Erica's Original Notes The following is a compilation of notes on one of the most important components of a plant: leaves. A simple leaf, so varied in design, forms the bond of water, air and sun so that we may live! It is the Perfect Power Plant run on Solar Energy! One never finds a man-eating plant, but all men are plant eaters.
Leaves are used to identify plants, much as our dimples, upturned nose, etc. do in humans. A leaf is 100 percent functional, the dynamo of the waterworks like a nozzle spraying water; it makes food for the plant out of water and air with a patent held by chlorophyll. Chlorophyll is similar to our blood pigment, but instead of having iron in the molecule, it has magnesium and is capable of making carbohydrate from sugar, fructose and starch. There is a major energy investment in spring by deciduous plants to start up their leaf food factories again, dropping them again in the fall when little food is needed, and then the plant goes dormant like a bear in winter.
Palisade Cells: the antennae of the leaf receiving radiant energy from the sun. The floor of the spongy cells has stomata with valves that can inflate and open, or, deflate and close in response to drought and light; they can shut within one-half hour of 'lights out'. They also act as the food factory ventilators. They close at night when photosynthesis stops, breathe out vapour, and adjust automatically to regulate air intake and water loss; they are recessed to conserve moisture and are also protected by hairs. There are one-quarter-million stomata per square inch of an apple leaf. Some leaves, such as eucalyptus, may turn on edge when the sun is too hot at mid-day, so one is unlikely to find shade in a forest of eucalyptus. On a warm day, 2,500 gallons of water may be evaporated per acre of forest, which makes the forests cool in summer. Insects - heat lovers - go to the top of the trees during the day, and, at night, as the forest cools, descend to the forest floor.
Fall: The plugging or corking of supply lines to deciduous leaves isolates the chlorophyll which deteriorates and reveals the other colours in the leaves; shorter daylight hours and colder temperatures trigger the formation of corky cells at the base of each leaf stem (petiole), forming the abscission layer. The leaves of nitrogen-fixing trees rarely change colour and drop their leaves late, as in alder.
Colours: Green colour of a leaf is due to the chlorophyll absorbing the red and blue colours from the spectrum, leaving green. Yellow may be due to: a) carotenoids, e.g. sugar maple and birch; b) xanthophylls - they may even dominate the green chlorophyll in summer; c) flavones, e.g. daffodils, carrots. Red and purple: anthocyanins, red in acid soil, as in maple; purple in alkaline soil, as in ash. This is the pigment found in cranberries, grapes and Cotinus. Brown: tannins, brown from waste cellular products.
Eastern fall colours are more brilliant because the sugars made during the day are not translocated at night to the roots, as the temperature is too low.
Leaf age: seen by comparing the basal leaves with the others. The bright green of the juvenile leaf is due to full light penetration of cells and each chloroplast operating fully; in older leaves, the dull green is due to the thickening of cell walls and accumulation of waste material.
Evergreens: leaves are protected against winter by thick skins (arbutus, ivy) or by size reduction (needles), with resin channels that act as an antifreeze against the formation of ice crystals. Evergreen leaves do fall eventually. Called 'leaf flag,' this varies from family to family; 10 years is the life span of the Araucaria leaf. During the winter with daytime sun, the temperature of leaves can vary as much as 50 degrees.
Plants with no green leaves:saprophytes, such as some orchids on dead or decaying plants.
Defense of leaves may be by: a) irritants such as oxalic acid (nettles); b) thorns; c) ants, fed by special personal nectaries to keep them on guard duty; d) poisons: these may be oxalic crystals as in skunk cabbage and rhubarb, resinoids as in the Ericaceae family or alkaloids found in groundsel, horsetail, bracken, aconite and laburnum.
Poisonous plants: useful to know if you have children or deer! Azalea, boxwood, buttercup, calla lilies, daffodils, daphne, Diefenbachia, hellebores, iris, kalmia, laburnum, lily of the valley, lupines, nerines, oleander, philodendron, privet, rhododendron, snowdrop and wisteria.
