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CHAPTER TWO

BOTANY FOR BEGINNERS


WHAT IS BOTANY? Botany – the scientific study of plants and flowers – leads to greater understanding of our botanical world and all of the complex ecosystems it supports. Historically, this field was regarded as key to decoding the very purpose of life. Today more than ever, a deeper understanding of nature is vital to Earth’s shared future.

In 1648, the results of a five-year experiment by the Flemish physician Jan Baptist van Helmont (1580–1644) were published by his son Franciscus Mercurius van Helmont. These results would help answer a mystery that had baffled philosophers and scientists for centuries – how exactly do plants grow? Van Helmont’s experiment, conducted while under house arrest by the Spanish Inquisition, involved a willow tree, a pot, a tin-coated iron lid and a lot of diligent watering and weighing, the aim being to prove or disprove a theory first proposed by Ancient Greek teachings that claimed plants grew by ‘eating’ soil (see page 111). His conclusion – that plants in fact grew by drinking water – was actually inaccurate, and officially rejected in the late 1600s by John Woodward, a professor and physician at Cambridge University. Some 130 years later, in 1779, on the back of the work of English chemist Joseph Priestley, the Dutch physiologist Jan Ingenhousz (1730–1799) was still trying to crack the question about how plants grow and the effect that they had on their environment. Two and a half centuries on, although much more is known about the process of photosynthesis (see page 111), scientists are still exploring its microscopic wonders and how we can harness its life-giving powers. What van Helmont, Ingenhousz et al. were searching for was, in fact, much bigger than the question of how plants grow. They wanted to know how the world worked and find scientific proof for how it had been created. They suspected, as we now know, that the Plant Kingdom was in part

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responsible for all life on Earth and, as Charles Darwin speculated in his On the Origin of Species (1859), that plants were also continuously evolving. This then is botany, the study of plant life from the tiny powerhouses of plant cells to the magnitude of giant sequoia trees. It investigates what plants are, how they appear (see page 90), what they do (see page 110), what they ‘know’ (see page 118), how they interact with all other life on Earth and what we need to do to conserve them (see page 143). It also strives to promote consistent ways in which plant knowledge can be recorded and shared (see page 123). As far as plants and people are concerned, Professor Kathy Willis, Director of Science at Royal Botanic Gardens, Kew, has it in a nutshell: ‘A detailed knowledge of plants is fundamental to human life on Earth. Plants underpin all aspects of our everyday life – from the food that we eat, to the clothes that we wear, the materials we use, the air we breathe, the medicines we take and much more.’ Botany is, as Willis continues, ‘the unique combination of beauty and science which can together provide some of the solutions for the global challenges facing humanity today’ – all of which can be a lot more fun in the context of a plant-based activity you already enjoy or are passionate about from gardening, cooking and wellbeing (see chapters 3–5) to botanical art, craft or design (see chapter 6). Opposite  Observe the number and form of a tulip’s petals, sepals, stamens, carpels and ovary to find distinguishing features of both the Tulipa genus and its plant family, Liliaceae.


UNDER THE MICROSCOPE How do some petals appear velvety? How does a leaf send oxygen into the world? Does a plant have a brain or central processing unit? What can flower fossils tell us about the world we live in today? Microscopy gives us the potential to look beneath the surface appearance of things – at cells, tissues and even gene expression – to look closer at the hidden complexities of plants and flowers and the questions posed by them, and to take plant exploration into exciting new realms, perhaps even the future.

