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     

Initiation of Procambial Strands in Leaf Primordia of Bread Wheat, Triticum aestivum L

In Triticum aestivum L. the median and lateral procambial strandsserving the primordia originate independently and in isolationfrom the vascular system of the rest of the plant. The medianstrand is initiated first, followed by a succession of lateralstrands during the next four or so plastochrones. The medianand first lateral strands have their point of origin in theaxis, in the disc of insertion of the primordium. The laterlaterals are initiated up in the primordium. Once initiatedthe procambial strands extend from their point of origin bothacropetally and basipetally, the latter extension eventuallylinking them to strands associated with older leaves. It wouldappear that the materials necessary for the growth of the apicaldome and of the first four leaf primordia are supplied by generaldiffusion and not via direct vascular connexions with the restof the plant.     

Initiation of Procambial Strands in Leaf Primordia of Dactylis glomerata L. as an Example of a Temperate Herbage Grass

This study is concerned with the examination of shoots of herbagegrasses, particularly Dactylis glomerata L., for comparisonwith earlier investigations on this species and on the cerealTriticum aestivum L. The main features of procambial strandinitiation are found to be the same in all the herbage grassesexamined and to be very similar to those in Triticum. Thus,in a vegetative shoot, all the procambial strands originatein the leaf primordia independently of the vascular system ofthe older parts of the plant, and extend downwards from theirfirst point of origin. For the median strands, the first pointof origin is in the disc of insertion of the primordium, butfor later strands it is progressively higher, so that the laststrands are initiated well up in the free portion of the primordium.The course of the median and first and second lateral strands,in the first two discs of insertion that they penetrate, isvery regular and predictable. However, two discs of insertionbelow the point of initiation of strands, their course is interruptedby the strands of the primordium directly below (there is onealternating primordium between them), and from this point downwardsvariations, both specific and between individual plants andprimordia, occur. This is the region where connexion with thevascular supply of the rest of the plant is first established.The timing of the arrival of young strands from above and ofthe development of successive nodal plexi from below, via whichmost interconnexions are established, will give some variationto the early development of this region.     

Identification of Shoot Apical Meristem Cells Committed to Form Vascular Elements in Pisum sativum L. and Vicia faba L.

A cytochemical study of naphthol AS-D esterases in vegetativeshoot apices of Pisum sativum and Vicia faba L. has shown thepresence of carboxyl esterases (E.C. 3.1.1.1 [EC] .) in those meristemcells already committed to form vascular elements. These cellsform a sequence linking the morphologically identifiable procambiumto the cells of the tunica layers at a site either already identifiableas the next primordium or which will form the next primordium.The implications of this result are briefly discussed in relationto the control of primordia formation and procambial cell development. Pisum sativum, Vicia faba, determination, vascular tissue, shoot apex, cytochemistry     

Surgical Experiments on the Differentiation of Vascular Tissue in the Shoot Apex of Carrot (Daucus carota L.)

Surgical techniques were applied to the shoot apex of carrot(Daucus carota L.) to test the interpretation that provasculartissue is the initial stage of vascular differentiation andto localize the sources of the influences that control its differentiation.If the apex is isolated laterally by vertical incisions leavingit at the summit of a plug of pith tissue, vascular differentiationproceeds normally and an independent vascular system is formedin the pith plug. If all leaf primordia are systematically suppressed,provascular tissue continues to differentiate as an acropetalextension of the pre-existing vascular system but no furtherdifferentiation occurs. When the apex is isolated laterallyand all leaf primordia are suppressed, provascular tissue continuesto be formed acropetally and is extended basipetally into thepith plug by redifferentiation of pith cells, but no furtherdifferentiation occurs. This tissue reacts positively to histochemicaltests for esterase indicating its vascular nature. If only oneleaf primordium is allowed to develop on an isolated shoot apex,its vascular system develops normally and extends basipetallyinto the pith plug, but there is no extension of provasculartissue into the pith plug. These results support the interpretationthat the initial stage of vascular differentiation is controlledby the apical meristem but that further maturation of vasculartissue depends upon influences from developing leaf primordia.Copyright 2000 Annals of Botany Company Provascular tissue, differentiation, carrot (Daucus carota L.), shoot apex, surgical techniques, leaf primordia     

