RADIAL GROWTH IN THE CYCADALES

The development of radial growth which leads to the pachycaulous form was investigated in eight of the 10 genera of the Cycadales; i.e., Ceratozamia, Cycas, Dioon, Encephalartos, Macrozamia, Microcycas, Stangeria, and Zamia. In all taxa, development of radial growth is essentially the same: a primary thickening meristem is differentiated in the stelar region of the cotyledonary node of the seedling at germination and produces derivatives mainly centrifugally. This primary thickening meristem (PTM) then differentiates acropetally and becomes continuous with the peripheral zone of the shoot apex. At first the PTM is a vertical cylinder, but as the seedling continues to grow into an adult plant, the PTM shows a more horizontal orientation (like an open umbrella) and produces the broad cortex. Secondary growth is by a vascular cambium which produces secondary xylem to the inside and secondary phloem to the outside. The broad pith originates from derivatives of the rib meristem of the massive shoot apex. The seedling and young plant is composed of a shortened shoot (i.e., no internodes) produced by the PTM and rib meristem, and a large fleshy primary root which results from a diffuse growth pattern. Individual cells in both the pith and cortex of the root divide. Their derivatives divide at right angles to the original division plane. Thus, quartets and even octets of cells are recognizable and can be traced to individual parent cells.     

PITH DEVELOPMENT IN NORMAL AND SHORT INTERNODE SEEDLINGS OF PRUNUS PERSICA VAR. LOVELL

Holmsen , Theodore W. (U. Florida, Gainesville.) Pith development in normal and short internode seedlings of Prunus persica var. Lovell. Amer. Jour. Bot. 47 (3): 173—175. Illus. 1960.–Investigation of pith cells of mature (no longer elongating) internodes of normal- and short-internode seedlings of Prunus persica var. ‘Lovell’ indicates that the short internode condition results largely from a suppression of cell division. The pith cells of normal seedlings were found to occur in an irregular fashion. The pith cells of short-internode seedlings were found to retain the linear order characteristic of the rib meristem. The relationship of these findings to the accepted long- and short-shoot conditions are discussed.     

Structure and seasonal study of the shoot apex of two species ofAraucaria

Summary and Conclusions The shoot apices ofAraucaria columnaris andA. bidwillii exhibit a tunica-corpus organization. In the former the tunica is one-layered during spring and summer months and becomes more stratified during winter months. InA. bidwillii it is usually two-layered, with a tendency to further stratification.The corpus is divisible into three zones, the corpus initial zone, the flanking zone and the rib meristem zone. The corpus initial zone is the seat of formation of the rest two. The flanking zone is concerned with the production of leaf primordia, in which the covering tunica layer has no part.The rib meristem zone varies in depth according to the season of the year and the pith cells mature gradually in these species.InA. columnaris, where a seasonal study was undertaken, no period of complete dormancy was observed.     

A SEASONAL STUDY OF THE VEGETATIVE SHOOT APEX AND THE PATTERN OF PITH DEVELOPMENT IN HIBISCUS SYRIACUS

Tolbert , Robert J. (West Virginia U., Morgantown.) A seasonal study of the vegetative shoot apex and the pattern of pith development in Hibiscus syriacus. Amer. Jour. Bot. 48(3): 249–255. Illus. 1961.—The shoot apex of Hibiscus syriacus L. is described as having a cytohistological zonation superimposed on a tunica-corpus configuration. The apex is flat-topped or may have a saddle-back or concave appearance as seen in median longitudinal section. The metrameristem, consisting of the tunica and corpus initials, is comprised of large, light-staining, vacuolate cells that have thick cell walls and exhibit much dark-staining intercellular substance. Surrounding the metrameristem is the flanking meristem, which is responsible for the outer layers of the shoot, and from which the leaf primordia arise. The pith rib meristem lies below the metrameristem and consists of files of cells that are responsible for the pith. There are no major seasonal changes in the structure of the apex during the yearly cycle. The pith displays a long-shoot type of development with the cells remaining in distinct files during the first flush of growth in the spring. As growth slows and internode elongation is gradually reduced, the pith displays the characteristic short-shoot type of development, consisting of a spongy tissue of rounded cells with many intercellular spaces and no distinct files of cells. A crown is differentiated across the top of the pith at the end of the growth period. This consists of a band of cells with thick, dark-staining cell walls, which separates by the apex from the last year's growth. In contrast to many gymnosperms, this crown is dispersed by renewed cell activity the following spring.     

