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.     

FLEXIBILITY IN ONTOGENY AS SHOWN BY THE CONTRIBUTION OF THE SHOOT APICAL LAYERS TO LEAVES OF PERICLINAL CHIMERAS

The development of leaves on apically stable, periclinal chimeras was studied in a number of dicot genera. The mutant cell layers of the shoot apex and the tissues derived from them were as active developmentally as the normal layers. Ontogeny was the same in these chimeras as in nonchimeras, and growth of their leaves can be outlined as follows. Formation of the buttress, the axis, and the lamina of simple dicot leaves were independent events. In each the first growth included derivatives of the apical layers, usually three in number, found in the apex of the shoot and the lateral buds. Most cell divisions in the outer layers (L-I and L-II) were anticlinal relative to the new structures. Therefore, in the proximal regions of the buttress, axis (petiole and midrib), and lamina, the derivative cells of L-I and L-II were usually present in single layers. The rest of the internal tissue was from L-III. As formation of the axis and the lamina proceeded, derivatives of L-II replaced L-III internally in the distal and marginal regions leaving cells of L-III behind. Both the determinate growth of leaves and the pattern of cell divisions at and near the leading edges of growth meant that no cells in the leaf were comparable to the initial cells of the shoot apex. As the lamina extended, there were extensive intercalary cell divisions, both anticlinal and periclinal, so that in any given region of a leaf the layers of internal cells were from either L-II or L-III. At any point along the axis, L-III participated or did not participate in laminar extension. At any given stage in laminar growth either of two sister cells in any internal layer divided either a few times or extensively. The extreme variability in direction and frequency of cell division during leaf development was under an overriding genetic control, which resulted in the normal or typical size, shape and thickness of leaves.     

The Structure,Elongation and Thickening of Rhizome in Nelumbo nucifera Gaertn

The seedling of Nelurnbo nucifera is erect and its internodes are very short with four Alternately arranged floating leaves. During the juvenile stage, the shoot elongates remarkably and forms the horizontal rhizome. Each leaf grows out from the dorsal side of the node of the rhizome. There are two kinds of terminal buds in the juvenile shoot. (1) vegetative bud and (2) mixed bud. The axillary scale is the derivative part of the leaf. It forms an ochrea around the terminal bud. The winter buds on the annual shoot are all mixed buds. The vessels are absent in the rhizome and no cambium exists. During tile early growth of the rhizome, the rib meristems contribute mainly to the internode elongation. Later however, divisions are seen to commence in the parenchymatous tissue of the internode. As a result of these divisions the internode becomes elongated. The tuberization of the rhizome is built up from cell divisions of three kinds of tissues: (1) primary thickening meristems, (2) cells of the vascular bundles and (3) parenchyma of cortex. But, the growth in thickness of the rhizome seems to be chiefly due to the enlargement of parenchymatous cells.     

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.     

莲的根茎构造,伸长与增粗

莲 (Nelumbo nucifera)种苗的茎短而直立,叶互生。幼苗期茎延伸成横卧根茎,其上生有营养芽及混合芽。腋生鳞片为叶的衍生部分,形如叶鞘状,包着预芽。年苗上的冬芽内全为混合芽。根茎内的维管束分散排列,无导管及形成层存在。节间延长通过肋状分生组织及节间内的薄壁组织细胞分裂与增长来完成。根茎可由初生加厚分生组织,维管束细胞,皮层薄壁细胞等的细胞分裂,使层次增加,但增粗主要是由皮层薄壁细胞体积显著增大而引起的。     

A Sequential Study of Cell Divisions and Expansion Patterns on a Single Developing Shoot Apex of Vinca major

During the growth of a single developing vegetative apex ofVinca major, both the orientation and frequency of cell divisions,and the pattern of cell expansion, were observed using a non-destructivereplica technique. Micrographs taken at daily intervals illustratethat the central region of the apical dome remains relativelyinactive, except for a phase of cell division which occurs after2 d of growth. The majority of growth takes place at the proximalregions of the dome from which develop the successive pairsof leaves. The developing leaf primordia are initiated by aseries of divisions which occur at the periphery of the centraldome and are oriented parallel to the axis of the subsequentleaves. The cells which develop into the outer leaf surfaceof the new leaves undergo expansion and these cells divide allowingfor the formation of the new leaf. This paper describes thefirst high-resolution sequential study of cell patterns in asingle developing plant apex. Sequential development, cell division, expansion patterns, SEM, Vinca major, apical dome, leaf primordium, leaf initiation     