Effects of Developing Leaves on Stelar Pattern Development in the Shoot Apex of Matteuccia struthiopteris

A developmental study of the normal shoot apex of Matteucciastruthiopteris suggested that patterned stelar differentiationis initiated immediately beneath the single layer of promeristemand occurs prior to the initiation of the youngest leaf primordium.A developmental study in which all leaf primordia were suppressed,with or without lateral isolation of the terminal meristem byvertical incisions, has confirmed this interpretation of stelardifferentiation. Experimentally-induced changes in the tissueimmediately below the promeristem were reflected in the resultingmature structure of the stele. Failure of leaf gap initialsto differentiate, if all leaf primordia were suppressed at theincipient stage, resulted in a mature stele without leaf gaps.Similarly the disappearance of pith mother cells after severalweeks of leaf removal was associated with the formation of astele without pith. Leaf influence was further assessed by allowingone primordium to develop while all others were suppressed.The developing leaf had a small promoting effect on caulinevascular tissue differentiation but its major impact on theexpansion of the parenchymatous tissues of the stele. Characteristicprotoxylem and protophloem failed to differentiate when allleaves were suppressed and, when leaf was allowed to develop,formed only in relation to the leaf.Copyright 1995, 1999 AcademicPress Leaf influence, vascular pattern formation, experimental surgery, shoot apex development, protoxylem, protophloem, Matteuccia struthiopteris     

Lateral Root Development in Attached and Excised Roots of Zea mays L. Cultivated in the Presence or Absence of Indol-3-yl Acetic Acid

The initiation of lateral root primordia and their subsequentemergence as secondary roots have been examined in attachedand excised roots of Zea mays grown in the presence or absenceof indol-3-yl acetic acid (IAA). Exposure to IAA enhanced anlageinception in both batches of roots. In the attached roots, theIAA-induced stimulation of primordium initiation was followedby a similar increase in lateral emergence. IAA treatment, however,had no effect on the number of laterals produced, per centimetreof root, in the excised primaries. Thus, exposure to IAA didnot directly enhance lateral emergence in the attached rootsnor did it stimulate such emergence in the excised ones. Nocorrelation was found between proliferative activity in themeristem at the apex of the primary or the rate of root elongationon the one hand, and either the number of primordia initiated,or the number of laterals produced, per centimetre of primary,on the other. Zea mays, maize, root, primordium, lateral, indol-3-yl acetic acid, meristematic activity     

Initial Differentiation of Vascular Tissue in the Shoot Apex of Carrot (Daucus carotaL.)

Although an initial stage of vascular differentiation precedingprocambium has been demonstrated in ferns, its presence in seedplants has not been accepted generally. In the shoot apex ofcarrot, a short cylinder of provascular tissue is recognizedas the initial stage of vascular differentiation. This firstbecomes apparent through the enlargement and vacuolation ofpith and cortical tissue rather than as a result of specificchanges in the provascular tissue itself. Procambium in discretestrands differentiates acropetally in the provascular tissuein relation to developing leaf primordia. Provascular tissueis not recognized above the axil of the youngest leaf primordiumbut it is distinct at or above the level at which the traceof the youngest primordium diverges. Support for the recognitionof provascular tissue is provided by a positive reaction tohistochemical tests for carboxylesterases in this tissue aswell as in procambium and later stages of vascular differentiation.Copyright1999 Annals of Botany Company. Provascular tissue, carboxylesterase, shoot apex, vascular differentiation, carrot (Daucus carotaL.).     

Initiation of Procambial Strands in Axillary Buds of Dactylis glomerata L., Secale cereale L., and Lolium perenne L.

In axillary buds of Dactylis glomerata L., Secale cereale L.,and Lolium perenne L., the first two procambial strands of theprophyll and the median strand of the first normal leaf areinitiated in the bud in isolation from the vascular system ofthe parent axis. They rapidly form connections with the vascularsystem of the parent axis, presumably by downward extension,as is the case of the strands of leaf primordia on the mainaxis.     

