Plant Kingdom, Morphology and Physiology

Plants did not appear all at once. They came out of water, learned to live on damp land, then grew vessels to carry water upward, then made seeds, and finally wrapped those seeds inside fruits. The Plant Kingdom (Plantae) follows exactly this story. Plant cells have a cellulose cell wall and chloroplasts, so plants are multicellular, photosynthetic autotrophs. Fungi, by contrast, have a chitin wall and bacteria a peptidoglycan wall.

The first three groups do not make seeds or flowers, so they are called cryptogams (hidden reproduction). Gymnosperms and angiosperms make seeds and are called phanerogams. Another handy split: bryophytes have no vascular tissue, while pteridophytes, gymnosperms and angiosperms are vascular plants (tracheophytes).

Why bryophytes and pteridophytes stay near water: their male gametes are swimming cells, and they can only reach the egg through a film of water. Seed plants solved this problem with pollen, which travels by wind, insects or water, and that freedom let them conquer dry land.

Now the biggest group. Angiosperms are divided into monocots and dicots by the number of seed leaves (cotyledons) in the seed. Once you remember that one difference, the rest of the table follows.

Plants also take special forms to match where they live or how they feed. Learn these pairs, because "Which of the following is correctly matched?" is a favourite framing.

Morphology means the outer form of a plant. A flowering plant has two systems: the root system below the ground, which anchors the plant and absorbs water and minerals, and the shoot system above it (stem, leaves, flowers, fruits), which makes food and reproduces. The key idea for this topic is simple: when a plant part has to do a second job, it changes its shape. That change is called a modification, and examiners love it.

The root. A root has no nodes and no leaves, and it grows towards gravity and water. Near the tip lie the root cap (protects the tip), the zone of cell division, the zone of elongation and the root-hair zone, where thin hairs absorb water. Dicots have a tap root (one main root with branches, from the radicle); monocots have a fibrous root (many equal roots). Roots that grow from any part other than the radicle, such as from a stem, are adventitious.

The stem. The stem bears nodes (where leaves arise) and internodes between them, along with buds. Think of it as the highway that carries water up and food down. Many stems turn into food stores or helpers.

The leaf. A typical leaf has a flat green blade called the lamina, a stalk (petiole) and a leaf base. The central thick line is the midrib, and the pattern of veins is the venation: parallel in monocots, reticulate in dicots. A simple leaf has one blade (mango); a compound leaf has a blade cut into leaflets (neem is pinnate, silk cotton is palmate). The arrangement of leaves on a stem is phyllotaxy: alternate (mustard, China rose), opposite (guava, Calotropis) or whorled (Alstonia, oleander).

Plasticity and heterophylly. A plant can change its form to suit its surroundings. When one plant bears leaves of different shapes depending on the environment, as in cotton, coriander and larkspur (or aquatic plants with submerged and floating leaves), it is called heterophylly, a classic example of plasticity. Phototropism is a movement, while vernalisation and photoperiodism are timing responses; they are not plasticity.

The flower. A flower is the reproductive organ of an angiosperm. Its four whorls, from outside to inside, are the calyx (sepals, usually green, protect the bud), the corolla (petals, coloured, attract insects), the androecium (male part: stamens made of filament and anther, which holds pollen) and the gynoecium or pistil (female part: stigma, style and ovary, which holds ovules). A flower with all four whorls is complete; one with both stamens and pistil is bisexual. After fertilisation the ovary becomes the fruit and the ovules become seeds.

The fruit. A true fruit develops only from the ovary (mango, tomato). A false fruit (accessory fruit) develops with other parts too, such as the thalamus in apple and pear. A fruit that forms without fertilisation, and so has no seeds, is parthenocarpic (banana, seedless grapes). Fruits are also grouped by how they form: simple (from one flower with one ovary: mango), aggregate (from one flower with many ovaries: strawberry, custard apple) and composite or multiple (from a whole inflorescence: pineapple, jackfruit, mulberry).

Every meal you have ever eaten began as sunlight caught by a leaf. Photosynthesis is the process in which green plants, algae and some bacteria use light energy to turn carbon dioxide and water into glucose and release oxygen. This is why the Sun is the primary source of energy for all life on Earth, and why plants are called producers.

