Middle Lamina Research Articles

Combustion behaviour of composite sandwich propellants containing RDX

Addition of nitramine particles (RDX) to composite solid propellants are promoted in advanced propulsion systems for better performances, high energy, high specific impulse, and environmentally benign combustion products. This study is performed for understanding the combustion mechanism of RDX based matrices and sandwich propellants. Four types of fine AP/Al/binder matrix mixtures are considered, two samples contain 5% and 20% RDX while another two mixtures are without RDX, for comparison. Burning rate experiments are carried out on sandwich propellants and matrices in the pressure range of 2–10 MPa. The condensed phase behaviour of RDX decomposition and RDX melt flow, their influence on the matrix flame and interaction with leading edge flames (LEFs) located over lamina interfaces of sandwich propellants are assessed from quenched surface profiles and SEM micrographs. The effect of middle lamina thickness and pressure on burning rates of sandwich propellants are investigated. It is noticed that different levels of addition of RDX to matrix induces either an increase or decrease in burning rates depending upon the composition and fine AP/binder ratio. The binder melt flow effect is reduced in RDX containing matrix lamina due to presence of near-surface premixed flame. The effect of middle lamina thickness on burning rates of RDX containing sandwich propellants is significant. The overall burning rates of RDX sandwich propellants are relatively high when compared to their corresponding non-RDX counterparts. This is due to the existence of RDX assisted matrix flame that re-establishes the interaction between the LEFs anchored over lamina interfaces. Such matrix flames that are located over the regions of matrix in inter-particle spaces of the propellant microstructure would have a positive effect on the LEFs and outer diffusion flames over coarse AP particles.

Dynamic mechanisms of lamina-induced fracture propagation in tight oil-sand formations

Lamina-induced fractures (LFs) are abundant in lacustrine formations, playing an important role in tight oil resource exploration. There is a lack of research on the dynamic mechanisms of LF propagation in heterogeneous formations characterized by complex combinations of lamina and surrounding rock. For this reason, we conducted triaxial compressive tests of 28 long column samples and Brazil split tests of 37 short column samples using six core samples with different lithologies, including nine column samples tested using a CT scanner and an acoustic emission apparatus. We designed this experiment to study the LF propagation process and its dynamic mechanisms within different lithologies. Our results show that (1) we classified three mechanical LF types based on the relationship between stress state and lamina angle: tensile LF (T-LF), shear LF (S-LF), and hybrid LF (H-LF); (2) H-LF only propagated in the middle lamina angle range (approximately 11°–64°); T-LF was inclined to propagate in the low lamina angles; and S-LF was inclined to propagate in the high lamina angles; and (3) in the Ordos Basin, T-LF, TF, and SF propagated mutually in outcrops with low confining pressures. T-LF, H-LF, and SF propagated mutually in the Yanchang Formation, an outcrop that greatly contributes to tight oil sweet spot exploration. Furthermore, unlike the in situ accumulation theory, we find that it is possible for hydrocarbon migration to occur in tight reservoir formations, in which the LF acts as a hydrocarbon migration pathway during the tectonic phase.

Effect of Curing Agent on the Plateau Burning Mechanism of Solid Propellant Sandwiches

This study focuses on further understanding of the plateau burning mechanism of sandwiches containing a matrix of fine ammonium perchlorate (AP) particles and low/high-melt binder as affected by the type of the curing agent. Pure binder sandwiches and matrix sandwiches of various fine AP/binder ratios with two different curatives, isophorone diisocyanate (IPDI) and toluene diisocyanate (TDI), are considered. The burning rate versus middle lamina thickness curves are obtained to examine the interaction between the leading-edge flames (LEFs) anchored over the lamina interface edges. Pressure variation on matrix sandwich burning rates is considered in 1–12 MPa range. High-magnification combustion images and quenched surface features and profiles of these sandwiches are also examined. The IPDI-cured binder causes larger LEF standoff and inter-LEF distances in both pure binder and matrix sandwiches than the TDI-cured one. Consequently, the disparity in the burning rates with middle lamina thickness of IPDI-cured matrix sandwiches is minimal. The binder melt-layer thickness and its melt viscosity determine the flammability limits and midpressure extinction regimes of matrices depending upon the curative. Plateau burning trends of matrix sandwiches with large middle lamina thickness (greater than ) are observed to coincide with midpressure extinction of their corresponding matrices.

Effect of binder melt flow on the leading edge flames of solid propellant sandwiches

This study reports the effect of binder melt flow on the burning behaviour, specifically the burning rate controlling sites referred to as the leading edge flames (LEFs), of different types of sandwich propellants, namely, pure, micro-aluminized and nano-aluminized binder. The distance between the LEFs anchored over the lamina interface edges of sandwiches is measured from the combustion images captured under high spatio-temporal resolution. Similarly, the extent of binder melt flow is also measured from the quenched surfaces of sandwiches. The burning rate experiments are performed as well on sandwiches with different middle lamina contents and thicknesses at pressures of 2, 4 and 7 MPa. Two different curing agents are considered to examine the melt flow behaviour of the binder. The curing agent significantly influences the inter-LEF distance mainly in the case of pure binder sandwiches, however, its effect is negligible in aluminized binder sandwiches because of the presence of Al particles that impedes the flow to appreciable extent. Substantial protrusion of the middle lamina relative to the lamina interfaces is observed in micro-aluminized binder sandwiches due to significant accumulation of Al particles on the burning surface. In the case of nano-aluminized binder sandwiches, such protrusion is relatively marginal since nano-Al particles burn quickly, which enables the gas phase flame to locate close to the burning surface, although the extent of Al accumulation is considerably more than in the former case. This causes the nano-aluminized binder sandwiches as a whole to burn significantly faster than the other two cases in the pressure range (<7 MPa) where the LEFs predominantly control the sandwich burning rates.