CYTOKININ- AND GIBBERELLIC ACID-INDUCED EFFECTS ON THE STRUCTURE AND METABOLISM OF SHOOT APICAL MERISTEMS IN OPUNTIA POLYACANTHA (CACTACEAE)

The dormant axillary buds of Opuntia polyacantha can be activated by either cytokinins or gibberellic acid. Under the influence of benzylaminopurine (BAP), the axillary bud meristem increases greatly in size and becomes mitotically active. The primordia produced by the meristem develop as normal photosynthetic leaves. Gibberellic acid (GA) also causes the meristem to become mitotically active, but the meristem does not increase in size. The primordia produced under the influence of GA develop as normal cactus spines. Leaf-producing meristems and spine-producing meristems have the same zonation, despite the differences in size. The meristems are composed of a uniseriate tunica, a central mother cell zone, peripheral zone, and a pith rib meristem. The mitotic activity of each of the zones in the leaf-producing meristem differs significantly from the mitotic activity of the corresponding zones in the spine-producing meristem.     

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.     

Cytokinesis, cell expansion, and the potential for cytokinin-autonomous growth in tobacco pith

Pith tissue from Nicotiana tabacum L. cv `Maryland Mammoth' or `Wisconsin 38' was isolated, free of vascular tissue, and cultured on a medium containing auxin but no cytokinin. Explants from the apical 1 cm of stem, within the pith rib meristem, initiated callus growth with 100% efficiency. Macroscopically visible callus was evident 5 days after the tissue was isolated, and the cultures grew persistently in the absence of cytokinin. Heat treatment, sometimes used to initiate cytokinin habituation, was not required. Explants from tissue basipetal to the pith rib meristem declined in the frequency of habituation with increasing distance from the shoot apex. Although pith tissue which was growing, in vivo, was more prone than mature tissue to establish cytokinin-habituated callus, the basipetal decline in habituation frequency extended well beyond the zone of cell expansion. Explants from mature pith 40 centimeters or more from the shoot apex grew in the absence of cytokinin with 18% frequency, although the response required at least 2 weeks of culture. Further analysis demonstrated that tissue near the periphery of mature pith was more prone to cytokinin-habituation than tissue from the pith center.     

STRUCTURAL AND HISTOLOGICAL STUDIES OF THE CAMBIUM AND SHOOT MERISTEMS OF SOYBEAN TREATED WITH 2,3,5-TRIIODOBENZOIC ACID

Flowering soybeans were sprayed at the tips with 50 ppm TIBA. Microscopic and macroscopic observations were made of the nodes, internodes, and shoot meristems every week for 4 wks after TIBA treatment. TIBA-treated plants produced open flowers at the upper nodes 1 week earlier than did control plants. Accompanying this early flower development, the following changes occurred in the upper internodes, as compared to controls: (a) increased activity of the procambium; (b) rapid development of thick-walled protophloem cells; (c) production of small vessels. Three weeks after treatment middle internodes of treated plants showed less cambial activity than did corresponding internodes of controls. The changes in the middle internodes of treated plants suggest a close correlation with increased flowering. Two weeks after treatment some lateral shoot apices at nodes nearest the main shoot apex exhibited the following changes, in contrast with controls: (a) development of conical apices with stack-of-brick-like peripheral cells; (b) shrinkage of protoplasmic contents in some rib meristem cells and young pith cells; (c) frequent thickening of primary walls in young pith cells. Three weeks after treatment cells of lateral shoot meristems with conical axillary buds showed a denser stain for protein than did cells of corresponding meristems of controls. Floral apices in these meristems also stained more densely for protein than did similar apices in control plants. Together with early flower production in the upper nodes of treated plants, less starch occurred 2–3 weeks after treatment than in corresponding nodes of controls. Two to three weeks after treatment lateral shoot apices of both treated and control plants had numerous, large starch grains in the rib meristem, young pith, leaf and bud primordia, and developing flowers but few starch grains appeared in the tunica, corpus, and procambium.     