Experimental and Analytical Studies of Pteridophytes: XXXIII. The Experimental Induction of Buds from Leaf Primordia in Dryopteris aristata Druce

When a young leaf primordium of Dryopteris aristata is isolatedon a plug of tissue at any time prior to the formation of alenticular apical cell, the tissue may be induced to developas a solenostelic bud instead of a leaf. The result dependson the rate of leaf development. In some apices, leaf determinationmay take place during the first plastochrone; in others it maybe postponed until the end of the third plastochrone. Isolationof very young primordia by deep incisions in close proximityto the primordium may result in the cessation of meristematicgrowth and the formation of parenchyma. The vascular structureof the isolated primordia and the phyllotaxis of the inducedbuds are described, and the significance of the findings isdiscussed.     

A Note on the Ontogeny of Cross Veins in Digitaria eriantha

The small cross veins that link the longitudinal bundles arisefrom single files of the ground meristem. The tangential divisionsof the cross vein initial produce two vascular parenchyma cellsand two conducting elements. Digitaria eriantha, ontogeny, cross vein, tracheary element, sieve element     

Determination and Differentiation of Leaf and Petal Primordia in Impatiens balsamina

The development of primordia as leaves, petals, or as organsintermediate between leaves and petals can be regulated by photoperiodin Impatiens. In intermediate organs only some parts of theorgan differentiated as petal, and then only in some cell layers.Allometric measurements of primordium shape suggested that intermediateorgans may begin development as petals, and that their intermediatecharacter at maturity resulted from a switch of some parts ofthe organs from petal to leaf development when the primordiawere between 0.5 and 1 mm long. In reverted apices made to re-flower,primordia were not completely determined as leaves until theywere about 750 µm long. Determination typically occurredfirst at the tips and last at the bases of these primordia.The determination of primordia as leaves or petals in Impatiensis discussed in relation to primordium determination in otherspecies. It is suggested that the lack of commitment to flowermay result in relatively late primordium determination in Impatiens. Impatiens balsamina, determination, differentiation, leaf and petal development, flowering, reversion     

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     

The Relationship Between the Distribution of Periclinal Cell Divisions in the Shoot Apex and Leaf Initiation

Periclinal cell divisions in vegetative shoot apices of Pisumand Silene were recorded from serial thin sections by mappingall the periclinal cell walls formed less than one cell cyclepreviously. The distribution of periclinal divisions in theapical domes corresponded to the distributions subsequentlyoccurring in the apices when the young leaf primordia were forming.In Pisum, periclinal divisions were almost entirely absent fromthe I₁ region of the apical dome for half a plastochron justafter the formation of a leaf primordium and appeared, simultaneouslyover the whole of the next potential leaf site, about half aplastochron before the primordium formed. In Silene periclinaldivisions seemed to always present in the apical dome at thepotential leaf sites and also round the sides of the dome wherethe ensheathing leaf bases were to form. Periclinal divisionstherefore anticipated the formation of leaf primordia by occuring,in Pisum about one cell cycle and in Silene two or more cellcycles, before the change in the direction of growth or deformationof the surface associated with primordial initiation. Pisum, Silene, planes of cell division, orientation of cell walls, leaf primordia, shoot apical meristem, plastochron     

Initiation of the Vascular System and the Transition to Flowering in Early Tassels of Zea mays Land Race Chapalote (Poaceae)

Serial transections of young tassels of (Zea mays land race) chapalote revealed relationships between the vascular system in its procambial state and the lateral primordia along the axis. A lateral tassel primordium usually consists of an indefinite rim with a prolongation that will become a tassel branch or spikelet pair. A lateral tassel primordium usually develops via modifications of the vegetative leaf primordium in which the leaf apex is enhanced but the leaf base and the bud it produces are suppressed. The clearest sign of the transition from the vegetative state to the tassel is the scale leaf, which is intermediate in form between a vegetative leaf and a lateral tassel primordium. Procambial traces differentiate in isolation in the tassel axis in response to the lateral tassel primordia. Adjacent procambial traces then link axially into sympodia to initiate the three-dimensional vascular system of the tassel axis. Older sympodia occur near the center of the axis interior to more recently initiated procambial traces. Procambial continuity does not occur between the tassel axis and the lateral primordia until isolated traces in the lateral primordia link with the sympodia in the tassel axis. The transition from distichy to polystichy by the lateral tassel primordia occurs as the narrowing of the leaf base makes space available on the tassel axis for lateral primordia out of the vegetative distichous plane.     

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.     

