The inhibition and stimulation of DNA synthesis in shoot apices ofChenopodium rubrum L. during photoperiodic induction of flowering

Three short-day inductive cycles bring about inhibition followed by transitional enhancement of growth, not only in roots and leaves but also in different zones of shoot apical meristem, as shown by measurement of DNA synthesis using³H-thymidine autoradiography. The first inductive cycle resulted in marked inhibition of the cells of the central zone (CZ), rib meristem (RM), and peripheral zone (PZ). Subsequent enhancement of DNA synthesis occurs in RM during the second inductive cycle, but in CZ only in the third cycle. The growth activation in PZ is counteracted by decrease in apical dominance which results in further inhibition of leaf primordia and increases in bud primordia. In plants induced only by one cycle, which later reverse the vegetative pattern of growth and differentiation, increased DNA synthesis in RM and CZ was not observed. The significance of inhibitory and stimulatory processes in particular zones of the shoot apex is discussed considering flower morphogenesis.     

Changes in the anatomical structure of the shoot apex ofSenecio vulgaris L. during ontogenis in relation to the formation of leaves and inflorescence

An investigation was made of the anatomical structure of the shoot apex ofSenecio vulgaris L. a photoperiodically neutral plant, and compared with the formation of successive leaf primordia along the axis up to the initiation of the terminal inflorescence. In the shoot apex of a germinating plant a central zone can first be distinguished from the peripheral zone which is composed of small and intensely stained cells. Later, a rib meristem appears. At the time of the initiation of the middle (the largest) leaves, the shoot apex has a distinct small central zone and a well developed peripheral zone and rib meristem. Between these zones there is a group of cells dividing in all directions, the subcentral zone. At the time of initiation of the last leaves, the central zone extends to the flanks and gradually ceases to be distinguishable. At the same time, the subcentral zone increases in size. This is caused first by cell division and later, with the initiation of the last, most reduced leaves, by enlargement of the cells. Vacuolization in the inner part of the apex and the arrangement of the superficial cells in rows parallel to the surface of the apex, is a preparatory step to the initiation of the inflorescence.     

ANATOMY,HISTOCHEMISTRY AND FINE STRUCTURE OF THE GERMINATING EMBRYO OF PAULOWNIA TOMENTOSA

The anatomy, morphology and gross histochemistry of the germinating embryo and seedling of Paulownia tomentosa (Scrophulariaceae) is described. Cytohistological changes in the shoot apex are correlated with anatomical and histochemical differences. During the first days of germination and before leaf primordia are formed, there is a gradual increase in staining for cytoplasmic protein and RNA. Once the first leaves are initiated, the staining for these compounds remains constant in the central mother-cell zone, and further increase in protein and RNA is confined to the developing leaf primordia. Starch grains are found in all areas of the very young apex, but with anatomical maturity the grains are restricted to the central mother-cell zone. Some electron micrographs point up special cytological features.     

TENDRILS OF PASSIFLORA FOETIDA: HISTOGENESIS AND MORPHOLOGY

Passiflora foetida bears an unbranched tendril, one or two laterally situated flowers, and one accessory vegetative bud in the axil of each leaf. The vegetative shoot apex has a single-layered tunica and an inner corpus. The degree of stratification in the peripheral meristem, the discreteness of the central meristem, and its centric and acentric position in the shoot apex are important plastochronic features. The procambium of the lateral leaf trace is close to the site of stipule initiation. The main axillary bud differentiates at the second node below the shoot apex. Adaxial to the bud 1–3 layers of cells form a shell-zone delimiting the bud meristem from the surrounding cells. A group of cells of the bud meristem adjacent to the axis later differentiates as an accessory bud. A second accessory bud also develops from the main bud opposite the previous one. A bud complex then consists of two laterally placed accessory bud primordia and a centrally-situated tendril bud primordium. The two accessory bud primordia differentiate into floral branches. During this development the initiation of a third vegetative accessory bud occurs on the axis just above the insertion of the tendril. This accessory bud develops into a vegetative branch and does not arise from the tissue of the tendril and adjacent two floral buds. The trace of the tendril bud consists of two procambial strands. There is a single strand for the floral branch trace. The tendril primordium grows by marked meristematic activity of its apical region and general intercalary growth.     