Characterization of Stelar Initiation in Shoot Apices of Ferns

Procambium is commonly recognized as a vascular meristem inshoot apices of vascular plants. Prestelar tissue comprisingprovascular tissue (PVT) and pith mother cells (PMCs) immediatelysubjacent to the single cell layer of promeristem has been consideredto represent the initial stage of stelar differentiation precedingprocambium and rib meristem in ferns. In addition to characterizationof PVT and PMCs on the basis of cell morphology, cytologicalfeatures and developmental continuity with procambium and ribmeristem, four lines of evidence from studies of shoot apicesof Matteuccia struthiopteris and Osmunda cinnamomea supportthis interpretation of initial differentiation. (1) Differentialstaining by safranin-fast green and crystal violet-erythrosinshows that PVT and PMCs differ in colour reactions from promeristemand resemble procambium and pith meristem, respectively. (2)Comparative ultrastructural study reveals qualitative differencesin the cell membrane system, nuclei, cytoplasm, vacuoles andplastids between promeristem and PVT but similarity of PVT toprocambium. (3) Large droplets of tannins occur in promeristembut not in PVT, PMCs and procambium. (4) Cytochemical studyof the shoot apex of Osmunda shows that carboxylesterase activityis strongly demonstrated in PVT and procambial cells but notin promeristem cells and PMCs. These observations further substantiatethe interpretation that PVT represents initial vascular differentiationand PMCs reflect a commitment to pith development.Copyright1995, 1999 Academic Press Initial vascular differentiation, provascular tissue, differential staining, ultrastructure, tannins, carboxylesterase, shoot apex, Matteuccia struthiopteris, Osmunda cinnamomea     

Histomorphological changes in shoot apices of <Emphasis Type="Italic">Lactuca sativa</Emphasis> treated with gibberellic acid

Lettuce plants were treated with gibberellic acid (GA₃) and uniconazole (UZ; a gibberellin synthesis inhibitor) to investigate the influence of GA₃ on cell division frequency in the shoot apical meristem (SAM) during stem elongation and flower initiation in lettuce (Lactuca sativa) grown in a greenhouse. GA₃ (0.1 mM) was sprayed on the surface of outer leaves and uniconazole solution (0.86 mM) was applied to the soil. GA₃ increased cell division frequency in the peripheral zone and the rib meristem of shoot apices, and this was associated with the stimulation of stem elongation. UZ treatment decreased cell division frequency in the peripheral zone, rib meristem and subapical pith, and this was associated with restricted stem elongation. Treatment with UZ and GA₃ together induced minor stem elongation. Flower induction occurred 3 d earlier in the GA₃ and UZ+GA₃ treatments than in the control, while the UZ treatment delayed flower initiation for more than 9 d relative to the control.     

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.     

