Apex (geometry)

Abstract: It may be true of men; it is certainly true of Sphagnum. The plants grow at the apex, as do most other mosses. The apex produces initials which develop into branches of determinate growth, though in a few cases (S. cuspidatum var. plumosum for example), the branches may themselves branch. Whilst the branches are increasing in length the internodes of the main stem do not elongate. This results in the branches and attendant leaves forming a compact hemispherical head — the coma or capitulum. Later, after branch growth has finished, the internodes do elongate; it then becomes obvious that the branches are borne in groups (fascicles) on the central stem. Each branch bears 30 to 150 spirally arranged imbricate leaves. These leaves are one cell thick, with the unique and well known differentiation into porose hyaline cells with thickened hoops, and enclosed or chlorophyllose cells. The branches may be spreading or pendent and clasping the stem (Fig. 8.1). The individual leaves live for a year or two, but by that time the growth of branches above has put them into dense shade and they die. The only parts left alive at this distance below the apex seem to be the axillary buds. These usually remain inactive, and eventually die, but if the apex is destroyed, either artificially (for example, by cutting if off) or accidentally (for example, by drought) then one or more lateral buds may begin to grow again from as far as 10 cm below the apex.

Abstract: Prior to root canal treatment at least one undistorted radiograph is required to assess canal morphology. The apical extent of instrumentation and the final root filling have a role in treatment success, and are primarily determined radiographically. Electronic apex locators reduce the number of radiographs required and assist where radiographic methods create difficulty. They may also indicate cases where the apical foramen is some distance from the radiographic apex. Other roles include the detection of root canal perforation. A review of the literature focussed first on the subject of electronic apex location. A second review used the names of apex location devices. From the combined searches, 113 pertinent articles in English were found. This paper reviews the development, action, use and types of electronic apex locators.

Abstract: For a better understanding of Al inhibition of root elongation, knowledge of the morphological and functional organization of the root apex is a prerequisite. We developed a polyvinyl chloride-block technique to supply Al (90 μm monomeric Al) in a medium containing agarose to individual 1-mm root zones of intact seedlings of maize (Zea mays L. cv Lixis). Root elongation was measured during a period of 5 h. After Al treatment, callose (5 h) and Al (1 h) contents of individual 1-mm apical root segments were determined. For comparison, callose and Al levels were also measured in root segments after uniform Al supply in agarose blocks to the 10-mm root apex. Only applying Al to the three apical 1-mm root zones inhibited root elongation after 1 h. The order of sensitivity was 1 to 2 > 0 to 1 > 2 to 3 mm. In the 1- to 2-mm root zone high levels of Al-induced callose formation and accumulation of Al was found, independently of whether Al was applied to individual apical root zones or uniformly to the whole-root apex. We conclude from these results that the distal part of the transition zone of the root apex, where the cells are undergoing a preparatory phase for rapid elongation (F. Baluska, D. Volkmann, P.W. Barlow [1996] Plant Physiol 112: 3–4), is the primary target of Al in this Al-sensitive maize cultivar.