

Glossary of plant terms

Or browse the descriptions and definitions using the A-Z.

oak gall wasps

Produce galls on leaves by various gall wasps.

oak phylloxera

In late spring, yellow spots develop on upper surface of oak which later turn white-brown and dry up.

oak powdery mildew

Rust coloured spots develop on young leaves which then form a layer of white fungal spores.

ob ...

This is a prefix to a word which generally means "inverted" or "unequal".


Inverted conical shape.


Inverted heart shape.


Narrow, base tapering, widest in distal (apical) half.

obligate parasite

This is a parasite which depends solely on one host for its life support.


Unequally sided.


Means "unequal-sided" in Latin.


Longer than wide with the widest part in the centre. Similar elliptic or oval.


Means "oblong" in Latin.


With the widest part in the distal (apical) half.


Blunt, as in the apex of a leaf or petal.


Can describe a leaf whose base is narrower than its tip.


Means "western" in Latin.


Meaning "completely closed in", such as where a wound in a tree bark has become totally calloused over.


Means "hidden" in Latin.


Having 8 sets of chromosomes.


Having a fragrance (implying that the fragrance is pleasant).


Means "odorous - fragrant" in Latin.


Having a smell or fragrance.


Sometimes known as dropsy, this is a disorder where leaves and stems develop wart-like growths. It is usually due to over-watering and excessive humidity, often where plants are grown in a greenhouse. This may occur in plants such as peppers, tomatoes, begonias, and pelargoniums.


Means "used in medicines" in Latin.


Side shoot capable of producing a new plant. This small plantlet arises by natural vegetative reproduction, usually at the base of the mother plant. (It is often used for propagation where the offset is removed and potted up for growing on.)


Plants produce various oils usually for food storage purposes. "Vegetable oils" are organic chemical compounds called lipids which are mostly high in polyunsaturates and low in mono-unsaturates, such as sunflower oil or rape seed oil used in cooking. Olive oil is a pale-green or yellowish oil obtained from the fruit of Olea europaea.

oldest plant

The longest living plant may be the King's Holly Bush of Tasmania, which has individual plants which live about 300 years but use only vegetative propagation and hence from the dispersion of the plant it appears that the same genetic individual plant has been growing for about 43,000 years.


Meaning "oily".

oleander scale

May cause loss of vigour and die-back caused by sap-feeding insects.


Means "producing oil" in Latin.


Means "used as a green vegetable" in Latin.


Meaning "bearing little fruit".


Refers to an environment which has low levels of nutrients, although good levels of oxygen.


Means "of Olympus, Greece" in Latin.

ombrophile and ombrophobe

An ombrophile is a plant which tolerates high rainfall, whereas an ombrophobe is intolerant of high rainfall.

onion bolting

Onions produce flower stems early, bulb size is smaller than normal.

onion downy mildew

Affected leaves turn grey, wither and collapse.

onion eelworm

Plants are stunted and swollen, may not produce bulbs.

onion fly

This is a fly (Delia antiqua) whose maggots feed on the bulbs and stems of the onion family, including shallots and leeks. Most damage may be caused in mid-summer, although 2 or 3 generations may occur in a growing season. The control of this pest includes the cultivation of soil during the winter to reduce the overwintering pupae, sterilisation of the soil and the treating of seedlings with an insecticide.

onion neck rot

Scales of infected onions become soft, pale, brown and semi-transparent. A dense furry growth develops and black fungal growths may develop.

onion thrips

Foliage develops a fine, white mottling during the summer.

onion white rot

White fungal growth develops around base of bulb and roots, fungal spores appears.

onion yellow dwarf

Yellow stripes on onion leaves caused by virus, later the leaves droop.


Sexual reproduction in plants where the female gamete is a large, non-motile cell, filled with food materials. This cell may either float freely in water (as in the seaweed algae Fucus), or be retained in a chamber (as in some algae and all land plants).


The condition where the female gamete is non-motile and the male is motile.

open-centre pruning

The pruning of a tree to leave the centre free of growth, such as that done to some fruit trees.

open soil

Another term for light soil.


A lid or cover, eg of a fruit. In mosses, it is the circular lid of the capsule.


Of two organs, arising at the same level on opposite sides of the stem eg opposite leaves.


A name given to a heated glasshouse, usually built onto the south side of a house, which is designed for growing citrus fruits in cool climates.


Rounded in outline with length and breadth about the same.


Means "disk shaped" in Latin.


