RELATION OF PLASTOCHRON TO ANATOMY AND GROWTH IN THE SHOOT APEX OF CHRYSANTHEMUM

The relation between plastochron stage, apical anatomy, and thymidine-C¹⁴ incorporation was studied in the shoot apex of Chrysanthemum morifolium ‘Albatross.’ Apices were sorted into early, middle, and late plastochron stage under a dissecting microscope, fixed, and sectioned longitudinally so that median sections included known sectors of the apical flank. Study of these sections revealed no discernible difference between apices in early, middle, or late plastochron with respect to regularity of cell pattern, presence of a cambium-like zone, appearance of the second tunica layer or staining pattern with pyronin or with toluidine blue. Likewise, apices that had been treated with thymidine-C¹⁴ for 2-4 hr showed no differences between the three stages in number or distribution of labeled cells.     

LEAF INITIATION RATES AND VOLUME GROWTH RATES IN THE SHOOT APEX OF CHRYSANTHEMUM

The leaf initiation rate and apical volume growth rate of lateral shoots of Chrysanthemum morifolium ‘Improved Albatross III’ were determined for plants growing under controlled environment conditions. After removal of the terminal bud, the top two lateral buds produced leaves at a rate of 1.4 to 1.6 leaves/day for the first 9 or 10 days. At about the 10th day there appeared to be an abrupt shift to a lower initiation rate of 0.7 to 0.8 leaves/day. Defoliation by periodic removal of leaves larger than 1 cm caused a statistically significant increase in initiation rate, but not to as high a rate as that of the first 10 days of bud growth. Volume growth rates of the apical region were determined from transverse sections of five apices, according to the method of Richards (1951). The apical region was found to have a radial expansion rate of 0.1623/plastochron and a vertical expansion rate of 0.0494/plastochron. The volume growth rate was calculated to be 0.3740/plastochron and the volume doubling time to be 1.85 plastochrons or 56 to 63 hr.     

THE SHOOT APEX OF BOUGAINVILLEA SPECTABILIS

The shoot apex of Bougainvillea spectabilis consists of five zones: A two- or occasionally three-layered tunica, a central mother cell zone, a cambium-like zone, a rib meristem (central meristem), and a peripheral meristem. The presence of a cambium-like zone is somewhat unusual in the apex of vascular plants, having only been reported for a few taxa. In B. spectabilis the cambium-like zone is consistently present throughout the plastochron and all yearly seasonal periods.     

THE CYTOHISTOLOGICAL AND CYTOHISTOCHEMICAL ZONATION OF THE SHOOT APEX OF BOTRYCHIUM MULTIFIDUM

Cytohistologically, the shoot apex of Botrychium multifidum is composed of three zones—a zone of surface initials in which there is usually a centrally located apical cell, a zone of subsurface initials, and a cup-shaped zone that is subdivided into a pheripheral zone and a rib meristem. The results of cytohistochemical tests for total protein, RNA, total carbohydrate, histones, and DNA localization support this concept. Thus, the cytohistological zonation of the apical meristem of the Ophioglossales is essentially identical to that of the Filicales, and furthermore, is comparable to that of the seed plants.