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Protoplast The living contents of a plant cell, bounded by the plasmalemma. Protoplast fusion A laboratory process whereby naked protoplasts of plants are released from cells (either mechanically or using enzymes), induced to fuse in pairs and then induced to regenerate a new plant. The process may be used to produce hybrids of plants that cannot be crossed by normal pollination (e.g. in making interspecific hybrids). PuMnus A swollen region at the base of the petiole of some species capable of causing leaf movement. Pteridophytes (Pteridophyta) A collective term used informally to refer to ferns, horsetails and their relatives. Quantum (pi. a) A discreet unit of electromagnetic energy. Quanta within the wavelengths of the visible spectrum are known as photons. Raceme An inflorescence in which flowers with stalks of equal length arise from a single axis. Radial symmetry Exhibited by a flower that is circular in outline (e.g. the flowers of Primula vulgaris, primrose) and in which the flower parts radiate from a central point. Radicle An embryonic root. Rate of metabolism The rate at which a cell or tissue is respiring and carrying out metabolic functions (the chemical reactions of a living cell). Rays Sheets of living cells extending radially across the secondary xylem and phloem. Those rays that link the pith with the cortex are called medullary rays, whilst those confined to the xylem and phloem are called vascular rays. Receptacle The enlarged, elongated, flat, concave or convex end of a stem that bears the flower or flowers and later the fruit. Recessive allele See allele. Recombination The process during sexual reproduction whereby genes in an offspring are rearranged in combinations that are different from those in either parent. Relative humidity The amount of water vapour present in the air expressed as a percentage of that which would be present if the air were saturated. Replication: semi conservative A form of repli-
Glossary
cation of DNA that uses an existing string of bases to create another double strand. Respiration The process, which normally takes place in the mitochondria, whereby organic molecules (those containing carbon), usually sugars, are oxidised (combined with oxygen) to release energy for metabolism (the chemical processes of a living cell). Carbon dioxide is a byproduct of respiration. Respiration rate The rate at which respiration is taking place. Rhizoid In, for example, mosses and liverworts, a thread-like multicellular or unicellular structure that serves to anchor the plant and to absorb water and minerals. Rhizome A specialised underground stem, which also serves as a storage organ. Rhizosheath The soil attached to plant roots. Rhizosphere The soil surrounding and influenced by plant roots. Rhodopsin See visual purple. Ribosomes Minute structures that are the sites of protein synthesis in a cell. RNA: ribonucleic acid A nucleic acid characterised by the presence of the simple sugar ribose and the base uracil. It may occur in three forms, messenger-RNA, ribosomal-RNA and transfer-RNA, all of which are involved in protein synthesis. Root cap The tissue, consisting mainly of large cells, many of which secrete mucilage, that covers the tip of the root, protecting it and facilitating its progress through the soil as the root grows. Rootstock The plant upon which is grafted a shoot or bud (the scion) from another specimen, thereby providing the root system of the new combined plant. Ruderal A plant with a short life cycle that is able to grow, flower and set seed rapidly if the ground is disturbed and competition is thus reduced. Many weeds are ruderals. Sapwood The outer wood of a tree trunk or branch, consisting principally of xylem tissues which both transport water and provide support. See also heartwood. Scale A modified leaf or leaf base that performs a storage function in a bulb. Scale leaf A reduced leaf. Scale/scaling/twin scaling Scales are the fleshy leaves of bulbs and can be used to propagate