THE SHELL ZONE: ITS DIFFERENTIATION AND PROBABLE FUNCTION IN SOME DICOTYLEDONS

Thirty-five species belonging to various dicotyledonous families were investigated to study the origin, development, and probable function of the shell zone, which is defined as an arcuate zone of cambiform cells delimiting the early axillary bud meristem. It is present in the majority of the investigated plants and five intergrading patterns of origin are described: (i) from the parenchymatized derivatives of the cells of the peripheral meristem of the shoot apex, adaxial to the bud meristem, (ii) from the peripheral meristem of the shoot apex along with the initiation of the early bud meristem, (iii) from the adaxial cells of the bud meristem, (iv) from the derivatives of the cells of the bud meristem at its base, and (v) partly from the parenchymatized cells of the peripheral meristem adaxial to the bud and partly from the adaxial derivatives of the bud meristem. The shell zone loses its identity at different stages of bud development in various species. Its cells ultimately contribute to the ground meristem, procambium, and pith cells of the axis. In Cuminum cyminum and lpomoea cairica the shell zone contributes in bringing about the axillary position of the bud from its early lateral position. In Solarium melongena, derivatives of the shell zone initiate the internodal elongation between the flower or inflorescence and the shoot apex, ultimately shifting the bud to an extra-axillary position on the internode.     

拐枣果序梗的发育解剖学研究

拐枣肉质膨大果序梗的发育过程可划分为前、中、后、末４个时期,前期为初生生长时期,内部结构类似一般双子叶植物茎的初生构造;中期为维管形成层活动时期,产生了不同其茎的次生木质部,由成片木质化的厚壁纤维细胞、一定量的木射线及星散在其中极少数的导管组成;后期为异常分生组织活动时期,初生木质部木薄壁组织及邻近少量髓细胞及邻近少量髓细胞转化为异常分生组织,向外产生切向排列的薄壁细胞,经扩大的切向伸长,使原导管     

白鲜根的发育解剖学研究

应用半薄切片、常规石蜡切片并结合离析法,对药用植物白鲜(Dictamnus dasycarpus Turcz.)根的发生发育过程进行了研究。结果表明:白鲜根的发生发育过程包括4个阶段,即原分生组织阶段、初生分生组织阶段、初生结构阶段以及次生结构阶段。原分生组织位于根冠内侧及初生分生组织之间,衍生细胞分化为初生分生组织。初生分生组织由原表皮、基本分生组织以及中柱原组成。原表皮分化为表皮,基本分生组织分化为皮层,中柱原分化为维管柱,共同组成根的初生结构;在初生结构中,部分表皮细胞外壁向外延伸形成根毛,皮层中分布有油细胞,内皮层有凯氏带,初生木质部为二原型或偶见三原型,外始式;根初生结构有髓或无。次生结构来源于原形成层起源的维管形成层的活动以及中柱鞘起源的木栓形成层的活动;白鲜次生韧皮部宽广,其中多年生根中可占根横切面积的85%,另外除基本组成分子外,还分布有油细胞;周皮发达,木栓层厚;初生皮层、次生木质部和次生韧皮部薄壁细胞中常充满丰富的淀粉粒。     

THE RELATIONSHIP BETWEEN THE PRIMARY THICKENING MERISTEM AND THE SECONDARY THICKENING MERISTEM IN YUCCA WHIPPLEI TORR. I. HISTOLOGY OF THE MATURE VEGETATIVE STEM

Anatomical observations were made on 1-, 2-, and 3-yr-old plants of Yucca whipplei Torr, ssp. percursa Haines grown from seed collected from a single parent in Refugio Canyon, Santa Barbara, California. The primary body of the vegetative stem consists of cortex and central cylinder with a central pith. Parenchyma cells in the ground tissue are arranged in anticlinal cell files continuous from beneath the leaf bases, through the cortex and central cylinder to the pith. Individual vascular bundles in the primary body have a collateral arrangement of xylem and phloem. The parenchyma cells of the ground tissue of the secondary body are also arranged in files continuous with those of the primary parenchyma. Secondary vascular bundles have an amphivasal arrangement and an undulating path with frequent anastomoses. Primary and secondary vascular bundles are longitudinally continuous. The primary thickening meristem (PTM) is longitudinally continuous with the secondary thickening meristem (STM). Axillary buds initiated during primary growth were observed in the leaf axils. The STM becomes more active prior to and during root initiation. Layers of secondary vascular bundles are associated with root formation.     