Correlative Growth in Young Terminalia superba in a Controlled Environment: Effect of the Leaves on Internode Elongation

Young Terminalia superba plants were cultivated in a controlledenvironment at the Phytotron. Effects of the excision of a youngleaf at definite elongation stages and at two given levels ofthe main axis were studied on the elongation of internodes inthese plants. Effects of the leaf did not seem to depend onits nodal position on the main axis but predominantly occurredon the immediate surrounding internodes. The excision of a youngleaf enhanced the growth rate of the internode located belowit and markedly decreased the elongation of the internode aboveit but slightly affected internode growth duration. This excisionenhanced the final length of the internode located below theleaf and decreased the final length of the internode locatedabove the removed leaf. Significant linear regressions werefound between the length of the excised leaf and the internodefinal lengths. Microscopic examination of epidermal cells ofcontrol and disrupted internodes revealed that the decreasedelongation after leaf excision could be attributed to reductionof cell divisions. The increased elongation after leaf excisioncould be attributed both to slight increase in the length ofcells (significant negative correlation was found with the lengthof the removed leaf) and to increase of cell divisions. Terminalia superba, leaf excision, cell division, internode elongation, correlative growth     

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.     

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.     

MORPHOLOGICAL AND ONTOGENETIC STUDIES ON TORREYA CALIFORNICA. II. DEVELOPMENT OF THE MEGASPORANGIATE SHOOT PRIOR TO POLLINATION

Kemp , Mahgaret . (Smith Coll., Northampton, Mass.) Morphological and ontogenetic studies on Torreya californica. II. Development of the megasporangiate shoot prior to pollination. Amer. Jour. Bot. 46(4): 249–261. Illus. 1959.—The development of the megasporangiate of Torreya californica during the first part of its maturation cycle of 26 months is described in detail. This first developmental period extends from the initiation stage in late July of one year through pollination of the young ovules in April of the following spring. At the end of this period, the reproductive shoot is a loosely organized, compound, determinate structure. It consists of a short primary axis which originated in the axil of one of the last formed bud scales or one of the first formed foliage leaves of the vegetative bud. This primary axis bears only 2 lateral and oppositely placed prophylls which stand at right angles to the subtending structure. In the axil of each prophyll is a short secondary axis which bears 2 successive pseudodecussate pairs of subopposite, sterile, scale-like perianth segments below a solitary, erect, terminal ovule. The integument of the ovule originates as a single lateral primordium, but its margins quickly merge and at pollination time it is a tubular envelope free from the nucellus. The nucellus, which is massive and contains a single deeply imbedded megasporocyte, terminates the secondary axis. Histogenetically, both primary and secondary axis systems of the megasporangiate shoot resemble a vegetative dwarf shoot. They both originate as axillary mounds of uniformly meristematic cells, whose apices soon exhibit a zonal pattern comparable to that of the apex of the vegetative shoot of the same species. The determinate nature of the primary axis is caused by cell senescence in its apex. The prophylls of the primary axis and the perianth segments of the secondary axes are comparable to bud scales of the vegetative bud in their arrangement, their origin from subsurface layers, the presence of apical and subapical initials which produce their first vertical growth, a basal intercalary meristem which completes their elongation, and marginal initials which produce a slender wing to the lamina of each type of cataphyll. At maturity all 3 types of cataphylls are basically similar in their histology. The apex of each secondary axis, at the initiation of the integument, shows an altered cellular pattern which rapidly becomes organized into a conspicuous fanshaped coaxial system as the central portion develops directly into the massive, cauline nucellus. This coaxial apical configuration differs markedly from the zonal pattern of the vegetative shoot apex and also from the similar zonal pattern in the apex of the primary axis of the megasporangiate shoot and its secondary axes during an earlier period of indeterminate growth.     