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UNDER THE MICROSCOPE

Cells

Tissues

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ooking at the Pathways of the Plant Kingdom (see page 24) reminds us that plants, animals and fungi are all eukaryotes – their cells are made up of membrane-bound organelles including a nucleus. So what sets plant cells apart? Cell wall This thick, rigid structure contains fibres of cellulose and provides protection, support and a permanent rectangular shape. Immediately inside of the cell wall is a selectively permeable membrane that surrounds a jelly-like cytoplasm. Within it are the organelles – specialised structures within the cell that perform specific and varied functions. Nucleus The nucleus contains the major portion of the plant’s DNA, composed of thread-like chromosomes made up of nucleic acids and protein. This hereditary information, in the form of genes, directs how plants appear, develop and behave. Chloroplasts Oval-shaped chloroplasts are exclusive to plants. They house stacks (grana) of thylakoid discs, on which sit the photosynthesising green pigment, chlorophyll. Energyrich molecules then move to nearby stroma, where carbon can be fixed and sugars synthesised. Mitochondria Rod-shaped mitochondria are relatively smaller and more plentiful. They use oxygen to release energy from sugars through aerobic respiration. Vacuole The largest, fluid-filled space is a vacuole. It helps keep the cell turgid, stores water and segregates waste. Endoplasmic reticulum This rapidly changing network of membranes connected to the nucleus is responsible for the synthesis and often storage of proteins and lipids.

lants grow and reproduce through a process known as cell division, either via mitosis or meiosis. This allows the cells to specialise, store nutrients and be replaced if damaged. Each specialised plant cell adapts its contents to perform a particular function. Certain cells then cluster together to form dermal (exterior surfaces), ground (nutrient manufacturing and storing) and vascular (transport) tissues. Plant tissues can also be divided into two types: meristematic tissue, with actively dividing cells that lead to growth; and permanent tissue, where cells have specific roles. Cell division Mitosis facilitates growth and repair and produces two diploid cells (cells with a full set of chromosomes) that are identical to each other and to the parent. Meiosis enables sexual reproduction and produces four haploid cells (cells with half the number of chromosomes) that are different to each other and to the parent. These cells become either the male pollen grains or the female egg cells.

Cell types 0

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Opposite Artwork of a plant cell shows a cell wall (outer band), chlorophyllfilled chloroplasts (large green ovals), a large central vacuole, a nucleus (in pink), mitochondria (small orange ovals) and peroxisomes (small blue spheres).

 arenchyma cells have thin primary cell walls and P large vacuoles; they carry out photosynthesis, stay alive when mature and provide the bulk of the soft part of plants.  clerenchyma cells have thick secondary cell walls S including lignin. They provide support and strength to roots, stems and vascular tissue and they die when mature.  ollenchyma cells have thin primary cell walls C with some secondary thickening and provide additional stretchable support, ideal for areas of new growth.  ater-conducting cells are narrow, elongated and W hollow. They die when mature but their cell walls remain, allowing free water flow within the xylem.  ieve tube elements are elongated, living cells S that transport carbohydrates in the phloem. They rely on companion cells for missing parts.

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WHAT DO PLANTS DO?

Respiration

Transpiration

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Why plants respire It’s helpful, first of all, to separate breathing from respiration. Breathing refers to the muscular movement of animals that sends oxygen to respiratory organs and removes carbon dioxide from them. Respiration is a biochemical process by which organic compounds are oxidised to liberate chemical food energy. When plants respire, they use oxygen to release energy from photosynthesised sugars, creating the byproducts of carbon dioxide and water vapour.

The profit of water loss If plants need water for photosynthesis, then why lose water through transpiration? One major profit – according to the Cohesion Theory of Sap Ascent – is that transpiration causes a sucking force when water at the top of xylem channels evaporates. Like sucking on a straw, it causes a negative pressure, which lifts the xylem sap to the leaf surface. Some of the sap’s water will be lost through stomata via transpiration but some water, complete with nutrients, will be used for photosynthesis. As transpiration occurs, plants cool down, making a better environment for growth.

espiration and transpiration don’t get nearly as much airtime as photosynthesis but they’re a vital team of processes that support the plant’s development. Respiration creates energy, allowing growth.

glucose + oxygen

carbon dioxide + water (+ energy)

How plants respire Plants carry out respiration by taking in oxygen from the air via the stomata in leaves and stems. This goes on day and night, although at a proportionally higher level in the dark when photosynthesis stops and carbon dioxide is generally not consumed. The oxidisation of sugars then takes place within tiny intra-cellular organelles called mitochondria. The resultant cellular energy creates the building blocks of new cells, allowing the plant to grow.

ranspiration is both the process of water movement through a plant and the evaporation of water vapour from aerial plant parts, such as leaves, stems and flowers.

