APPLICATION OF THE 22.5 MEV DEUTERON MICROBEAM TO THE STUDY OF MORPHOGENETIC PROBLEMS WITHIN THE SHOOT APEX OF OSMUNDA CLAYTONIANA

Excised shoot apices of Osmunda claytoniana were grown under controlled sterile conditions. Histological examination of the normal shoot apex shows that it is comprised of: (1) a promeristem, which possesses 1 or more apical initiating cells at its center; (2) a prestelar tissue consisting of an incipient vascular tissue which flanks the pith-mother-cell zone; the pith-mother-cell zone gives rise to the pith rib meristem and subsequently to the fundamental parenchyma of the pith; (3) the fundamental parenchyma of the cortex and the fundamental parenchyma of the dermal system both arising from flank cells of the promeristem. Apical initial cells of meristems irradiated with a 127,000 rad acute exposure of a deuteron beam having a diameter of 25μ, histologically examined at 7-day intervals for a 12-week period, as early as 3 weeks’ postirradiation, showed the apical initiating cell(s) together with certain of the cells of the pith-mother-cell zone to be destroyed. A wound response develops peripherally to the destroyed initials. In addition, an isolated, organized growth center is observed to develop from normal promeristem cells. Incipient vascular tissue and a new pith-mother-cell zone are also observed to develop in association with the new center of growth. Implications of the role of the interrelationships between apical initiating cell(s) and other cells of the meristem and the role they may play in maintenance of meristematic integrity within the shoot meristem are discussed.     

AN ELECTRON MICROSCOPIC EVALUATION OF CYTOHISTOLOGICAL ZONATION IN THE SHOOT APICAL MERISTEM OF PINUS BANKSIANA

Shoot apical meristems of jack pine (Pinus banksiana) were examined by light and electron microscopy. Cytohistological zonation was evident when meristems were fixed in Craf IV, embedded in paraffin, and stained with Chlorazol Black E. When meristems were fixed for electron microscopy the cytoplasm of the apical initials and central mother cells each contained numerous lipid bodies and their nuclei contained little, if any, heterochromatin. The cytoplasm of the peripheral zone was rich in ribosomes. The nuclei of the peripheral zone and rib meristem were heterochromatic. Thus, the lack of heterochromatin in the nuclei and the dissolution of lipids in the cytoplasm of the apical initials and central mother cells appeared to contribute most to the organization and appearance (cytohistological zonation) of the shoot apex when standard histological techniques are used.     

SHOOT APEX ORGANIZATION AND ORIGIN OF THE RHIZOME-BORNE ROOTS AND THEIR ASSOCIATED GAPS IN DENNSTAEDTIA CICUTARIA

The shoot apex of Dennstaedtia cicutaria consists of three zones—a zone of surface initials, a zone of subsurface initials, and a cup-shaped zone that is subdivided into a peripheral region and central region. A diffuse primary thickening meristem, which is continuous with the peripheral region of the cup-shaped zone, gives rise to a broad cortex. The roots occurring on the rhizomes are initiated very near the shoot apex in the outer derivatives of the primary thickening meristem. The roots that occur on the leaf bases also differentiate from cortical cells. Eventually, those cortical cells situated between the newly formed root apical cell and the rhizome procambium (or leaf trace) differentiate into the procambium of the root trace, thus establishing procambial continuity with that of the rhizome or leaf trace. Parenchymatous root gaps are formed in the rhizome stele and leaf traces when a few of their procambial cells located directly above the juncture of the root trace procambium differentiate into parenchyma. As the rhizome procambium or leaf trace continues to elongate, the parenchyma cells of the gap randomly divide and enlarge, thus extending the gap.     

EVIDENCE FOR APICAL INITIAL CELLS IN THE VEGETATIVE SHOOT APEX OF HEDERA HELIX CV. GOLDHEART

We examined changes in the pattern of leaf variegation in a periclinal chloroplast chimera of Hedera helix L. cv. Goldheart to determine whether stable apical initials exist in the shoot apex. Additional data were obtained by histological analyses. All of the data indicate that four apical initials are present in the third layer of the apex, supporting the model of a structured apical meristem.     

