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.     

DURATION OF CELL CYCLES IN CULTURED ROOTS OF CONVOLVULUS

The durations of the cell cycle in physiologically different regions of the meristem of cultured roots of Convolvulus arvensis were determined by the metaphase-accumulation technique involving colchicine. The cell cycle in the root cap increases from 13 hr in the actively dividing initials of the first tier to 155 hr in the slowly dividing initials of tiers 2–4 to an indeterminate value for derivatives of the initials in the root cap columella. The cycle times for the cells of the central cylinder and cortex are 21 and 27 hr, respectively. The cells of the quiescent center have a cycle of an estimated 420 hr. The duration of the cell cycle in these different regions is discussed in relation to the increased duration of G₁ in slowly or non-dividing cells. The possible regulation of cell division by the synthesis of a cell-division factor in the quiescent center is also discussed.     

Three maize root-specific genes are not correctly expressed in regenerated caps in the absence of the quiescent center

The quiescent center is viewed as an architectural template in the root apical meristem of all angiosperm and gymnosperm root tips. In roots of Arabidopsis thaliana (L.) Heynh., the quiescent center inhibits differentiation of contacting initial cells and maintains the surrounding initial cells as stem cells. Here, the role of the quiescent center in the development of the maize (Zea mays L.) root cap has been further explored. Three maize root-specific genes were identified. Two of these were exclusively expressed in the root cap and one of them encoded a GDP-mannose-4,6-dehydratase. Most likely these two genes are structural, tissue-specific markers of the cap. The third gene, a putative glycine-rich cell wall protein, was expressed in the cap and in the root epidermis and, conceivably is a positional marker of the cap. Microsurgical and molecular data indicate that the quiescent center and cap initials may regulate the positional and structural expression of these genes in the cap and thereby control root cap development. Received: 22 September 1999 / Accepted: 9 November 1999     

THE ROOT APEX OF LINUM

Several developmental stages of primary roots of 13 species of Linum were studied. The basic pattern of root apex organization in three species consisted of a single tier of cortical initials. Ten species had a basic pattern of two tiers of cortical initials. Variations, manifested by either an increase or a decrease in the tiers of cortical initials, were observed in some roots in those species that had a basic pattern of two tiers of cortical initials. Although these variations were interpreted as being ontogenetic, there was no total reorganization of the root apex. There was anatomical and cytological evidence that a quiescent center is established in Linum roots.     

QUANTITATIVE STUDIES OF THE ROOT APICAL MERISTEM OF EQUISETUM SCIRPOIDES

An investigation was made of the meristematic activity of the apical cell, its immediate derivatives (merophytes), and of other selected cell populations of the root of Equisetum scirpoides Michx. The plane of the first division of a derivative of the apical cell is radiallongitudinal, which provides evidence that merophytes immediately adjacent to the apical cell cannot be the ultimate root initials. The apical cell is as active mitotically in roots 20–40 mm long as it is in roots that are 0.25–1 mm in length. The mitotic activity of the apical cell and of other cell populations was determined from the mitotic index, and from determination of the durations of the cell cycle and of mitosis of the apical cell by using the colchicine method of metaphase accumulation. Microspectrophotometric measurements of DNA content indicated that there was no consistent increase in DNA (endopolyploidy) in the apical cell or in the other meristematic cells as roots increased in length. Conclusion: there is no evidence that the apical cell becomes quiescent or undergoes endopolyploidy as a root increases in length.     

THE QUIESCENT CENTER IN CULTURED ROOTS OF CONVOLVULUS ARVENSIS L.

Cultured roots of Convolvulus arvensis were incubated in 0.2–0.3 μc/ml methyl-³H-thymidine for different intervals of time. In roots supplied with tritiated thymidine for 12 hr, 14 hr, 48 hr, or 14 hr followed by transfer to fresh medium for 24 hr, autoradiographs prepared of serial, longitudinal sections of the root tips showed the presence of a subterminal quiescent center in the root proper at the distal poles of the central cylinder and cortex. In addition, a zone of unlabelled cells in the columella, distal to the root cap initials, was present. In roots supplied continuously with tritiated thymidine for 64 hr, 96 hr, and 120 hr, the quiescent center was either reduced in size or was not present.     