6CO2 + 6H2O + light energy (with chlorophyll) gives C6H12O6 (glucose) + 6O2. It is an anabolic, endothermic process: light energy is stored as chemical energy in food. Carbon dioxide enters through stomata, water comes up from the roots, and the leaf is the main factory. Green stems and even the guard cells also photosynthesise.

Four things are needed: sunlight, chlorophyll, carbon dioxide and water. The pigment hunts for light. Chlorophyll a is the main pigment and absorbs mostly blue and red light; it reflects green, which is why leaves look green. Helper pigments (chlorophyll b, carotenoids, xanthophylls) pass the energy to chlorophyll a. Chlorophyll contains magnesium, and it sits inside the chloroplasts.

Photosynthesis runs in two linked stages inside the chloroplast. The light reaction happens in the grana and needs light: chlorophyll captures light, water is split (photolysis) to give oxygen, and the energy is stored in two small carriers, ATP and NADPH. The dark reaction (Calvin cycle) happens in the stroma and does not need light directly: the enzyme RuBisCO fixes carbon dioxide and, using the ATP and NADPH, builds sugar. So the first step of photosynthesis is the absorption of light by chlorophyll.

Three ways of fixing carbon. Most plants (rice, wheat, gram) are C3 plants: the first stable product is a 3-carbon compound (3-PGA). C4 plants (maize, sugarcane, sorghum) first make a 4-carbon compound (oxaloacetic acid), have a special leaf anatomy called Kranz anatomy, and photosynthesise very efficiently in hot, bright conditions. CAM plants (Opuntia, pineapple, Agave, Bryophyllum) live in dry places: they open stomata at night to take in CO2 and store it as malic acid, and use it during the day with stomata closed to save water.

Blackman's law of limiting factors: when several factors affect a process, the rate is controlled by the one in shortest supply. If a leaf has plenty of light but little CO2, adding more light will not help.

A tall tree lifts hundreds of litres of water from the soil to its topmost leaf every day, with no heart and no pump. How? The trick is that the leaves do the pulling. Follow the water step by step: it enters the root hairs, climbs through the xylem, and leaves as vapour through the stomata. Meanwhile the food made in the leaves travels down through the phloem.

Osmosis is the movement of water through a semi-permeable membrane from a region of high water concentration (dilute solution) to low (concentrated solution). Root hairs have a more concentrated cell sap than the soil water, so water flows in. A cell that is full of water is turgid; when it loses water the cell shrinks away from the wall (plasmolysis) and the plant wilts. Diffusion is the spreading of molecules (like CO2 and O2) from high to low concentration.

How does water rise? Two forces work together. Root pressure is a gentle push from the roots; it is strongest at night and in the morning, and it causes guttation, the oozing of water drops from leaf tips through special pores (hydathodes). But the main force during the day is transpiration pull: as water evaporates from the leaves, it creates a pull that draws the whole water column up, because water molecules stick together (cohesion) and to the vessel walls (adhesion). This is the cohesion-tension theory (Dixon and Joly, 1894). Root pressure alone cannot lift water to the top of a tall tree.

Transpiration is the loss of water as vapour from the aerial parts of a plant, mostly through stomata (stomatal transpiration, the largest share), and also through the cuticle and the lenticels of woody stems. It looks wasteful, but it does three useful jobs: it creates the pull that lifts water and dissolved minerals, it cools the leaf, and it keeps the leaf cells supplied with water. Most of the water a plant absorbs is lost this way; only a small part is used in photosynthesis. A plant prefers to lose some water rather than starve of CO2, because the same pores must open for both.

Stomata are tiny pores in the leaf epidermis, each guarded by two guard cells which open and close it. Guard cells are the only epidermal cells with chloroplasts. In light they take in potassium ions and water, swell (become turgid) and bend apart, so the pore opens; at night, or when the plant is short of water, they lose water and the pore closes. The hormone abscisic acid (ABA) closes stomata during drought. Guard cells are kidney-shaped in dicots and dumb-bell-shaped in grasses.

Transport of food: translocation. Soluble food made in the leaves (mainly sucrose) is carried to every part of the plant by the phloem. This is called translocation. The phloem has sieve tubes (living cells with pores in the end walls) and companion cells that supply energy. Food moves from the source (leaf) to the sink (root, fruit, growing tip), in both directions, and it needs energy, unlike the one-way upward flow in xylem. If a ring of bark is cut off (girdling), food piles up and the stem swells above the cut, which proves that the phloem lies in the bark.