THE DEVELOPMENTAL ANATOMY OF LONG-BRANCH TERMINAL BUDS OF PINUS BANKSIANA

A study of the composition of long-branch terminal buds (LBTB) of Pinus banksiana Lamb. and the yearly periodicity associated with their formation, development, and elongation was undertaken. Each LBTB has lateral bud zones and zones of cataphylls lacking axillary buds. When present, staminate cone primordia differentiate from the lowest lateral buds in the lowest lateral bud zone of the LBTB. Ovulate cone primordia and lateral long-branch buds can differentiate from the upper lateral buds in any lateral bud zone. When both types of buds are present, lateral long-branch buds are uppermost. Dwarf-branch buds occur in all lateral bud zones. During spring LBTB internodes elongate, new cataphylls are initiated, dwarf branches elongate, needles form and elongate, pollen forms and is released, and ovulate cones are pollinated. During summer buds form in the axils of the newly formed cataphylls. By early fall the new LBTB are in overwintering condition and the four types of lateral buds are discernable. The cytohistological zonation of the LBTB shoot apex is similar to that of more than 20 other conifer species. Cells in shoot apices of pine are usually arranged in distinct zones: apical initials, subapical initials, central meristem, and peripheral meristem. Periclinal divisions occur in the surface cells of the apex; therefore no tunica is present. At any given time, shoot apex volume and shape vary among LBTB in various positions on a tree. In any one LBTB on a tree, shoot apex shape changes from a low dome during spring to a high dome during summer to an intermediate shape through fall and winter.     

ZONATION IN THE APICAL CELL OF ZONARIA

Light and electron microscopic techniques were used to study the row of apical cells that form the meristem of the dictyotalean brown alga, Zonaria farlowii. A distinctly polar pattern occurs in the cells. Four ultrastructurally different cytoplasmic zones have been seen: an apical zone, a nuclear zone, a compound vacuolar zone, and a vegetative zone. The apical cell of Zonaria is a differentiating cytoplasmic unit where striking intracellular gradients occur.     

ONTOGENETIC REORGANIZATION OF THE ROOT MERISTEM IN THE COMPOSITAE

The organization of the root meristem in selected Compositae was investigated to determine whether changes in the pattern of cell arrangement occurred during root growth in species other than Helianthus annuus. Embryonic, short, and long primary roots of one species of each of twelve genera were prepared for microscopic examination. Additional intermediate growth stages were prepared for Echinacea pallida. The meristem of embryonic roots showed layers of initials typical for dicotyledons. The meristem in many of the short roots of eight species was reorganized by the development of a secondary columella. The long roots showed patterns similar to the embryonic roots. In three species which maintained closed meristems, two layers of cortical initials were common in the embryonic root, and as a general trend, a single layer of cortical initials became more common during root elongation. The cellular changes that resulted in the initiation of a secondary columella are characterized by the conversion of cortical initials to secondary columella initials by a shift in their plane of cell division. It is proposed that the size and shape of the quiescent center changes as the conversion takes place. No intermediate stages were observed which could account for the reduction of two layers of cortical initials to one layer.     

THE CONCEPT OF INCIPIENT VASCULAR TISSUE IN FERN APICES

A comparison of shoot apices of runners and rosettes of Nephrolepis with shoot apices of Adiantum indicates that a uniform concept of apical organization can be applied throughout. The region directly below the apical initials is designated a “central zone,” by analogy with that found in Lycopodium. The central zone is regarded as undifferentiated meristematic tissue rather than incipient vascular tissue.     

MITOTIC ACTIVITY IN THE ROOT APICAL MERISTEM OF AZOLLA FILICULOIDES LAM., WITH SPECIAL REFERENCE TO THE APICAL CELL

The meristematic activity of the apical cell and its derivatives (merophytes) in the unbranched, determinate roots of Azolla filiculoides Lam. was investigated. The plane of division of the apical cell indicates that it is the initial of each merophyte. The division plane of each newly formed merophyte is strictly periclinal to the root surface and provides confirmation that the immediate derivatives of the apical cell cannot be the ultimate root initials. The frequency of cell division as determined by the mitotic index, and by the duration of the cell cycle as determined by the colchicine method, confirmed the meristematic activity of the apical cell. As roots increase in length, the duration of the cell cycle in the total meristem increases, with the apical cell possessing the longest cell cycle, whereas the immediate derivatives maintain approximately the same cycle duration as in shorter roots. In determinate Azolla roots, cell division appears to play a major role up to a certain root length, then increase in length is produced mainly by cell elongation.     