Leaf Development in Narcissus pseudonarcissus L.: With twenty Figures in the Text: The Comparative Development of Scale and Foliage Leaves

Details are given of the distribution of cell division and cellelongation in various tissues of the daffodil leaf. The development of the vascular system is also described, andrelated to the intercalary growth of the leaf. The productionof a new longitudinal vascular strand appears to be determinedby the number of cells between the existing strands. The scale and foliage leaves appear to originate from similarprimordia. Their developments diverge when they are about 1mm. long; a scale leaf is developed where most cell divisionsoccur in the sheath, and a foliage leaf is formed where thereis a region of more rapid cell division at the base of the blade.     

COMPARATIVE FOLIAR HISTOGENESIS IN ACORUS CALAMUS AND ITS BEARING ON THE PHYLLODE THEORY OF MONOCOTYLEDONOUS LEAVES

A comparative histogenetic investigation of the unifacial foliage leaves of Acorus calamus L. (Araceae; Pothoideae) was initiated for the purposes of: (1) re-evaluating the previous sympodial interpretation of unifacial leaf development; (2) comparing the mode of histogenesis with that of the phyllode of Acacia in a re-examination of the phyllode theory of monocotyledonous leaves; and (3) specifying the histogenetic mechanisms responsible for morphological divergence of the leaf of Acorus from dorsiventral leaves of other Araceae. Leaves in Acorus are initiated in an orthodistichous phyllotaxis from alternate positions on the bilaterally symmetrical apical meristem. During each plastochron the shoot apex proceeds through a regular rhythm of expansion and reduction related to leaf and axillary meristem initiation and regeneration. The shoot apex has a three- to four-layered tunica and subjacent corpus with a distinctive cytohistological zonation evident to varying degrees during all phases of the plastochron. Leaf initiation is by periclinal division in the second through fourth layers of the meristem. Following inception early growth of the leaf primordium is erect, involving apical and intercalary growth in length as well as marginal growth in circumference in the sheathing leaf base. Early maturation of the leaf apex into an attenuated tip marks the end of apical growth, and subsequent growth in length is largely basal and intercalary. Marked radial growth is evident early in development and initially is mediated by a very active adaxial meristem; the median flattening of this leaf is related to accentuated activity of this meristematic zone. Differentiation of the secondary midrib begins along the center of the leaf axis and proceeds in an acropetal direction. Correlated with this centralized zone of tissue specialization is the first appearance of procambium in the center of the leaf axis. Subsequent radial expansion of the flattened upper leaf zone is bidirectional, proceeding by intercalary meristematic activity at both sides of the central midrib. Procambial differentiation is continuous and acropetal, and provascular strands are initiated in pairs in both sides of the primordium from derivatives of intercalary meristems in the abaxial and adaxial wings of the leaf. Comparative investigation of foliar histogenesis in different populations of Acorus from Wisconsin and Iowa reveals different degrees of apical and adaxial meristematic activity in primordia of these two collections: leaves with marked adaxial growth exhibit delayed and reduced expression of apical growth, whereas primordia with marked apical growth show, correspondingly, reduced adaxial meristematic activity at equivalent stages of development. Such variations in leaf histogenesis are correlated with marked differences in adult leaf anatomy in the respective populations and explain the reasons for the sympodial interpretation of leaf morphogenesis in Acorus and unifacial organs of other genera by previous investigators. It is concluded that leaf development in Acorus resembles that of the Acacia phyllode, thereby confirming from a developmental viewpoint the homology of these organs. Comparison of development with leaves of other Araceae indicates that the modified form of the leaf of Acorus originates through the accentuation of adaxial and abaxial meristematic activity which is expressed only slightly in the more conventional dorsiventral leaf types in the family.     