Photosynthesis stores energy; respiration releases it. Every living cell of a plant, in roots, stems, leaves and seeds, breaks down food (mostly glucose) to make ATP, the energy currency of the cell. This goes on day and night. A plant has no lungs, so each part exchanges gases directly: leaves through stomata, woody stems through lenticels, and roots through the air spaces in the soil (which is why waterlogged roots suffocate).

Glucose + oxygen gives carbon dioxide + water + energy (ATP). The first stage, glycolysis, happens in the cytoplasm without oxygen: one glucose (6 carbon) splits into two pyruvate (3 carbon) with a net gain of 2 ATP. If oxygen is present, pyruvate enters the mitochondria for the Krebs cycle and electron transport chain, which release most of the ATP (aerobic respiration). If oxygen is absent, fermentation follows (anaerobic respiration) and the yield is small.

Compensation point: the light intensity at which the CO2 used in photosynthesis exactly equals the CO2 given out in respiration, so there is no net exchange of gases. Below it the plant is losing more food than it makes. At night, with no photosynthesis, a plant takes in oxygen and gives out only CO2.

What do plants use and store? The food made in photosynthesis is moved around as sucrose and stored mainly as starch, a large polysaccharide made of long chains of glucose, in roots, tubers and seeds. Seeds of groundnut, mustard and castor store fats (oils), and pulses store proteins. Cellulose, another glucose polymer, is not stored for energy but builds the cell wall. Plants store starch, while animals store glycogen.

A plant cannot run, but it can grow towards light, bend away from danger and ripen fruit on time. It does this with chemical messengers called plant hormones (plant growth regulators). They are made in tiny amounts in one part, act on cells in the same or another part, and work at very low concentration. There is no nerve in a plant, so hormones and electrical-chemical signals do the coordination.

Apical dominance: the growing tip makes auxin, which flows down and stops the side buds from growing. Cut the tip (pruning a hedge) and the side buds sprout, so the plant becomes bushy. Cytokinin pushes the other way and encourages side buds to grow, so the balance between the two decides the shape of the plant.

Plant movements. A tropism is a growth movement whose direction is set by the stimulus. A nastic movement is a response where the stimulus does not decide the direction; for example, a flower opening in the morning. Growth bends happen because one side grows faster than the other. When a shoot gets light from one side, auxin collects on the shaded side and makes those cells elongate more, so the shoot bends towards the light. Roots are more sensitive: the same extra auxin slows root cells, so roots grow away from light and down towards gravity.

Plants make sugar from air and water, but sugar alone cannot build proteins, DNA or chlorophyll. For that they need mineral nutrients from the soil. An element is called essential when the plant cannot complete its life cycle without it, nothing else can replace it, and it acts directly in the plant's body. Carbon, hydrogen and oxygen come from air and water; all the others are taken from the soil.

Mobile and immobile elements. Nitrogen, phosphorus, potassium and magnesium are moved out of old leaves to feed new growth, so their deficiency shows first in the older leaves. Calcium, iron and sulphur are fixed in place, so their deficiency shows in the young leaves. It is a neat way to guess the missing element from the picture.

The nitrogen puzzle. Air is about 78 per cent nitrogen gas (N2), yet plants starve for nitrogen. The reason is that the N2 molecule has a very strong triple bond, and plants can only take nitrogen as nitrate (NO3-) or ammonium (NH4+) ions. So nitrogen has to be "fixed", that is, converted into these forms. Only some microbes have the enzyme nitrogenase (which contains molybdenum and iron) to do this. In the soil, nitrogen is mainly supplied by decomposition of dead organic matter and by these bacteria; phosphorus comes mainly from the weathering of rocks.

Farming uses this knowledge. In crop rotation, farmers grow a legume such as gram, pea or a fodder legume in one season and wheat in the next; the legume leaves behind nitrogen that the wheat can use, so less fertiliser is needed. Green manuring with dhaincha (Sesbania) or sunn hemp does the same. Biofertilisers (Rhizobium, Azotobacter, Azospirillum, blue-green algae, Azolla; and mycorrhiza for phosphorus) are cheap, eco-friendly alternatives to chemical fertilisers. Hydroponics is growing plants in a nutrient solution without soil, which is how scientists proved which elements are essential.