Transpiration factors Many plants have adapted their design to create the perfect balance of photosynthesis, respiration and transpiration. Plants with more or bigger leaves and stomata will exhibit greater transpiration. This works for larger or taller plants that need more sucking pressure to distribute water to their various parts. To reduce evaporation and procure shade, some plant species, such as cacti (pictured below), have evolved spiny, photosynthesising, waxy stems in the place of true leaves, along with the added facility to store water.

Opposite Mariposa Grove in Yosemite National Park is home to some of the world’s largest giant sequoia (Sequoiadendron giganteum), which use transpiration to pull water and nutrients from root to towering crown.

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COMPELLED TO CLASSIFY

The need to name

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he often overlooked beauty of binomial nomenclature is that many features associated with common names (such as allusion to appearance, use, mythology, habitat, history or discovery) are intrinsic parts of botanical names, too. The fact that botanical names are written in Latin (though daunting at first) bestows the additional superpower of making these names universal. Thus French-speaking plant lovers can discuss specimens with those from India; plant scientists can discuss their findings with paleobotanists; gardeners can more easily choose preferred species for their plots; lists can be compiled of rare or protected plants; and publications can be produced for a global audience. Of course, there’s certainly a charm in referring to plants by their common names – names that conjure up childhood walks through the countryside when such names were first learned, or that beautifully describe how a plant looks or behaves in language that is easily understood – but it can also be immensely confusing and sometimes even dangerous. Anthriscus sylvestris, for example, has numerous nicknames (cow parsley, mother die, wild chervil, wild-beaked parsley and keck among them), some of which are also used to describe other plants as well. Its lacy, umbelliferous flowers are also easy to confuse with Queen Anne’s lace or wild carrot (both common names actually refer to Daucus carota), edible parsley (Petroselinum crispum) and carrot (Daucus carota subsp. sativus), plus species of skin-irritating hogweed (Heracleum) and the highly toxic, absolutely not edible hemlock (Conium maculatum). Before the universal adoption of Carl Linnaeus’s system of binomial nomenclature (using a genus name followed by a species name), plant names could also be extremely lengthy: Plantago foliis ovato-lanceolatis pubescentibus, spica cylindrica, scapo tereti meaning ‘plantain with pubescent ovate-lanceolate leaves, a cylindric spike and a terete scape’, for example. In the binomial system this plant is now referred to as Plantago media, in line with the current governing body for botanical plant names, the International Code of Nomenclature for algae, fungi, and plants (ICN). Following in the footsteps of the Lois de la nomenclature botanique – the ‘best guide to follow for botanical nomenclature’ – established in 1867 at the International Botanical Congress, in Paris, its guidelines

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are vital for the standardisation of botanical names. These can then be shared through resources, such as the International Plant Names Index (IPNI), which shows who named the plant and when, or The Plant List (www.theplantlist.org), a working database of all the known plant species in the world, created as a response to the Global Strategy for Plant Conservation (GSPC) to help conserve biodiversity. As per the descriptive language used by early botanists, ICN-governed names are in Latin, although often heavily drawn from Greek or other languages. Each assigned name then includes an initial-capped genus name (see page 130) followed by a lowercase species name (see page 133), the whole written in italics. Subspecies, variety, form, cultivar and hybrid (see page 134) may also be included, each written according to the ICN code. It’s also helpful to know that the genus name is a noun and the species name usually an adjective. This is where the romance lies – scientific names, like common names, tell stories. The word Rosa, for example, probably stemmed from an Ancient Greek word for the rose plant, while canina (see diagram on page 120) translates as dog, from the belief that the root was effective against the bite of a mad dog. Rosa x damascena is a hybrid of Rosa gallica (French rose), Rosa moschata (musk rose) and Rosa fedtschenkoana (a mountain rose named after Russian botanist Olga Fedtschenko) that was introduced to Europe from Damascus, Syria. Meanwhile the story behind the naming and indeed creation of the hugely popular Rosa ‘Madame A. Meilland’, or Peace rose, stems from cuttings sent abroad by French horticulturist Francis Meilland during World War II. Successfully propagated and then introduced as a cultivar in 1945, the Peace rose – a pale-yellow, pink-tinged hybrid tea rose – was given to all delegations of the inaugural meeting of the United Nations in San Francisco. The accompanying note read: ‘We hope the “Peace” rose will influence men’s thoughts for everlasting world peace.’