Morpho-Histological Investigation of Plantlet Formation In Vitro in Taiwania flousiana Gaussen

Histological events during adventitious shoot formation in cultured shoot apex of 10–12-day-old seedlings and adventitious root formation in the elongated shoot of Taiwania floudana Gaussen were examined. Ceils of the peripheral subsurface layers of the shoot apex responded to cytokinin and divided into meristematic cells from which the shoot primordia were proliferated. A few bud primordia also originated from the epidermis and hypodermis of the adaxial surface of the cotyledon. The parenchyma of leaf gap of the shoots cultured in rooting medium dedifferentiated to regain the capacity of division and form adventitious root. Besides, cells that had relatively low potential of differentiation, such as the cortex parenchyma, pith ray, phloem parenchyma and cambium zone, albeit initiated to divide, but seldom formed root primordium. The origin of the adventitious roots in the leaf gap facilitated the establishment of the vascular connection between the shoot and root.     

Growth and Development of the Banana Plant: II. The Transition from the Vegetative to the Floral Shoot in Musa acuminata cv. Gros Michel

The changes that occur in the shoot apex of the banana, as itpasses from the vegetative to the flowering stage, are described.The crucial events occur well before floral primordia are evident,and they require a redistribution of activity in the variousgrowing regions. The vegetative shoot apex is in a central depressionin the rhizome; there is virtually no internodal growth in theaxis, the most active growth is in the leaf bases; vegetativebuds do not form in the leaf axils but only appear adventitiouslyfar from the tip of the shoot. With the onset of flowering thisis changed; growth in the axis itself, previously suppressed,occurs and flower buds arise as primordia in the axils of subtendingbracts. The bracts do not show the market growth in their baseswhich is so characteristic of leaves. Thus, the shoot apex risesto the level of the rhizome and then above it; as it does so,its tip changes in shape from a broad flattened some to a pointedcone. At the transitional stage, more activity occurs in thecells of the mantle, or tunica, which now consists of 3 to 4layers over the central dome. Below, in the central or mothercell zone of the corpus, which was quiescent in the vegetativeshoot, the cells spring into greater activity, becoming moreprotoplasmic and stain more deeply. Directly below this regionin the rib meristem, cells show transverse divisions. Bractprimordia occur high on the flanks of the apex, and, thoughthey originate in the manner of leaves, their subsequent growthis different. Flower primordia occur even in the axils of bractsclose to the shoot tip. Thus, the problem now is to designatethe source, nature, and mode of action of the stimuli whichinitiate and control this quite different distribution of growthin the floral, as contrasted with the vegetative, shoot. Thesignificance of the previously more quiescent central, or mothercell zone, of the apex as the source of such stimuli, is stressed.Thus, flowering first requires that the limiting controls whichapply to the vegetative shoot be released, and, secondly, thatthe apex of the shoot, rather than the leaf base, becomes themain centre of growth and development.     

AXILLARY AND DICHOTOMOUS BRANCHING IN THE PALM CHAMAEDOREA

In both Chamaedorea seifrizii Burret and C. cataractarum Martius each adult foliage leaf subtends one axillary bud. The proximal buds in C. seifrizii are always vegetative, producing branches (= new shoots or suckers); and the distal buds on a shoot are always reproductive, producing inflorescences. The prophyll and first few scale leaves of a vegetative branch lack buds. Transitional leaves subtend vegetative buds and adult leaves subtend reproductive buds. Both types of buds are first initiated in the axil of the second or third leaf primordia from the apex, P2 or P3. Later development of both types of bud tends to be more on the adaxial surface of the subtending leaf base than on the shoot axis. Axillary buds of C. cataractarum are similarly initiated in the axil of P2 or P3 and also have an insertion that is more foliar than cauline. However, all buds develop as inflorescences. Vegetative branches arise irregularly by a division of the apex within an enclosing leaf (= P1). A typical inflorescence bud is initiated in the axil of the enclosing leaf when it is in the position of P2 and when each new branch has initiated its own P1. No scale leaves are produced by either branch and the morphological relationship among branches and the enclosing leaf varies. Often the branches are unequal and the enclosing leaf is fasciated. The vegetative branching in C. cataractarum is considered to be developmentally a true dichotomy and is compared with other examples of dichotomous (= terminal) branching in the Angiospermae.     