Refers to an area of land used for growing fruit trees.

orchard house

An unheated greenhouse used for growing fruits.

orchid viruses

Various symptoms depending on cultivar-yellow or dark brown-black streaking spots or rings may appear.


The classification between class and family.

Ordovician period

A period of time in the Earth's development 440 to 505 million years ago. Marine organisms develop markedly with a variety of algae-like plants. Early animals such as graptolites and trilobites are abundant.


Refers to a functional part of the plant, such as a leaf, bud, ovary, stamen etc.


A sub-component of a cell, such as the photosynthetic chloroplasts in plant cells.


Used to describe substances that are the "product of" or "part of" living organisms. Often meaning that no artificial chemicals have been used to produce the food plant or animal. An "organic chemical" refers to a chemical containing carbon.

organic farming

Refers to the growing of plants and raising of animals without the use (as far as possible) of artificial chemicals. Not using synthetic pesticides, antibiotics, herbicides, fungicides etc is intended to avoid the residual chemicals in the farm produce, lessen the impact on the environment and reduce running costs. Organic farming favours biological control, use of naturally resistant crops and more ecological awareness to improve overall use of the land.


An individual plant or animal, or other living entity on planet Earth.

organo-phosphorus chemicals

These chemical compounds form the basis of many insecticides and other treatments. Special care should be taken if chemicals of the organo-phosphorus group are being used, since an accumulation of these chemicals within the body can be harmful.


Means "eastern" in Latin.


Refers to the regions in the world where the plant grows naturally or where the first specimens of cultivated plants were discovered.


This term can be used as an alternative to decorative. The term ornamental may be used as a general name to refer to any plant species or variety valued for their pleasing appearance, including plants such as ornamental cabbage.


Means "showy" in Latin.


Refers to growth in the direct line of the stimulus. An example is the geotropism of most roots.


A term for a willow (such as Salix viminalis) whose branches can be used for basket making.


The passage of water (or other chemicals) through a "semi-permeable" membrane (such as those in cell walls), because of differences in concentrations of chemicals in the solutions on each side of the membrane. This is an important process in living tissues. Osmotic pressure enable plant cells to maintain their turgor and hence retain their shape.


Meaning "brittle or bony".

ounce or oz

A measure of weight (oz) equivalent to 28.6 gm (gram).


Refers to any offshoot, lateral shoot or possibly a mutant shoot or one caused by a canker.


Longer than wide with the widest part in the centre. Similar to elliptic or oblong.


Means "oval" in Latin.


The part of the flower containing the ovules and later the seeds.


Egg-shaped; the widest part in the basal half.


Means "egg shaped" in Latin.


The process of a plant surviving winter. Tender plants unable to withstand freezing temperatures will need protection.

oviform or ovoid

Meaning "egg-shaped".


A structure containing the egg (or ovum), which after fertilization becomes the seed.

ovuliferous scale

In gymnosperms, the ovules develop on the ovuliferous scale which in the female cone arises in the axis of the bract scale.


A name for the female gamete or egg, which is part of the ovule. In flowering plants the ovum and ovule is contained in the carpel. The plural of ovum is ova.


This is an organic systemic fungicide available as a wettable powder and is often used for controlling rust diseases.


This is a systemic organophosphorus insecticide used in aerosols to control aphids on outdoor plants.


This is a chemical element (symbol O) which as a gas comprises about 20% of the air and which together with hydrogen makes up water molecules. It is essential for organic life processes, and is absorbed as water and as a gas (gaseous exchange) by both plants and animals for respiration. Plants release oxygen gas during photosynthesis. On balance, plants release more oxygen to the atmosphere than they absorb. (Oxygen gas is consumed by a fire when wood or other organic matter is burned, and carbon dioxide is produced.)


Refers to the addition of oxygen to ponds, rivers etc. Water absorbs oxygen quite readily when air is bubbled through it, or where water flows over a waterfall or is otherwise agitated.

oxygenator plant

Refers to a water plant which will give off oxygen and so help support fish and other pond life, whilst reducing the growth of algae. An example oxygenator is Canadian waterweed (Elodea canadenis).


Means "sharp-petalled" in Latin.


Means "sharp-leaved" in Latin.

ozone layer

This is a region of the world's atmosphere between about 20 to 40 km above the surface where ozone gas (molecules of 3 oxygen atoms) is an important part of the Earth's defence against potentially harmful ultraviolet radiation from the sun. Damage to the ozone layer due to the release of certain pollutant gases such as chlorofluorocarbons, results in increasing ultraviolet radiation and affecting the global greenhouse effect.

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