Opposite The Flowering Plants and Ferns of Great Britain (1846) illustrates similarities between cow parsnip (Heracleum maximum), species of hartwort (Tordylium), coriander (Coriandrum sativum) and hemlock (Conium maculatum).


COMPELLED TO CLASSIFY

Species

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he English naturalist John Ray was the first to give the concept of species a biological definition in his Historia Plantarum (1686): ‘ . . . no surer criterion for determining species has occurred to me than the distinguishing features that perpetuate themselves in propagation from seed. Thus, no matter what variations occur in the individuals or the species, if they spring from the seed of one and the same plant, they are accidental variations and not such as to distinguish a species . . . Animals likewise that differ specifically preserve their distinct species permanently; one species never springs from the seed of another nor vice versa.’ In short, species bred true and did not change. For basic botanical purposes, species is the basic unit (smallest taxon) of plant taxonomy and in most cases denotes organisms that breed with each other (the potential for hybrids blurs the boundaries somewhat). Species ranks below genus and, in common parlance, refers both to the specific epithet that follows the genus in most two-part botanical names and to the full two-part name. Thus the striking magnolia species also known as bull bay or southern magnolia (Magnolia grandiflora) is written out as the genus Magnolia followed by the specific epithet grandiflora. The specific epithet is an adjective, as most species names are, derived from the Latin words grandis, meaning ‘big’ and flor, meaning ‘flower’. Other species in the Magnolia genus therefore need different combinations of names to set them apart: Magnolia stellata (star magnolia) or Magnolia macrophylla (bigleaf magnolia), for example. Once species names from across the Plant Kingdom start to become more familiar, it’s not unusual to come across the same specific epithet being used to describe different genera. Hydrangea macrophylla shares its second name with Magnolia macrophylla but the two are distinct species validated by the precedence of their genus name. Often such epithets refer to structure (arboreus = tree-like), form (caespitosus = tufted), habit (altus = tall), colour (alba = white) or habitat (alpinus = alpine) but

some reference people (hookeri = after William J. Hooker) or places (japonica = from Japan). Not only are species names vital for the often ultimate identification of a plant, they can be a fun way to learn more about the plants, too. Just remember to keep the genus name initial-capped, the species name lowercased and both italicised when put together to impress fellow speciesloving colleagues or friends.

MAKE THE CONNECTION

Commonly known specific epithets: (the second part of the species name): annua (annual); arvensis (of the field); borealis (from the north); digitata (leaves like a hand); elegans (elegant); floribundus (free-flowering); grandiflora (largeflowered); grandis (large); leiocarpus (with smooth fruits); mexicana (from Mexico); multicaulis (with many stems); nanus (dwarf); nocturna (nocturnal); odoratus (fragrant); officinalis (with herbal uses); orientalis (eastern); peregrinus (exotic); perfectus (complete); purpuratus (purple); racemosus (with flowers in racemes); rudis (wild); sanguineus (blood-red); sempervirens (evergreen); simplex (unbranched); stellaris (star-like); sylvestris (found wild); tinctorius (used for dyeing); undulatus (wavy); variegatus (variegated); virens (green); viridescens (becoming green); vulgaris (common)

Opposite Magnolia is an ancient genus of more than 200 species, named after the French botanist Pierre Magnol. Right The Orchidaceae family, meanwhile, boasts 20,000–30,000 different species, including 60 or so kinds of moth orchid.

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