initial capitulum (pl. capitula), as in many Asteraceae, e.g. daisy (Bellis), the inflorescence comprises many unstalked florets inserted on the flattened disklike end of the peduncle and surrounded by a ring of sterile bracts, the involucre. The internodes are not recognizable. Each floret may or may not be borne in the axil of a bract on the disk. Many species of composite have two distinct types of floret in the capitulum: disk florets in the center are tubular florets ending in five short teeth, while ray florets have a straplike extension to the tube and occur around the edge of the capitulum, rather like petals. Some species, such as thistles, have only ray florets, others have only disk florets, e.g. chicory (Cichorium), and many have both disk and ray florets, e.g. sunflower (Helianthus). In racemose inflorescences the flowers typically open from below upwards. An umbel is a type of inflorescence in which the stem axis is not elongated and individually stalked flowers appear to arise from same point on the stem. These flowers are massed on one plane, giving the appearance of an umbrella, with the oldest flowers on the outside and the youngest in the middle. The umbel is typical of the carrot family (Apiaceae). Umbels may be grouped into a compound umbel, composed of umbels of smaller umbels, as in onion (Allium). A corymb is an inflorescence with flower stalks of different lengths, the lowest being the longest. This gives a flat-topped cluster of flowers at the same level that is characteristic of many brassicas, e.g. candytuft (Iberis). Umbels, corymbs, and capitula can in turn be grouped into cymose inflorescences, e.g. onion, Viburnum, and scabious (Scabiosa) respectively.
initial A cell permanently in a MERISTEM, actively dividing and adding new cells to the plant body. An initial never becomes differentiated. One of its daughters remains in the meristem as an initial, while the other differentiates to form a nonmeristematic cell. Two basic groups exist: apical initials at root and shoot apices, and lateral meristem initials whose position depends on the location of the meristem,
e.g. intrafascicular CAMBIUM between xylem and phloem. Apical initials are rarely single cells except in certain lower plants, and initiate more than one structure, e.g. lateral shoots, flowers, and leaves, as well as new stem tissues. Vascular cambium possesses two distinct types of initials, ray initials forming medullary rays, and fusiform initials producing xylem and phloem elements.
initiation factor See translation. inositol An optically active cyclic sugar alcohol, synthesized from glucose 6-phosphate. A derivative of iniositol, phytic acid, is an important storage compound in seeds, which is believed to be converted during germination to inositol, and thence to glucuronic and uronic acids for the synthesis of cell-wall components. insectivorous plant See carnivorous plant.
integument A layer surrounding the nucellus in the ovules of gymnosperms and angiosperms. Most angiosperm ovules possess two integuments, while gymnosperms usually have only one. Enclosure of the nucellus by the integuments is incomplete, the micropyle remaining to allow access to the embryo sac or, in gymnosperms, to the archegonium. intercalary meristem A region of actively growing primary tissue clearly separate from the apical meristem. Intercalary meristems occur at the internode and leafsheath bases (joints) of many monocotyledons including grasses, and at the internodes of horsetails, where they serve to increase longitudinal growth.
intercellular Describing materials found and processes occurring between cells. Compare intracellular.
interfascicular cambium A single layer of actively dividing cells between the vascular bundles in stems. It is formed when parenchyma cells resume meristematic
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