Development of shoot inHydrangea macrophylla I. Terminal and axillary buds

The structure of shoots, in particular of winter buds, ofHydrangea macrophylla was examined. The non-flower-bearing shoot is usually composed of a lower and an upper part, between which a boundary is discernible by means of a distinctly short internode. This internode is the lowermost of the upper part, and it is usually shorter than the internodes immediately above and below, although the internodes tend to shorten successively from the proximal to the distal part of the shoot. Variations exist in the following characters among the terminal bud, the axillary bud on the lower part of the shoot and the axillary bud on the upper part: (1) length of bud; (2) character of the outermost pair of leaf primordia; (3) degree of development of secondary buds in the winter bud; and (4) the number of leaf primordia. Usually, the terminal bud contains several pairs of foliage leaf primordia with a primordial inflorescence at the terminal of the bud, but the axiallary bud contains only the primordia of foliage leaves in addition to a pair of bud scales.     

Periods of organogenesis in shoots of Nothofagus dombeyi (Mirb.) oersted (Nothofagaceae)

The organogenetic cycle of main-branch shoots of Nothofagus dombeyi (Nothofagaceae) was studied. Twelve samples of 52-59 parent shoots were collected from a roadside population between September 1999 and October 2000. Variations over time in the number of nodes of terminal and axillary buds, and the length, diameter and number of leaves of shoots derived from these buds (sibling shoots) were analysed. The number of nodes of buds developed by parent shoots was compared with the number of nodes of buds developed, I year later, by sibling shoots. The length, diameter and number of leaves of sibling shoots increased from October 1999 to February 2000 in those shoots with a terminal bud. However, extension of most sibling shoots, including the first five most distal leaf primordia, ceased before February due to abscission of the shoot apex. Axillary buds located most distally on a shoot had more nodes than both terminal buds and more proximal axillary buds. The longest shoots included a preformed part and a neoformed part. The organogenetic event which initiated the neoformed organs continued until early autumn, giving rise to the following year's preformation. The absence of cataphylls in terminal buds could indicate a low intensity of shoot rest. The naked terminal bud of Nothofagus spp. could be interpreted as a structure less specialized than the scaled bud found in genera of Fagaceae and Betulaceae.     

Developmental Anatomy of the Inflorescence of Bread Wheat (Triticum aestivum L.) during Normal Initiation and when Affected by 2, 4-D

In wheat, the tip of the shoot apex normally consists of a coreof irregularly arranged cells covered by two uniseriate, selfperpetuating, layers (the dermatogen and the hypodermal layer):no third, inner layer (sub-hypodermal layer) is present. Leafinitiation involves periclinals in the cells of the dermatogenand hypodermal layers, but not the core. Buds involve many periclinalsin the outer cells of the core, a few occasionally in the hypodermallayer but never any in the dermatogen. The appearance of ‘double-ridges’signals inflorescence initiation. Each double-ridge is the equivalentof an axillary bud (the future spikelet bud) and its subtendingleaf primordium. The initiation of the subtending leaf is normal:the initiation of the spikelet bud is characterized by periclinaldivisions in the outer cells of the core, though some may alsooccur in cells of the hypodermal layer immediately outside:no periclinals are observed in the neighbouring dermatogen cells.All the above events concerned with leaf and bud initiationoccur in an easily recognizable, strictly distichous, pattern.In plants affected by 2, 4-dichlorophenoxyacetic acid the cellularpattern where double-ridges would have been arising, is badlydisrupted, due mainly to increased cell divisions in the hypodermallayer and outer part of the core, though possibly includingsome in the dermatogen. The apex tip itself is unaffected, probablyexplaining why, when growth is resumed, it produces a successionof normal spikelets in the normal phyllotaxis. Triticum aestivum L, bread wheat, shoot apex, double-ridge primordia, inflorescence initiation, spikelet buds, 2, 4-dichlorophenoxyacetic acid     

