pennybacker et al., Phyllotaxis: some progress, but a story far from over, Physica D 306: 48-81, 2015;
J. Swinton et al., Novel Fibonacci and non-Fibonacci structure in the sunflower: results of a citizen science experiment, R. Soc. open sci. 3:160091; 2016.
The color of flowers
Flowers display a variety of different colors thence the question: what contributes to flowers coloration? There are two main contributions: (1) the pigments inside the cells of the petals which absorb specific groups of wavelengths (bands) and transmit the non-absorbed ones and, (2) the structure and thickness of the petals which scatters the light. A third, minor contribution to flower coloration is (3) iridescence which arises from nanoscale patterns on the surface of petals.
The blue color of Hydrangea (see above) is due to the presence in the flower's petals of delphinidin-3-glucoside (Myrtillin), an anthocyanin (see below). This molecule is made of two parts connected by a chemical bond, delphinidin (an anthocyanidin) and glucose (a monosaccharide). The delphinidin moiety is rich in pi-electrons which can absorb visible light and transmit the blue component of the spectrum. Another important pigment is beta-carotene (see below), an hydrocarbon with alternate single and double bonds which is responsible for the yellow-orange color of leaves, flowers, and carrots.
Variations in the colors of petals (or sepals) are due to the effect of pH (in other words, how acidic or basic the soil is) and the presence of metal ions in the soil; metal ions such as Ca(2+) or Al(3+) can coordinate one or more pigment molecules and the resulting complexes are characterized by their own colors. For example, besides blue colored Hydrangeas (see above), both pink and white colored Hydrangeas are fairly common as shown in the pictures below (for further details see the paper by M. Kodama et al., Analyses of Coloration-related Components in Hydrangea Sepals Causing Color Variability According to Soil Conditions, Hort. J. 85: 372-379, 2016).
Pigments are often localized on the cells of the adaxial and abaxial sides of the petal, as shown in the cross section below (however, other distribution patterns of pigments are known). The incident radiation (I) is partially reflected by cone cells while one part of it crosses the mesophyll layer and is then transmitted (T) by the abaxial side. The thickness of the petal, which ranges from 100 to 400 micrometers, also affects the wavelength of the transmitted light that our eyes perceive when watching flowers. As far as iridescence is concerned, it was first observed in the flower Hibiscus trionum whose petals are characterized by striations with a separation of about 1.3 micrometers that behave as a diffraction grating toward visible light. Iridescence (structural coloration) is commonly observed in beetles and morpho butterflies both of which display metallic shades.
What about white or black flowers? White flowers do reflect all the wavelengths of the visible spectrum (400-800 nm) and stimulate all the photoreceptor cells (blue, green and red cones) in our eyes thus they appear white to us. However, they do not necessarily appear white to insect pollinators whose eyes are sensible to the UV portion of the electromagnetic spectrum. Black flowers, on the other hand, do not exist in Nature hence botanists selected flowers with high levels of anthocyanins in their petals and through breeding they created "black" tulips and petunias. These flowers actually are dark purple colored but they appear black to our eyes thanks to the unique texture of their petals. If you want to know more about the chemistry and physics of flower coloration, check the following references:
P. Kevan et al., Why are there so many and so few white flowers? Trends Plant Sci. 1:280-284, 1996;
E. Grotewold, The genetics and biochemistry of floral pigments, Annu. Rev. Plant Biol. 57: 761-780, 2006;
K. Yoshida et al., Blue flower color development by anthocyanins: from chemical structure to cell physiology, Nat. Prod. Rep. 26: 884-915, 2009;
S. Vignolini et al., The flower of Hibiscus trionum is both visibly and measurably iridescent, New Phytologist 205: 97-101, 2015.
van der Kooi et al., How to colour a flower: on the optical principles of flower coloration, Proc. R. Soc. B 283: 20160429, 2016.
The fragrance of flowers
Many flowers, albeit not all, are characterized by their distinctive fragrances (scents) which are mixtures of several volatile and non-volatile organic compounds. These substances can be analyzed and identified with the aid of analytical chemistry methods such as gas chromatography (GC), mass spectrometry (MS), and molecular spectroscopy. Let us take as an example the rose flowers (Rosa damascena), the major constituents are the following compounds: citronellol (35.9%), geraniol (25.7%), nerol (3.7%), 2-phenylethanol (1.1%), and rose oxide (0.9%) - see the figure. Also, there are several minor components such as beta-ionone (1.8%) and beta-damascone (0.3%) that contribute to the overall scent of this type of rose flowers. For further details, see: A. Mannschreck and E. von Angerer, The scent of roses and beyond: molecular structures, analysis, and practical applications of odorants, J. Chem. Educ. 88: 1501-1506, 2011.
Books on flowers
Hereafter are the titles of some excellent books devoted to the study of flowers and plant morphology:
Floral Biology by M. Percival, Pergamon Press (1965);
Morphology of Flowers and Inflorescences by F. Weberling, Cambridge University Press (1989);
Plant Form: an Illustrated Guide to Flowering Plant Morphology by A.D. Bell, Oxford University Press (1991);
The Development of Flowers by R.I. Greyson, Oxford University Press (1994);
Phyllotaxis: A Systemic Study in Plant Morphogenesis by R.V. Jean, Cambridge University Press (1994);
Flowers: Evolution of the Floral Architecture of Angiosperms, by G. Tcherkez, Science Publishers Inc. (2004)
Floral Diagrams: An Aid to Understanding Flower Morphology and Evolution by L.P. Ronse De Craene, Cambridge University Press (2010);
The Hidden Geometry of Flowers: Living Rhythms, Form and Number by K. Critchlow, Floris Books (2011);
The Reason for Flowers: their History, Culture, Biology, and How they Change our Lives by S. Buchmann, Scribner (2015).
Floral Mimicry by S.D. Johnson & F.P. Schiestl, Oxford University Press (2016).
Flora: Inside the Secret World of Plants by Smithsonian and The Royal Botanic Gardens, DK (2018).