The inhaling or absorbing of air for the purpose of obtaining oxygen. Plants normally respire at night, absorbing oxygen through the stomata. This is in direct contrast to their behaviour during daylight when they absorb carbon dioxide and expel oxygen in order to photosynthesise and produce carbohydrates. Resting spore

The spore of a microscopic organism that remains dormant during an adverse season such as a temperate winter or a tropical dry season. Resting spores are usually produced following sexual recombination and this provides a wide variety of phenotypes when the dormancy ends. This is important, for example, with vertical pathotypes, which have to match a wide variety of vertical pathodemes in the system of locks and keys of a vertical subsystem. Reverse osmosis

Osmosis is the passage of a solvent (e.g., water) through a semi-permeable membrane from a less concentrated solution to a more concentrated solution. This is the phenomenon that keeps plant cells turgid. Without water, turgidity is lost and the plant wilts. Reverse osmosis involves forcing a solvent in the opposite direction with physical pressure. It is an artificial process used for purifying water, and for concentrating fruit juices. Rheum rhaponticum

Rhubarb. An ancient clone that does not breed true. Not recommended for amateur breeders . Rhizobium

This is the bacterium that forms a symbiotic relationship in the root nodules of plants belonging to the family Leguminosae. The plant provides the bacterium with carbohydrates while the bacterium provides the plant with nitrates obtained by nitrogen fixation. The chemistry of this process remains a scientific mystery which industrial chemists would dearly like to imitate. A further scientific mystery is the fact that the nitrogen-fixing nodules are pink inside, and this colour is due to a form of haemoglobin that is closely related to mammalian haemoglobin. Rhizoctonia

One of the soil-borne fungi that can cause damping-off of seedlings, and other root and stem rots. Rhizome

An underground stem with buds that produce new roots and shoots, and which often acts as a food storage organ, and as a survival mechanism from one season to the next. Rhizomorph

See: Armillaria. Rhizosphere

The micro-environment that surrounds a root and is influenced by that root. Rhodes grass

See: Chloris gayana. Rhubarb

See: Rheum rhaponticum.

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