DIFFERENTIATION OF LATERAL SHOOTS AS THORNS IN ULEX EUROPAEUS

Ulex europaeus is a much-branched shrub with small, narrow, spine-tipped leaves and axillary thorn shoots. The origin and development of axillary shoots was studied as a basis for understanding the changes that occur in the axillary shoot apex as it differentiates into a thorn. Axillary bud primordia are derived from detached portions of the apical meristem of the primary shoot. Bud primordia in the axils of juvenile leaves on seedlings develop as leafy shoots while those in the axils of adult leaves become thorns. A variable degree of vegetative development prior to thorn differentiation is exhibited among these secondary thorn shoots even on the same axis. Commonly the meristems of secondary axillary shoots initiate 3–9 bracteal leaves with tertiary axillary buds before differentiating as thorns. In other cases the meristems develop a greater number of leaves and tertiary buds as thorn differentiation is delayed. The initial stages in the differentiation of secondary shoot meristems as thorns are detected between plastochrons 10–20, depending on vigor of the parent shoot. A study of successive lateral buds on a shoot shows an abrupt conversion from vegetative development to thorn differentiation. The conversion involves the termination of meristematic activity of the apex and cessation of leaf initiation. Within the apex a vertical elongation of cells of the rib meristem initials and their immediate derivatives commences the attenuation of the apex which results in the pointed thorn. All cells of the apex elongate parallel to the axis and proceed to sclerify basipetally. Back of the apex some cortical cells in which cell division has persisted longer differentiate as chlorenchyma. Although no new leaves are initiated during the extension of the apex, provascular strands are present in the thorn tip. Fibrovascular bundles and bundles of cortical fibers not associated with vascular tissue differentiate in the thorn tip and are correlated in position with successive incipient leaves in the expected phyllotactic sequence, the more developed bundles being related to the first incipient leaves. Some secondary shoots displayed variable atypical patterns of meristem differentiation such as abrupt conversion of the apex resulting in sclerification with limited cell elongation and small, inhibited leaves. These observations raise questions concerning the nature of thorn induction and the commitment of meristems to thorns.     

Glucose-6-phosphate-dehydrogenase activity in the shoot apical meristem,leaf primordia,and leaf tissues of Dianthus chinensis L.

The oxidation of carbohydrate by the pentose-phosphate pathway in the shoot apical meristem and developing leaf primordia of Dianthus chinensis was assessed by measuring the activity of glucose-6-phosphate dehydrogenase (EC 1.1.1.49). On a kg^-1 dry weight h^-1 basis, activity rose from 250 mmol in the apical meristem to 550 mmol in the first two leaf primordia and then declined to 350 mmol in the sixth pair of leaf primordia, and finally to 200 mmol in leaves just emerged from the shoot bud. Measurements of activity in the sixth leaf pair from the apex showed differential distribution in leaf tissues. Epidermal and mesophyll tissue had about the same activity as whole-leaf tissue, but vascular bundles had 70% greater activity. Within the vascular tissue, activity in the phloem was twice as high as in the xylem. When activity was expressed on a per-cell basis, there was a continuous increase from 20 fmol in the apex to 2 pmol in the sixth leaf pair. Activity on a per unit cell volume basis showed that cells of the apical meristem and the epidermis, mesophyll and xylem of the sixth leaf pair had similar values, about 30 amol; only the two youngest pairs of primordia and the phloem had values two or three times this amount.     

