The colours found in the flowers and leaves of flowering plants [Angiosperms] can be ascribed to four major 'families' of pigments; the chlorophylls, carotenoids, flavonoids and betalains.  The chlorophylls are perhaps the most familiar as they are the main photosynthetic pigments, absorbing blue and red wavelengths of light. Chlorophyll in flowers is relatively unusual.  Indeed, green flowers are quite rare and often associated with wind pollination. Examples of green flowers include some species of Euphorbia, Hedera and Fritillaria. Green flowers, despite their less conspicuous nature, can still attract insect pollinators.  This is due partly to differences in light scattering and brightness (achromatic contrast) as revealed recently by researchers at the Univeristy of Seville. The carotenoids are pigments belonging to the isoprenoid group of chemicals.  They are commonly present in flowers, absorbing mainly blue wavelengths of light. They lend yellow, orange and very occasionally red colour to flowers.  Carotenoids are the petal pigments of many yellow-flowered plants of the Daisy (Asteraceae) and Bean (Fabaceae) families.  The flavonoids offer the most diverse range of pigments.  They are water-soluble polyphenols found in nearly all vascular plants. They are located in the vacuoles of cells.  Certain flavonoid groups, such as, the catechins, flavonols, flavones, isoflavones absorb in the ultraviolet region of the spectrum.  They are invisible to humans but can be recognised by many bees, flies, butterflies and most birds.  The anthocyanins, also part of the flavonoid group, absorb green light and reflect shades of purple, blue, and red. They occur in many tissues of flowering plants, including leaves, roots, and fruits (think blueberries and raspberries). The last group are the betalains. The name derives from the beetroot (Beta vulgaris).  They are nitrogen containing compounds, derived from the amino acid tyrosine. Betalains give rise to yellow to pink and red colours. The deep red-purple colour of beets, bougainvillea, amaranth, and many cacti comes from certain betalain pigments. Interestingly, plants that produce betalain pigments do not form anthocyanins.   Apart from these four major pigment types, other rarer pigments do exist. For example, the xanthones found in some species of irises.  Flower colours may be generated from one specific pigment or through the combination of different pigments. Thus, red petals may be result of red anthocyanins, or red betalains, red carotenoids, or even by the combination of orange carotenoids with purple anthocyanins.  The carotenoids and chlorophylls are stored in chromoplasts and chloroplasts of the petals respectively. Chromoplasts are membrane-bound, fluid filled  vesicles in which pigments may be stored.  Flavonoids and betalains, which are water-soluble compounds, are found in the vacuoles of cells. White petals result from the absence of coloured pigments and thus reflect all wavelengths of visible light, though UV light may be absorbed.  Most plants have a distinctive flower colour that is stable, despite the vagaries of climate.  Sometimes the flower colour can darken or even change.  For example, the colour may deepen over time or even alter.  The Purple Mistress [Moricandia arvensis, found in the mediterranean region] has lilac coloured flowers in spring, but these change to white flowers in the summer. [caption id="attachment_42267" align="aligncenter" width="675"] Iris[/caption]  

Trees and shrubs that grow in temperate regions, where the seasons alternate (warm / cold, dry /wet) create annual rings.  The rings formed in a deciduous tree (like beech, oak, lime) are generally quite noticeable when the tree is felled.  They may be counted to give an indication of the age of the tree.  Annual rings are formed because there is a difference in the creation of ‘wood’ / xylem tissue when growth is fast in the Spring and slow as Autumn progresses.  The thickness of the rings from year to year reflects the changing climate and environment that the tree experiences during its life. Xylem tissue is one component of a tree’s vascular tissue.  The xylem tissue conducts water and minerals around the plant, whereas phloem tissue transports sugars and other organic molecules.  Lying between these two tissues is the cambium.  This is a layer of dividing cells, which becomes active in the Spring forming new cells some of which will form new phloem tissue and others develop into xylem tissue. The cells that will form the xylem tissue undergo a series of dramatic changes.   The walls of the cells that will form the long tubes of the xylem are made of cellulose to begin with, but then they are strengthened with lignin.  Lignin is the ‘stuff of wood’.  It is a complex material - made from polyphenols and other substances such as pectins and hemicelluloses.  It is a waterproofing material that is highly resistance to decay.  It lines the tubes of the xylem so that water can be transported from the roots, up the trunk / stem to the leaves etc.  The xylem vessels that form in the Spring [early wood] have a greater diameter than those formed later in the year [late wood].  It is this size difference in the vessels that results in the visible ‘rings’ when a tree is felled. Careful study of tree rings can reveal information about climate, sometimes extending back through the centuries   using species such as the long lived Bristlecones. It has given rise to the discipline of dendrochronology [link opens / downloads a PDF].  This information can then be ‘combined’ with tree ring data from intact remains in cold, dry (and often high altitude) environments and material from archaeological sites.   Apart from measuring the ‘width’ of the annual rings by creating thin section of the wood that can be examined under microscope, it is also possible to use staining techniques to reveal which xylem tissue has a higher / lower, lignin / cellulose content.  By using a double staining technique with the dyes Safranin and Astra Blue, it is possible to identify which xylem vessels are rich in lignin, and which have more cellulose.  Tree rings which stain largely blue are formed from cells which have not lignified properly.  Lignin stains red.  A recent study of blue rings in Pine trees and Juniper shrubs suggests that blue rings are indicators of cold summers. These two species are typical of the upper tree line in Northern Norway. Furthermore, blue rings have the potential to weaken the pine trees, leaving them more susceptible to mechanical damage and / or disease.  This study has identified blue rings associated with the cold summers of 1877 and 1902, which might have been caused by the eruptions of volcanoes as far away as Ecuador and Martinique. Note : The xylem tissue in conifers is different to that of broad leaved deciduous trees.  It is made up of shorter structures called tracheids, which pass water from one to the next via pits - ‘pores’ in their lignified walls. For more information on Blue rings in Black Pine, click here  

We come across tannins in various foods and drinks.   They contribute to the taste of a cup of tea, a mug of coffee or a glass of wine.  Tannins contribute an astringent taste - a sensation of “dryness” in the mouth. Tannins are molecules made by plants - they are complex polyphenols built from several phenolic molecules.   A phenol is  made from a hexagon-shaped carbon ring with one or more hydroxyl groups (-OH) attached to it (see diagram). They are generally water-soluble molecules and they can combine with proteins, cellulose, pectins etc.  Tannins are generally stored in the vacuole of plant cells (as are various oils, resins, crystals of calcium oxalate etc).  Collectively, these materials are sometimes referred to as ergastic substances. Read more...