Adventitious buds ofdryopteris sparsa complex (dryopteridaceae): Developmental anatomy and systematic implication

Adventitious buds of theDryopteris sparsa complex were examined anatomically and taxonomically. While no buds are found inD. hayatae andD. sparsa, they occur inD. sabaei, D. yakusilvicola, and in putative hybrids of which one parent seems to beD. sabaei. The buds function as a means of vegetative reproduction in the species and hybrids. The buds arise as a pair on stipes of abortive leaves without lamina. InD. sabaei the youngest bud primordium observed consists of a small group of surface and subsurface meristematic cells surrounded by differentiated tissue cells, and the meristematic cells appear to be quiescent. As the bud primordia develop, the inner and then outer parenchymatous cells below the meristematic cells divide each into several small cells, among which the procambial strands are later differentiated to connect the bud primordium to the vascular strand of the leaf. The meristematic cells also undergo cell divisions, and the bud primordium becomes larger. A shoot organization of the bud primordium is later established. The bud-bearing, uniquely abortive leaves and delayed development of the buds support the taxonomic relationship of agamosporousD. yakusilvicola having been derived from hybridization betweenD. sabaei andD. sparsa.     

Epichlo? endophytes grow by intercalary hyphal extension in elongating grass leaves.

A fundamental hallmark of fungal growth is that vegetative hyphae grow exclusively by extension at the hyphal tip. However, this model of apical growth is incompatible with endophyte colonization of grasses by the symbiotic Neotyphodium and Epichlo? species. These fungi are transmitted through host seed, and colonize aerial tissues that develop from infected shoot apical meristems of the seedling and tillers. We present evidence that vegetative hyphae of Neotyphodium and Epichlo? species infect grass leaves via a novel mechanism of growth, intercalary division and extension. Hyphae are attached to enlarging host cells, and cumulative growth along the length of the filament enables the fungus to extend at the same rate as the host. This is the first evidence of intercalary growth in fungi and directly challenges the centuries-old model that fungi grow exclusively at hyphal tips. A new model describing the colonization of grasses by clavicipitaceous endophytes is described.     

Floral transition as a sequence of growth changes in different components of the shoot apical meristem ofChenopodium rubrum

The changes in cell division rate were studied in different components of the shoot apex ofChenopodium rubrum during short-day photoperiodic induction and after the inductive treatments. Induced and vegetative apices were compared. Accumulation of metaphases by colchicine treatment was used to compare the mean cell cycle duration in different components of the apex. A direct method of evaluating the increase in cell number obtained by anticlinal or periclinal divisions was applied if the corresponding components of induced and non-induced apices had to be compared. The short-day treatment prolonged the cell cycle more in the peripheral zone than in the central zone and still more in the leaf primordia. The importance of changing growth relations for floral transition was shown particularly if the induced plants were compared with the vegetative control with interrupted dark periods. Induced plants transferred to continuous light showed further changes in the rates of cell division. The cell cycle was shortened more in the central zone than in the peripheral zone,i.e. there was a further shift in growth relations within the apical dome. The cell cycle in the leaf and bud primordia was also shortened if compared with the vegetative control, the acceleration being stronger in the bud primordia. There was a subsequent retardation in cell division in the leaf primordia formed during and after the inductive treatment if the plants were fully induced. An inhibition of the oldest bud primordia was observed in fully induced apices, as well.     

Histological study of in vitro development of adventitious buds on leaf explants of oriental beech (Fagus orientalis lipsky)

Summary Adventitious shoots were induced on the proximal portion of leaves excised from Fagus orientalis shoot cultures derived from a 2-mo.-old or a 4-yr-old seedling. Up to 90% of the explants formed between 13 and 19 buds after culture on Woody Plant Medium containing 2.9 μM indole-3-acetic acid and 4.5 μM thidiazuron. Adventitious buds developed mostly on petiole stub callus, but also on the midvein. The histological events leading to shoot organogenesis were examined. Some shoots developed directly from subepidermis or epidermis, but most originated indirectly from cell file proliferation produced by periclinally dividing cells subadjacent to the epidermis. Some cells in the outermost layers of these files became meristematic and divided extensively, resulting in the formation of meristemoids after 16 d of culture. Dedifferentiation into meristematic cells, which exhibited a large, prominent nucleus, densely-stained cytoplasm, and a high nucleus-to-cell area ratio, was generally associated with anticlinal divisions in cells previously originated by periclinal division. Subepidermal cell proliferation occurred mainly in the abaxial surface of the explant, which initially formed a diffuse cambium and later evolved to a phellogenic cambium. Some meristemoids were also differentiated by lenticel phellogen. Organized cell divisions in meristemoids gave rise to bud primordia that emerged from the explant surface and differentiated a protoderm. The progressive structural differentiation of the apical meristem, leaf primordia, and procambial strands led, after about 28 d of culture, to shoots with vascular connections with treachery elements previously differentiated in adjacent tissues.     