Root apical organization inArabidopsis thaliana 1. Root cap and protoderm

Summary We investigated the development of the root cap and protoderm inArabidopsis thaliana root tips.A. Thaliana roots have closed apical organization with the peripheral root cap, columella root cap and protoderm developing from the dermatogen/calyptrogen histogen. The columella root cap arises from columella initials. The initials for the peripheral root cap and protoderm are arranged in a collar and the initiation event for these cells occurs in a sequential pattern that is coordinated with the columella initials. The resulting root cap appears as a series of interconnected spiraling cones. The protoderm, in three-dimensions, is a cylinder composed of cell files made up of packets of cells. The number of cell files within the protoderm cylinder increases as the root ages from one to two weeks. The coordinated division sequence of the dermatogen/calyptrogen and the increase in the number of protoderm cell files are both features of post-embryonic development within the primary root meristem.Abbreviations RCP root cap/protoderm - CI columella initial - PI protoderm initial     

THE ROOT APEX OF MALVA SYLVESTRIS. III. LATERAL ROOT DEVELOPMENT AND THE QUIESCENT CENTER

Both histological and autoradiographic procedures were used to follow lateral root initiation and development. Lateral roots of M. sylvestris were initiated in the pericycle, and although the endodermis became multiseriate, endodermal derivatives were not incorporated into the lateral root primordium. Apical organization of pre-emergent roots, characterized by two tiers of cortical initials, did not change with growth. During pre-emergent development there was no evidence of cortical lysogeny or quiescent center formation. Quiescent centers were present in both secondary and tertiary roots longer than 0.5 cm.     

PROPOSAL OF THE GRACILARIALES ORD. NOV. (RHODOPHYTA) BASED ON AN ANALYSIS OF THE REPRODUCTIVE DEVELOPMENT OF GRACILARIA VERRUCOSA1

The mode of division of vegetative cells, formation of spermatangial parent cells, initiation of the carpogonial branch apparatus, and formation of tetrasporangial initials are homologous developmental processes that are documented for the first time in the type species of the economically important family Gracilariaceae, Gracilaria verrucosa (Hudson) Papenfuss from the British Isles. G. verrucosa is characterized by a supporting cell of intercalary origin that bears a 2-celled carpogonial branch flanked by two sterile branches, direct fusion of cells of sterile branches onto the carpogonium, formation of an extensive carpogonial fusion cell through the incorporation of additional gametophytic cells prior to gonimoblast initiation, gonimoblast initials produced from fusion cell lobes, schizogenous development of the cytocarp cavity, inner gonimoblast cells producing tubular nutritive cells that fuse with cells of the pericarp or floor of the cystocarp, absence of cytologically modified tissue in the floor of the cystocarp, and carposporangial initials produced in clusters or irregular chains. Spermatangial parent cells are generated in flaments from intercalary cortical cells that line an intercellular space forming a ‘pit’ or ‘conceptacle’. Tetrasporangial initials are transformed from terminal cells derived through division of an outer cortical cell. Tetrasporangia are cruciately divided. The Gracilariaceae is removed from Gigartinales and transferred to the new order Gracilariales. Their closest living relatives appear to be agarophytes belonging to the Gelidiales and Ahnfeltiales.     

THE ROOT APEX OF MALVA SYLVESTRIS. I. STRUCTURAL DEVELOPMENT

The apical organization of the primary root of Malva sylvestris was analyzed at several growth stages, beginning with the embryo, to determine the structural changes that occurred during growth. Seeds were germinated, and plants were grown under controlled conditions. There were three discrete groups of initials in the embryonic root: those of the central cylinder, cortex, and secondary columella. The secondary columella initials consisted of a plate of cells flanked by a ring of cortical initials. The lateral portion of the rootcap shared a common origin with the epidermis. During growth both the initials of the secondary columella and outer cortex produced rootcap cells. The first indication of the outer cortical initials participating in rootcap formation was observed in roots 3 cm long. In 6-, 9-, and 16-cm roots the cellular continuity between the outer cortex and rootcap was marked, but in 23- and 33-cm roots the histogenic continuity between the outer cortex and rootcap was not evident. In all growth stages the initials of the central cylinder and inner cortex retained their histogenic integrity.     