Aromatic leaves: Clerodendron - foetid - butternut and walnut - fragrant; Katsura - burnt sugar in the fall; Gaultheria procumbens - wintergreen; Houttuynia cordata - Seville oranges. In conifers there is the smell of terpenes wafting through our forests, giving a blue haze and acting as an insecticide and herbicide.
Stress: leaves are subject to bending stress and are built like bridges, the xylem and phloem forming the joists which enable them to support their weight in rain or snow; the blade can rotate on the petiole, thus avoiding the full force of the wind; the edges are strengthened against laceration.
Leaf shape: very varied and used in identification; A leaf may be simpl,e as in privet, or compound, as in horse chestnut and some Acacia. (See Note 1 below.)
Mosaics: arrangement of pattern in leaves, such as ivy, ensure that there is maximum exposure to light for each leaf.
Edges: usually notched in the northern zone, and smooth in the tropics.
Veins: may be parallel, as in monocotyledons (e.g. grasses, lilies, orchids) or form a net, as in dicotyledons. In rainforest and tropical plants the veins help channel the rain off the drip-tip to prevent fungi forming,
Leaf surfaces: carry a vast population of fungi, yeasts and bacteria existing non-parasitically.
Hairs: protection against too much solar radiation and consequent transpiration; they are usually hollow and contain air.
Stinging hairs: as in the nettle, have a lime- or silica-hardened tip which breaks off on contact, the sharp edge penetrating and formic acid forced into the skin like a needle, explaining why the Romans felt 'warm' after flailing themselves in chilly Britain with nettles!
Spines: stipule or leaf reduction typical of arid conditions and useful as a protection from hungry animals; they also give much needed shade and deflect raindrops from the hot skin of the cactus.
Carnivores: insect-eating plants live in water-logged sites with insufficient nitrogen which they need to make protein. The leaves carry glands on their surface which exude an enzyme that digests insects, as in the sundew (Drosera).
Galls: on leaves, such as oak, are often formed by a wasp laying eggs on a leaf; this irritates the leaf to form a layer rich in protein to feed the developing larva. Galls are used in dyeing.
Movement of leaves: Eucalyptus hangs vertically to avoid the sun; sensitive plants, such as sweet briar, fold when disturbed as a protection against browsing animals; even a cloud may initiate this movement. Some have sleep rhythms: the oxalis folds at night, the nasturtium 'tips' at night owing to the closing down of photosynthesis; the clover leaf also folds when the gland at its base ceases to function.
Chlorophyll: forms in light with germination of seed leaves, but conifer seedlings can manufacture it in the dark.
Snow: care should be taken in removing it from leaves, as it may cause cell damage.
Sounds: leaves are the most articulate part of a tree, singing different songs in the wind to those who are ready to hear.
Tropism: growth response, or 'feeling around' in plants as a response to stimuli outside the plant. Geotropism is movement down towards the earth, (a response to pull of gravity) as in roots. Phototropism is in response to light. The bending results from differences in growth rates of different parts of the plant; it may be positive (towards stimulus) or negative (away from stimulus). The study of tropism led to the discovery of auxins (see also Plant Hormones).
Thigmotropism: term used for vines that 'try' to twine in order to get support and more light for its leaves; the stem ridges rub against a structure, giving sensory information to the plant to twine. This circumnutation was first noticed by Darwin who did an experiment with a bean seedling over which he had placed a glass plate, noting the changing position of the bean tip each hour. There are different rates of making these turns: it takes a hop vine 2 hours to make a revolution, a trumpet vine 6-1/2 hours, honeysuckle 8 hours, but the passion flower, true to its name, starts turning immediately! (See Note 2 below.)
NOTES Updated 2013 (RP):
1. If you would like to see the variety of leaves and botanical descriptions a good starting point with good illustrations is The Cambridge Illustrated Glossary of Botanical Terms (2000) by Michael Hickey and Clive King. (This is a reference book in the Garden library.)
2. The Power of Movement in Plants (1880) Charles Darwin