LEAF HISTOGENESIS IN LACTUCA SATIVA WITH EMPHASIS UPON LATICIFER ONTOGENY

The leaf primordia of Lactuca sativa ‘Meikoningen’ develop from a subapical initial in the second layer of the tunica on the side of a fiat shoot apex. Subsequent growth of the subsurface lamina is initiated by submarginal initials which divide anticlinally to produce an adaxial layer and ***a biseriate abaxiallayer, and periclinally to produce a middle layer from which procambium differentiates. The protoderm is derived from the first tunica layer by continuous anticlinal divisions. The activity of the subapical and submarginal initials is completed when the leaf is 0.3 mm in length and 4.0 mm in width, respectively. Continued growth of the leaf to 130-150 mm results from intercalary cell division and enlargement. The mature venation is visibly delineated when the leaf is 25-30 mm in length. Laticifer and phloem cells are initiated by the same mother cells in the ***procambium. The former become non-septate laticifers by resorption of cross walls. They mature concurrently with the phloem and before the xylem.     

Developmental pattern formation of somatic embryos induced in cell suspension cultures of cowpea [Vigna unguiculata (L.) Walp]

The sequence of events in the functional body pattern formation during the somatic embryo development in cowpea suspensions is described under three heads. Early stages of somatic embryogenesis were characterized by both periclinal and anticlinal cell divisions. Differentiation of the protoderm cell layer by periclinal divisions marked the commencement of somatic embryogenesis. The most critical events appear to be the formation of apical meristems, establishment of apical-basal patterns of symmetry, and cellular organization in oblong-stage somatic embryo for the transition to torpedo and cotyledonary-stage somatic embryos. Two different stages of mature embryos showing distinct morphology, classified based on the number of cotyledons and their ability to convert into plantlets, were visualized. Repeated mitotic divisions of the sub-epidermal cell layers marked the induction of proembryogenic mass (PEM) in the embryogenic calli. The first division plane was periclinally-oriented, the second anticlinally-oriented, and the subsequent division planes appeared in any direction, leading to clusters of proembryogenic clumps. Differentiation of the protoderm layer marks the beginning of the structural differentiation in globular stage. Incipient procambium formation is the first sign of somatic embryo transition. Axial elongation of inner isodiametric cells of the globular somatic embryo followed by the change in the growth axis of the procambium is an important event in oblong-stage somatic embryo. Vacuolation in the ground meristem of torpedo-stage embryo begins the process of histodifferentiation. Three major embryonic tissue systems; shoot apical meristem, root apical meristem, and the differentiation of procambial strands, are visible in torpedo-stage somatic embryo. Monocotyledonary-stage somatic embryo induced both the shoot apical meristem and two leaf primordia compared to the ansiocotyledonary somatic embryo.     

The pattern of histone H4 expression in the tomato shoot apex changes during development

Two histone H4 cDNA clones were isolated from a tomato (Lycopersicon esculentum Mill.) shoot-tip cDNA library using a heterologous probe from barley (Hordeum vulgare L.). Both cDNAs, which are 81% identical in the coding region, are polyadenylated and belong to a small gene family in the tomato genome. Histone H4 message is abundant in young tissues and rare in older tissues. In the shoot apical meristem, the distribution of H4-expressing cells changes during development. In a juvenile vegetative apex, H4 message is detectable in the central region and the peripheral parts of the meristem. In a mature vegetative apical meristem, H4-expressing cells are localized in the peripheral zone extending into the provascular strands and the rib meristem whereas the central zone is almost devoid of H4 mRNA. After floral transition, H4 mRNA is found throughout the floral meristem, indicating a second change in the pattern of H4 expression. The observed changes in H4 expression are indicative of changes in the distribution of mitotic activity in the shoot apical meristem during plant development. In addition, H4-expressing cells were found to occur frequently in clusters, which may indicate a partial synchronization of cell divisions in the shoot apex.