Reproductive Development of the Apex of Brachiaria decumbens Stapf

The development of Brachiaria decumbens tillers, as based onapex morphology, may be conveniently divided into six phases.These are the (1) vegetative, (2) raceme initiation, (3) spikeletinitiation, (4) spikelet differentiation, (5) inflorescenceexsertion and anthesis, and (6) seed maturation phases. Theonset of reproductive development is characterized by an increasein apex length and proceeds with the expansion of a bud in theaxil of the most recently initiated leaf primordium. This budgives rise to the first raceme and further racemes are formedin basipetal succession. Changes in apex morphology during thefirst four phases of development are described and illustratedwith scanning electron microscope and median longitudinal sectionphotomicrographs. Brachiaria decumbens, signal grass, apex morphology, SEM, median longitudinal sections, developmental phases     

The anatomy of the shoot apex of wheat (Triticutn aestivum L.) during transition from the vegetative to the reproductive state and the determination of the primordia

An investigation was made of the anatomical structure of the shoot apex of wheat in the first four stages of organogenesis according toKuperman (1961). It was found that the shoot apex is first covered only with dermatogen (first stage). Then the hypodermis gradually differentiates (second stage) followed by differentiation of the subhypodermis (third stage). In the first stage, the central core of the apex is formed by more or less uniform isodiametric cells so that no zones are distinguishable. During the initiation of the primordia of the assimilating leaves, i.e. in the second stage, a group of larger cells was observed in the apical part of the hypodermis and can be compared with the central zone described in dicotyledons. Under it there is a characteristic group of smaller cells. In the third stage the differences between these groups of cells become less clear and in the fourth stage are no longer observable. No differences were found in the manner of initiating the leaf and bud primordia during the period of ontogenesis studied. There is, however, an alteration in the extent of growth between the bud primordium and the corresponding leaves. Short-day photoperiodic inhibition, always started on the days when the shoot apices were collected for anatomical study, showed that the determination of the primordia of the leaves and axillary buds as parts of the inflorescence is complete by the end of the third stage, at the time when the primordia in the central part of the ear are initiated     

PERIANTH DEVELOPMENT OF POTAMOGETON RICHARDSONII

Interpretation of the Potamogeton flower is complicated by the attachment of the “perianth segment” to the stamen connective. Developmental studies show that the perianth segments are not outgrowths of the stamen connectives. They are initiated on the floral apex acropetally before the (superposed) primordia of the stamens. After the inception of the stamen primordia, growth occurs in the regions between the primordia of each perianth segment and stamen. Thereby the bases of the developing perianth segment and stamen become united, and in the adult flower eventually the perianth segment is inserted on the connective of the stamen. The primordium of the perianth segment develops from the 2 outer layers (tunica) of the floral apex, in contrast to the stamen primordium which originates from the 3 outer layers. The vascular bundles for each perianth segment–stamen region develop acropetally from 1 common bundle which bifurcates into 1 bundle for the perianth segment and 1 for the stamen. The bundle leading into the perianth segment branches in a more or less dichotomous manner. The veins form none or only 1 or 2 anastomoses at the base of the lamina, whereas the vein endings remain free. The interpretation of the perianth segments is discussed in terms of the classical and the gonophyll theory. Since both theories rest on an ambiguous methodological basis, interpretation is postponed until a new approach to comparative morphology has been worked out and until the floral development of other Helobiales has been studied.     

Water Stress and Apical Morphogenesis in Barley

The formation of new primordia on the apical meristem of barley(Hordeum vulgare L.) was inhibited at levels of soil water potential(-1 bar and less) which had little or no effect on growth ofthe plant. Both leaf growth and morphogenesis of lateral spikeletson the developing ear proceeded at reduced rates at water-potentiallevels which completely suppressed primordium formation. Thissensitivity of the apex to stress was unaffected by floral initiationand was apparently not due to changes in water potential inthe apical tissues. Primordium formation was inhibited eventhough apical water potential was unchanged. Water stress suppressedthe response of the apical meristem to an increase in the photoperiodto which the plant was exposed. In some circumstances, however,the photoperiodic response was displayed after the stress wasrelieved. During an episode of moderate water stress it waspossible to observe an increase in the rate of primordium formationin response to an increase in light intensity.