THE ROOT APICAL MERISTEM OF OSMUNDA REGALIS

Roots of the Osmundaceae differ from most ferns in having more than one apical cell. The size of the apical initial group, which includes cells that are considered to be apical cells, varies directly with root diameter in Osmunda regalis L. Mitotic indices were 6.63% for apical cells, 7.45% for the entire apical group, 6.25% for distal derivatives, and 7.15% for developing cortical cells. Cytophotometric measurements of Fuelgen-stained nuclei indicated no endopolyploidy in the populations of cells studied. These findings demonstrate that there is no quiescent center in the roots of O. regalis.     

栝楼初生根静止中心的研究

用~3H-胸腺嘧啶核苷处理栝楼(Trichosanthes kirilowii Maxim.)初生根不同生长阶段的根尖,并用放射自显影技术测定根尖静止中心的存在。在各个生长阶段中,静止中心缺乏标记的细胞核。在生长过程中,静止中心的体积发生变化;静止中心所在部位也有发展。最初仅占中柱顶端和皮层顶端最内层的部分。当根长大时;静止中心扩大到皮层比较靠外的部分,但静止中心从来不越过次生冠轴原始细胞。     

Origin of the quiescent center in the root of Capsella bursa-pastoris (L.) Medik

The origin of the quiescent center in the embryonic radicle of Capsella bursa-pastoris was investigated by in-situ hybridization to cellular polyadenylic-acid-containing RNA using [³H]polyuridylic acid as a probe. In the globular embryo, autoradiographic silver grains were localized in all cells of the presumptive root apex except in the hypophysis. As the inner cell formed by a transverse division of the hypophysis cut off new cells toward the central procambial cylinder of the embryo, these cells remained characteristically unlabeled, in contrast to the labeled cells of the rest of the embryo. In the embryonic radicles of mature seeds and of seedlings, cells derived from the hypophysis appeared as a nonmeristematic, unlabeled, hemispherical group, bounded by the procambium to the inside and the root epidermis to the outside. When root tips excised from 2-d-old seedlings were incubated in [methyl-³H]thymidine, sectioned, and autoradiographed, cells derived from the inner cell of the hypophysis were found to be unlabeled, thus showing that they constitute the specific cells of the quiescent center. These results present evidence for the single-cell origin of the quiescent center in an angiosperm root and a role for the hypophysis in it.Abbreviations poly(A)⁺RNA polyadenylicacid-containing RNA - [³H]poly(U) [³H]polyuridylic acid - QC quiescent center This work was supported in part by National Science Foundation grants PCM-7902898 and DCB-8709092.     

ANATOMY OF SEEDLING ROOTS OF PSEUDOTSUGA MENZIESII

The seedling root system of Pseudotsuga menziesii (Mirb.) Franco consists of the primary root, active long laterals, long laterals that become mycorrhizal, and short roots that may or may not become mycorrhizal. Numerous adventitious roots arise from the pericycle in young roots and from the vascular cambium and pericycle in older roots following pruning. All actively growing apices have a single plate of initials, a complex zonation of mother cells, and a similar pattern of primary tissue differentiation. Short roots and mycorrhizal short roots have 2 plates of initials, one producing the stele and the other the root cap and cortex, and differentiation occurs close to the apex. Primary and adventitious roots are usually triarch, while long laterals are usually diarch as are all short roots. The latter lack secondary xylem, but mycorrhizal short roots may produce a small amount of secondary phloem.     

RELATION OF DWARFMISTLETOE (ARCEUTHOBIUM) TO THE XYLEM TISSUE OF CONIFERS. II. EFFECT OF THE PARASITE ON THE XYLEM ANATOMY OF THE HOST12

Srivastava , L. M., and K. Esau , (U. California, Davis.) Relation of dwarfmistletoe (Arceuthobium) to the xylem tissue of conifers. II. Effect of the parasite on the xylem anatomy of the host. Amer. Jour. Bot. (48(3): 209–215. Illus. 1961.—The changes in the xylem anatomy induced by dwarfmistletoe infection were studied in 7 coniferous species. The most pronounced abnormalities are observed in the shape and size of the infected rays. Because of the presence of parasite tissue, the rays assume a hypertrophied appearance; moreover, they fuse to form large composite rays. The union of rays involves intrusive growth of ray cells and displacement of fusiform initials. Some division of fusiform initials also occurs. Rays may increase in number and they may contain more host cells than normal rays. Axial tracheids in infected host woods differ more or less strongly from those of noninfected woods. They may be shorter, wider, and more irregular in shape than the axial tracheids in healthy wood. The samples of xylem from infected pines had a larger number of resin canals than those from healthy trees. Resin canals were also found in infected Tsuga, which normally lacks these structures.     