THE APICAL MERISTEM OF SPLACHNIDIUM RUGOSUM (PHAEOPHYTA)1

The meristem of Splachnidium rugosum consists of a central apical cell surrounded by a region of actively dividing cells, many of which bear hairs. Conceptacle initials are scattered throughout the surface layer of the meristematic region. Conceptacle initials and apical hairs differentiate adjacent to the apical cell. The apical cell and the conceptacle initials are distinctive, pear-shaped cells possessing similar cytological features that are consistent with significant metabolic activity. They have a nucleus surrounded by dictyosomes, a stellate chloroplast, mitochondria, and numerous vesicles and physodes. When the apical cell is damaged as a result of experimental manipulation, growth ceases. It is inferred that the apical cell controls cell division in the meristematic region and also the differentiation of conceptacle initials and apical hairs. The apical meristems of Splachnidium and species of the Fucales have several important features in common, including the growth-regulatory role of the apical cell and the process of conceptacle initiation. The taxa may possibly have a common evolutionary origin. The problematic and unresolved taxonomic status of Splachnidium is discussed.     

STUDIES ON THE ONTOGENY OF THE DWARF MISTLETOES,ARCEUTHOBIUM. II. HOMOLOGY OF THE ENDOPHYTIC SYSTEM

Cohen , L. I. (Washington State U., Pullman.) Studies on the ontogeny of the dwarf mistletoes, Arceuthobium. H. Homology of the endophytic system. Amor. Jour. Bot. 50(5): 409–417. Illus. 1963.—Development of the seedling in Arceuthobium, as well as the mode of penetration and infection, is described. The promeristem of the root apex of the seedling, as in the embryo, is composed of 4 zones: (1) a uniseriate layer of apical surface initials; (2) a subapical zone of central initials; (3) a peripheral zone; and (4) a zone of procambial initials. After germination, the seedling grows on the host surface until the root apex is confronted by a spur shoot or by a fragment of raised bark. The root apex becomes attached to the host by means of a massive holdfast. It is the procambial initials which actually invade the host tissue. After penetrating the host, the individual procambium initials proliferate in the cortex, each giving rise to a cortical strand. It was concluded that the endophytic system of Arceuthobium cannot be interpreted in terms of classical concepts of plant homology; it is, in fact, an organ sui generis.     

HISTOCHEMICAL STUDY OF ENZYME ACTIVITY IN THE SHOOT APICAL MERISTEM OF BRASSICA CAMPESTRIS DURING TRANSITION TO FLOWERING. IV. GLUCOSE-6-PHOSPHATASE

Glucose-6-phosphatase (G6P) activity was determined in fresh-frozen, cryostat sections in the shoot apical meristem of Brassica campestris L. Enzymatic activity was differentially distributed in a zonate pattern in the vegetative meristem, but not in the transition and floral meristem. Vegetative apices showed a heterogenous localization with the highest activity in the central zone and the pith-rib meristem zone. At the early transition stage of development, G6P activity in the peripheral zone increased slightly. At the late transitional (prefloral) stage, G6P activity was not localized within the peripheral zone in island-like areas of activity. This is the first demonstration of G6P in shoot apical meristem at the vegetative, transition, and floral stage. The results indicate that G6P activity 1) is an accompanying event of evocation, but 2) does not mark incipient floral primordia. G6P may play an important role in the maintenance of glucose-6-phosphate homeostasis in an evoked shoot apical meristem.     

DEVELOPMENTAL ANATOMY IN ROOTS OF IPOMOEA PURPUREA. I. RADICLE AND PRIMARY ROOT

The developmental anatomy of the primary root of Ipomoea purpurea was studied at several growth stages, beginning with the radicle. The radicle is generally composed of three superimposed tiers of initials, which produce the vascular cylinder, cortex, and columella; and a peripheral band of lateral rootcap-epidermal initials. The radicular cortex contains 16–19 immature laticifers; none of the tissue regions in the radicle contains mature cells. Following germination and during the first 2–3 cm growth of the primary root the apical meristem and its derivative tissues undergo a series of modifications. Root apical diameter decreases as cells in lateral portions of the rootcap elongate; meanwhile, the columella enlarges vertically. The relationship between cortical and columellar initials changes as fewer mitoses occur in the former while the latter remain active. In longer roots the columellar initials are directly in contact with the vascular initials. Cortical size diminishes during early root growth as cortical laticifers and their associated cells cease to be produced by the outer cortical initials and ground meristem. Early procambium, at the level of vascular pattern initiation, decreases in diameter by cellular reorientation, and the vascular cylinder decreases in overall diameter although the tetrarch pattern remains unchanged.     