Preformation and neoformation in shoots of Nothofagus antarctica (G. Forster) Oerst. (Nothofagaceae) shrubs from northern Patagonia

The size (length and diameter) and number of leaf primordia of winter buds of Nothofagus antarctica (G. Forster) Oerst. shrubs were compared with the size and number of leaves of shoots derived from buds in equivalent positions. Buds developed in two successive years were compared in terms of size and number of leaf primordia. Bud size and the number of leaf primordia per bud were greater for distal than for proximally positioned buds. Shoots that developed in the five positions closest to the distal end of their parent shoots had significantly more leaves than more proximally positioned shoots of the same parent shoots. The positive relationship between the size of a shoot and that of its parent shoot was stronger for proximal than for distal positions on the parent shoots. For each bud position on the parent shoots there were differences in the number of leaf primordia per bud between consecutive years. The correlations between the number of leaf primordia per bud and bud size, bud position and parent shoot size varied between years. Only shoots produced close to the distal end of a parent shoot developed neoformed leaves; more proximal sibling shoots consisted entirely of preformed leaves. Leaf neoformation, a process usually linked with high shoot vigour in woody plants, seems to be widespread among the relatively small shoots developed in N. antarctica shrubs, which may relate to the species' opportunistic response to disturbance.     

Growth rhythms at different stages of shoot morphogenesis in woody plants

Research data on the rhythms of shoot growth in woody plants obtained in the second half of the 20th century are reviewed. Analysis of these data demonstrated different regulation of shoot growth processes at three stages of its development: (1) initiation of shoot primordia, (2) primordia development into phyllome primordia, and (3) visible shoot growth. The growth rhythm after the first stage was realized at the level of apical shoot meristem; at the second stage, at the individual shoot level; and at the third stage, at the whole plant level.     

Growth rhythms at different stages of shoot morphogenesis in woody plants

Research data on the rhythms of shoot growth in woody plants obtained in the second half of the 20th century are reviewed. Analysis of these data demonstrated different regulation of shoot growth processes at three stages of its development: (1) initiation of shoot primordia, (2) primordia development into phyllome primordia, and (3) visible shoot growth. The growth rhythm after the first stage was realized at the level of apical shoot meristem; at the second stage, at the individual shoot level; and at the third stage, at the whole plant level.     

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.     

Bud Structure in Relation to Shoot Morphology and Position on the Vegetative Annual Shoots of Juglans regia L. (Juglandaceae)

The length and basal diameter of all lateral and terminal budsof vegetative annual shoots of 7-year-oldJuglans regia treeswere measured. All buds were dissected and numbers of cataphylls,embryonic leaves and leaf primordia were recorded. Each axillarybud was ranked according to the position of its associated leaffrom the apex to the base of its parent shoot. Bud size andcontent were analysed in relation to bud position and were comparedwith the size and number of leaves of shoots in equivalent positionswhich extended during the following growing season. Length andbasal diameter of axillary buds varied according to their positionon the parent shoot. Terminal buds contained more embryonicleaves than any axillary bud. The number of leaves was smallerfor apical and basal axillary buds than for buds in intermediatepositions on the parent shoot only. All new extended shootswere entirely preformed in the buds that gave rise to them.Lateral shoots were formed in the median part of the parentshoot. These lateral shoots derived from buds which were largerthan both apical and basal ones. Copyright 2001 Annals of BotanyCompany Juglans regia L., Persian walnut tree, branching pattern, preformation, bud content, shoot morphology     

Bud induction inSesamum indicum by splitting shoot apices followed by treatment with Amo-1618

Couples of buds were induced at the eccentric sites in the axial of the cotyledon inSesamum indicum by treating the embryo with a growth retardant Amo-1618 after the embyro shoot apex was split in the intercotyledonary plane, or incised between the primordia of the first opposite leaves. They appeared at first to have been transformed from the primordia of the first leaves. Developmental studies of the buds, however, revealed that they did not arise from the primordia, but from their adjacent area which is the presumptive stem tissue situated between the primordia and the cotyledon. Buds could occur only when the original shoot apex of the embryo as well as the first leaf primordia were degenerated. From experimental and circumstantial evidences, they were interpreted as axially buds induced at an unusual site.     