GROWTH STUDIES OF THE ROOT OF INCENSE CEDAR,LIBOCEDRUS DECURRENS. I. THE ORIGIN AND DEVELOPMENT OF PRIMARY TISSUES

Wilcox , Hugh . (State U. Coll. of Forestry, Syracuse, New York.) Growth studies of the root of incense cedar, Libocedrus decurrens. I. The origin and development of primary tissues. Amer. Jour. Bot. 49(3): 221–236. Illus. 1962.—The anatomical features of active and dormant roots of incense-cedar seedlings are described and discussed in relation to various problems of differentiation and morphogenesis. Autoradiographs confirm the presence of a group of relatively inactive cells at the site of the apical initials. During periods of maximum growth activity, the presence of a quiescent center is accentuated by a peak in number of divisions in adjacent tissues. With diminution in growth activity, the peak occurs closer to the quiescent center and the size of the meristem appears to diminish. During dormancy, the configuration of the initial region seems to indicate the existence of apical initial cells which coincide with a minimal constructional center, as determined by studies of cell lineage. Roots whose apical cells retain their meristematic appearance are able to resume growth after a period of dormancy, whereas roots whose apical cells undergo vacuolation are likely to perish. Graphs are presented to show the functional relationships between growth rate and the varying distances from the apical meristem at which the tissues of the root differentiate and mature. Although early differentiation of precursory phloem could be discerned almost as soon as early vacuolation of metaxylem, its recognition was more dependent upon subjective judgment. The functional relationship between differentiation and growth rate was most pronounced in the maturation of protoxylem elements, the development of Casparian strips in the endodermis, the development of suberin lamellae in the endodermis, and by the development of phi layers in the inner cortex.     

SHOOT ORGANIZATION IN THE FILICALES: THE PROMERISTEM

The classical pteridophyte apical cell theory of meristem organization is not flexible or realistic enough to encompass the variation encountered in a comprehensive anatomical survey of fern meristems. The meristems of 28 taxa of ferns (both eu- and leptosporangiate) were studied. This analysis has led to the formulation of the concept of a promeristem composed of two zones: the surface and subsurface initials. This concept is flexible and sufficient to describe the variation encountered in the ferns as a group. No differences in promeristem organization were observed in plants with various rhizome morphology (e.g., upright and radial; prostrate and dorsiventral). Marking experiments, performed on living surface cells of fern promeristems, correlate with anatomical observations of division planes in the promeristem and indicate that the central, surface initials are not quiescent. Feulgen determinations indicate that Nephrolepis stolons have no endomitotically polyploid cells in the promeristem. Additional work is needed before a generalization can be made concerning ploidy levels in a more typical fern apex. Ferns, in general, have a zoned promeristem which is parallel to the zonation described for higher vascular plants.     

THE ROOT APICAL MERISTEM OF ASPLENIUM BULBIFERUM: STRUCTURE AND DEVELOPMENT

The root apical meristem of Asplenium bulbiferum Forst. f. has a prominent four-sided pyramidal cell with its base in contact with the rootcap. Derivatives (merophytes) that contribute to the main body of the root are produced from the three proximal faces of the apical cell. The rootcap has its origin from the fourth (distal) face of the apical cell. The first division in a proximal merophyte is periclinal to the root surface, separating an outer cell and an inner cell. The outer cell is the origin of the outer part of the cortex and the epidermis; the larger inner cell is the origin of the inner cortex, endodermis, pericycle, and vascular tissue. After the establishment of the basic number of cells in a unilayered merophyte, the cells undergo transverse divisions forming longitudinal files of cells. The mitotic index of the apical cell indicates that it is not a quiescent cell. Also, the first plane of division in a newly formed merophyte dictates that the apical cell is the originator of merophytes.     

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.