Developmental anatomy of the root cortex of the basal monocotyledon, Acorus calamus (Acorales, Acoraceae)

BACKGROUND AND AIMS: The anatomical structure and development of adventitious roots were analysed in the basal monocotyledon, Acorus calamus, to determine to what extent those features are related to phylogenetic position. METHODS: Root specimens were harvested and sectioned, either with a hand microtome or freehand, at varying distances from the root tip and examined under the microscope using a variety of staining techniques. KEY RESULTS: Roots of Acorus calamus possess a unique set of developmental characteristics that produce some traits similar to those of another basal angiosperm group, Nymphaeales. The root apical meristem organization seems to be intermediate between that of a closed and an open monocotyledonous root apical meristem organization. The open-type root apical meristem consists of a curved zone of cortical initials and epidermal initials overlying the vascular cylinder initials; the epidermal part of the meristem varies in its association with the cortical initials and columellar initials of the promeristem. The cortex develops an endodermis with only Casparian bands, a dimorphic exodermis with Casparian bands and suberin lamellae, and a polygonal aerenchyma by differential expansion, as also observed in the Nymphaeales and some dicotyledonous species. The stele has characteristics like those of members of the Nymphaeaceae. CONCLUSIONS: Specific anatomical and developmental attributes of Acorus roots seem to be related to the phylogenetic position of this genus.     

MORPHOLOGY OF HYPOMYCES TRICHOTHECOIDES

Large, spirally coiled initials embedded in a subiculum develop into multicellular, multinucleate ascogonia. Hyphae grow up around them to form a prosenchymatous perithecial wall. The ascogonia give rise to multinucleate ascogenous cells from which croziers and asci form. As the ascocarp develops, an apical meristem produces uninucleate cells that elongate downward into long, slender filaments, the apical paraphyses. From a basal layer of ascogenous cells, asci grow up among the apical paraphyses, which disintegrate as the ascocarp matures. Ascospores are verrucose, with obtuse apiculi. This pattern of development is typical of the Nectria-type of Luttrell.     

Structure and Cytogenesis of the Shoot Apex of Syringa oblata var. affinis Lingelsh

The structure, growth and mitotic activity of 211 shoot apices of developing sprouts of Syringa oblata var. affinis Lingelsh. in longitudinal sections and 67 in transverse sections have been studied with the view to understanding the nature of zonation patterns and cytogenesis of the apical meristems during a double plastochron. The external morphology and the anatomical structure of the apices in 4 plastochronic stage-early, middle, late Ⅰ and late Ⅱ stages are described. In the shoot apices examined, especially those at late plastochronic stage, the following zones may be delimited: Zone of tunica initials, zone of corpus initials, peripheral zone and zone of rib meristem. The location and orientation of mitotic figures observed in longisections of the apices in 4 plastochronic stages are plotted in diagrams and the mitotic frequency has been calculated. Information obtained from these investigations reveals that the tunica and corpus inititals constitute an active region of the apex, but their mitotic activity changes periodically within the double plastochron. In the middle plastochronic stage when the apex is at its minimal area and the cells of peripheral zone and rib meristem zone have been completely transformed into constituent parts of foliar primordia and the subjacent tissues of the stem and the pith mother cells respectively, the mitotic frequency of the initials is at its maximium and its intensity of mitotic activity is not much lower than that of any other meristematic zone at any stage. When the apical dome is reformed by the activity of these initials in late plastochronic stages, the mitotic frequency of the initials gradually drops and the region of high mitotic frequency shifts to the flank of the apex, the peripheral zone. Anticlinal divisions are predominant in this zone. On the other hand, those cells directly left behind by the corpus initials, which constitute the rib meristem, are vacuolated and marked by the pre- dominance of transverse divisions. Thus the entire zonation pattern reappears. In the next early plastochronic stage, the mitotic frequency of the tunica and corpus initials drops to its mimimium, but other regions of the apex still maintain a high mitotic frequency. It may be concluded that the tunica and corpus initials form a cytogenerative center of the shoot, and the cytohistological zonation is actually a result of the fact that different regions of apical meristems are different in mitotic activety, different in state of cell differentiation and different in their function in morphogenesis.     