Shoot preformation in clones of Fraxinus pennsylvanica in relation to site and year of bud formation

Summary Shoot preformation was investigated in buds of four clones of Fraxinus pennsylvanica var. subintegerrima (Vahl) Fern. at two sites in Manitoba in the second (1988) and third (1989) growing seasons after grafting. More preformed primordia were produced in terminal buds in 1989 compared to 1988 at each site. Both terminal and lateral buds at Morden contained significantly more primordia than those at Winnipeg. The numbers of preformed primordia were significantly different among clones. Clone 3 produced the most and clone 1 the fewest primordia in terminal buds. Despite quantitative variation, the pattern was similar among clones for terminal buds at each site and in each year. A similar pattern was evident for lateral buds at the Winnipeg site in 1989 but at Morden, clones 4 and 1 had the largest number of preformed primordia. Data from 1989 revealed that numbers of primordia were correlated with bud dimensions, parent shoot length, diameter and number of leaves, and location of the bud on the parent. Shoot dry weight was also related to these variables and revealed a non-linear increase in dry weight with shoot length. Multiple regression, with parent shoot length and location of buds along the parent axis as independent variables provided a reliable indicator of preformation in the crown. Although there is a genotypic component to preformation, variation between sites, years and crown locations suggests plasticity in bud development.     

The Structure of the Plumule of Nelumbo Nucifera Gaertn. and the Nature of its Scale

The structure of the plumule of Nelumbo nucifera Gaertn. and its feature covered with scale are seldom seen in dicotyledon. The fact that the plumule possesses scale is even more uncommon. This particular phenomenon is investigated by observing the differentiation of the plumule apex and the development of the leaf organs. After the seed is formed, the embryo has two young leaves and a terminal bud covered with scale. In the bud it has already differentiated the 3rd and the 4th leaf primordium and a shoot apex, the differentiation of which is very complex. So the structure of the plumule passes through 4 plastochrons altogether. It is made clear through observation and analysis that, before the 4th leaf primordium is formed, the transforma- tions of the shoot apex of the embryo in each plastochron are fundamentally alike. After the 4th leaf primordium is developed, the shoot apex becomes complex and there appear 3 different active cell regions which become the bases of vegetative bud of the seeding apex. The development of these 3 active cell regions will be stated in “The Structure of the Vegetative Bud of Nelumbo nucifera Gaertn. and the Nature of its Scales.” The apices of the plumule are almost slightly domed in structure. As a rule, their width is from 95 to 107 μ. Their height is from 17 to 20 μ during one plastochron. Before the 3rd leaf initiation, the anatomical structure of apices is examined and the fol- lowing zones may be delimited: zone of tunica initials, zone of corpus initials, peripheral zone, and zone of rib meristems. It is frequently observed that the cell of corpus in subapical peripheral zone develops periclinal division, which is the initial cell of leaf primordium; Procambium will appear before the stage of the appearance of leaf buttress. The apex of the plumule is in an apical position, but when the seedling is formed, as the developing leaves are alternate, the directions of the shoot apex are changed, simultaneously the base part of the leaf encloses the axis, and the adaxial meristem also differentiates the scale which encloses the terminal bud, thus placing the bud in axillary of the leaf and forming a zigzag phenomenon of the axis of the seedling. Above the basal adaxial side of the leaf primordium develops the scale of the plumule with meristem periclinal division of closely attached protoderm as its base. So the scale of the plumule of Nelumbo nucifera Gaertn. and the axillary stipule are of the same origin. To sum up, the scale of the embryo of Nelumbo nucifera Gaertn. is differentiated from the adaxial meristem of the basal part of the leaf primordium, and is the derivative part of the leaf. It has the same function as the coleoptile of the monocotyledon. Whether they are homologous organs or not is still to be investigated.