FOLIAR ONTOGENY AND HISTOGENESIS IN MAGNOLIA GRANDIFLORA L. I. APICAL ORGANIZATION AND EARLY DEVELOPMENT

Foliar ontogeny of Magnolia grandiflora was studied to elucidate possible unique features of evergreen leaves and their development. The apex of Magnolia grandiflora is composed of a biseriate or triseriate tunica overlying a central initial zone, a peripheral zone and a pith rib meristem. Leaf primordia are initiated by periclinal divisions on the apical flank of the tunica in its second layer. This initiation and expansion is seasonal just as in related deciduous magnolias. Following leaf initiation, a foliar buttress is formed and the leaf base gradually extends around the apex. As growth continues, separation of the leaf blade primordium from the stipule proceeds by intensified anticlinal divisions in the surface and subsurface layers near the base. Marginal growth begins in the blade primordium when it reaches approximately 200 μm in height and results in the formation of two wing-like extensions, the lamina. This young blade remains in a conduplicately folded position next to the stipule until bud break.     

DIFFERENTIAL RNA SYNTHESIS IN THE SHOOT APEX OF PHARBITIS NIL DURING FLORAL EVOCATION

Early events in the evocation of the flower in Pharbitis nil Chois seedlings were investigated by following the incorporation of tritiated uridine into the shoot apex. The uridine was applied to the expanded cotyledons of seedlings at 8 hr into the inductive dark period. The shoot tips were fixed at 20 hr (a 12-hr labeling period). After the 12-hr labeling period there was considerable label throughout the shoot tips of both control plants (dark period interrupted with 5 min of red light at 8 hr) and plants induced to flower. Both RNase and acid hydrolysis removed the nonexchangeable label and the Azure B staining, thus leading to the conclusion that the uridine was incorporated into RNA. Induction in the cotyledons was followed quickly by an increased synthesis of RNA in the rib meristem region of the receptor bud at the time when the floral stimulus is assumed to be arriving. The increase in RNA synthesis is revealed by an increase in the rib meristem/central zone ratio of counts due to the incorporation of tritiated uridine. A comparison of counts in each of the two regions revealed that the change in ratio was due to an increase in the rib meristem and not due to a decrease in the central zone in induced shoot apices. The initial activation of the rib meristem probably occurred by 16 hr from the beginning of the dark period. Tendencies in the literature to disregard the role of the rib meristem in giving rise to part of the flower are discussed.     

木姜子油细胞的发育解剖学研究

利用薄切片法对木姜子茎叶油细胞的发育以及油细胞分布的研究结果表明：油细胞最早发生于第一叶原基以及茎端皮层和髓的基本分生组织中,在未出现油细胞以痛,上述器官的基本分生组织和原分生组织,难以区分油细胞的原始细胞与周围细胞。当油细胞原始细胞呈现出体积较大,液泡化程度较低,细胞核大而明显的特征才明显可辩,以后经过液泡融合,油细胞成熟和油细胞细胞质解体阶段而成为一贮油的囊,油细胞中未出现杯形构造。叶和茎中,     

寄生植物锁阳茎的发育解剖学研究

锁阳茎的初生分生组织由原表皮、基本分生组织以及在基本分生组织中呈波浪式环状排列的原形成层束组成。茎的增粗是由于呈波浪式环状排列的维管束,其“波浪”上下幅度逐渐增大,即从“浪”的基部到“浪”顶端维管束数目由4个逐渐增加到10－12个。维管束数目不断增加是由于：（1）由髓射线薄壁细胞反分化产生分生组织束,分生组织束活动产生新的维管束;（2）维管束中分化出一列或几列薄壁细胞,导致该维管束被分化出的薄壁细胞分成2－3个独立的维管束。