SHOOT APICAL MERISTEMS AND MUTATION: FIXATION OF SELECTIVELY NEUTRAL CELL GENOTYPES

Shoot apical meristems are interpreted as either structured, that is having a permanent set of apical initials, or stochastic, having apical initials which represent “... momentary representatives of the continuous meristematic residue at the apex of the relevant layer or zone” (Newman, 1965). The two main parameters of stochastic growth are the average number of apical initials (α) and the number of mitotic cycles (r) of the initials and their daughter cells prior to the random selection of subsequent initials. Mathematical analysis and computer simulation studies of stochastic growth have shown that if one starts with 1 mutant initial and α-1 nonmutant initials, eventually a mosaic plant results. The frequency of shoot apices composed of mutant cells is 1/α and the frequency of shoot apices composed of only nonmutant cells is (1 – α)/α. These asymptotics are only attained after considerable growth, thus mericlinal chimeras can persist for many nodes and give the appearance that a permanent set of initials is present.     

A SURVEY OF THE VEGETATIVE SHOOT APICES IN THE FAMILY MALVACEAE

The present study compares the structure of the vegetative shoot apex in 40 species of the Malvaceae. There is a wide range of size, shape, and zonation within the apices of the family. Although many of the apices are domed, some are flat-topped and do not extend above the axil of the youngest leaf primordium. Also, most of the species investigated are recorded as having a more or less marked cytohistological zonation superimposed on the tunica-corpus configuration. The tunica is single-layered in a majority of species, but stratification of the upper corpus is common. In an effort to give a more accurate concept of apical structure and activity, the apex is described as the metrameristem and its derivatives: the flanking meristem, and the pith rib meristem or pith mother cells. The metrameristem, consisting of the tunica initials and the co pus initials, is the focal point of the study of the zoned apices. Data are presented for the measurements of the metrameristem, measurements of the apical dome, type of flanking meristem, origin of the pith, and growth habit of the plant. There appears to be a correlation between growth habit and the distinctness with which the metrameristem is marked off from the surrounding tissue. Most of the herbaceous species have an indistinctly marked metrameristem, whereas the shrubby trees and trees have a distinctly marked metrameristem. Zonation in shrubs and suffrutescent plants may be of either type.     

STUDIES ON THE ONTOGENY OF THE DWARF MISTLETOES,ARCEUTHOBIUM. I. EMBRYOGENY AND HISTOGENESIS

Cohen , L. I. (Washington State U., Pullman.) Studies on the ontogeny of the dwarf mistletoes, Arceuthobium. I, Embryogeny and histogenesis. Amer. Jour. Bot. 50(4): 400–407. Illus. 1963.—A complete developmental study on the origin and organization of the main tissue systems in the embryo of Arceuthobium reveals a greater degree of internal differentiation in the mature embryo than hitherto reported. The mature embryo consists of rudimentary cotyledons, a well-defined hypocotyl and a highly organized radicle. The latter develops from the remaining undifferentiated portion of the globular embryo soon after the embryonic cortex and procambium are differentiated. The radicular promeristem is organized into 4 initiating zones: (1) a uniseriate layer of apical surface initials; (2) a subapical zone of central initials; (3) a peripheral zone; and (4) a zone of procambium initials. Comparison between the dwarf mistletoe embryo and those of several nonparasitic angiosperms indicates that their ontogenies, in many respects, are essentially similar.     

INTERCALARY MERISTEMATIC ACTIVITY IN THE SPOROPHYTE OF FUNARIA (MUSCI)

Marking procedures were combined with anatomical techniques to establish that in Funaria (1) the apical region does not act as an apical meristem contributing to seta growth, and (2) the subapical region contains an intercalary meristem the derivatives of which account for the elongation of the seta. In sporophytes that are 8 mm long there is a distinctive difference in the pattern of cell division in the apical and subapical regions. Large, undivided endothecial cells exist in the apical region, and a central strand of elongated cells occupies the analogous position in the subapical region. The apical region is earmarked to form the operculum and spore sac and part of the apophysis. There is an ontogenetic continuity between the seta and the lower portion of the apophysis, but the uppermost cells of the subapical region do not contribute to seta formation. Instead, these cells and those at the base of the apical region form a transitional zone between apical and subapical influences, and they account for most of the stomates that develop on the apophysis.