Is endoscopic endonasal transsphenoidal surgery increases the susceptibility to rhinosinusitis.

The aim of the study was to analyze whether the measurement of changes in the anatomical position and volume of middle concha, the volume changes in the area between the middle concha and lamina papyracea, the evaluation of opacification in major paranasal sinuses, and osteomeatal complex occlusion in cases with middle concha by out-fracture technique during endoscopic endonasal transsphenoidal approach is a minimally invasive surgery, and also to find out whether these changes lead to the development of tendency to rhinosinusitis. It was a retrospective clinical study. Forty-five cases, between 2013 and 2015, planned for endoscopic endonasal transsphenoidal surgery due to hypophyseal pathology at the Neurosurgery Departments of Marmara University Hospital were evaluated retrospectively. The patients were evaluated for the changes in the anatomy of the middle concha and the effects of these changes to paranasal sinuses by paranasal computed tomographies were studied at the preoperative second week and postoperative 12 month. The Lund-Mackay scoring system was used for the evaluation of opacification in the five major paranasal sinuses and occlusion of the osteomeatal complex in the pre- and postoperative period. The Lund-Mackay scoring system was used to analyze the paranasal computed tomography of the patients at the preoperative 2weeks and postoperative first year. According to the Lund-Mackay scoring system, no significant difference was detected between the preoperative and postoperative opacification of paranasal sinuses (p>0.05). Besides, there was also no significant difference between the preoperative and postoperative osteomeatal complex occlusion (p>0.05). Considering the distance between middle concha and lamina papyracea following the out-fracture of the middle concha, a significant lateralization of 0.5mm between the preoperative and postoperative period was observed (p<0.05). In addition, a significant change was also detected in the volume of middle concha (p<0.05). The volume of the area between the middle concha and lamina papyracea was decreased with a statistical significance (p<0.05). The endoscopic endonasal transsphenoidal surgery causes some variations in the structures of the middle concha, paranasal sinuses, and OMC, but these changes do not lead to significant rhinologic pathologies.

Combustion mechanism of composite solid propellant sandwiches containing nano-aluminium

Combustion of sandwiches made of alternating laminae of ammonium perchlorate (AP) and a matrix of hydrocarbon binder mixed with fine AP and nano-aluminium particles is considered in the 1–12MPa pressure range. Companion non-aluminized sandwiches are studied for comparison. The nano-Al particles are produced from the electrical wire explosion process in the size range ∼45nm, and added to the matrix containing either 5 or 45µm AP particles. Three categories of middle lamina thicknesses—thin (100–170µm), intermediate (200–280µm), and thick (370–480µm)—are examined in all the sandwiches. High-speed high-magnification imaging of the combustion is performed, and surface profiles and features of quenched sandwiches are investigated, besides burning rate measurements. The sandwich burning rates are compared with those of the corresponding matrices and AP laminae burning alone as well. The results imply the close location of lamina leading edge flames (L-LEFs) over the lamina interface edges, leaving the outer AP laminae inclined steeply relative to the matrix middle lamina, in the nano-aluminized cases relative to the non-aluminized versions. This observation suggests that the L-LEFs control the burning rates of the sandwiches even in the case of the fast burning matrix that contains nano-Al. The non-aluminized sandwiches containing the 5µm AP show higher burning rates than their 45µm AP counterparts because of the enhanced heat feedback available to the matrix burning surface from the extended fuel-rich branches of adjacent L-LEFs in the former case. Therefore, the enhancement in burning rate is minimal with the inclusion of nano-Al in sandwiches with smaller fine AP size, whereas the nano-aluminized sandwiches containing larger fine AP particles exhibit such enhancement. These effects affirm previous results on propellants with smaller fine and coarse AP sizes, where the burning rate is not enhanced by nano-Al.

Gas phase flame structure of solid propellant sandwiches with different reaction mechanisms

The gas phase flame structure of composite solid propellant sandwiches with two ammonium perchlorate (AP) laminae sandwiching a middle lamina containing a homogenized mixture of hydroxyl terminated poly-butadiene (HTPB) binder and fine AP particles is numerically simulated. Experimentally obtained surface profiles and burning rates are used to decouple the gas phase from the condensed phase. In the literature, three global gas phase reactions for (1) the AP mono-propellant flame, (2) the primary diffusion flame, and (3) the final diffusion flame between gaseous products of AP decomposition/deflagration and the binder have been considered. Alternatively, multi-step detailed/reduced reaction mechanisms have been adopted to represent all these flames together. In the present study, three different reaction mechanisms are evaluated: (1) global chemistry using symbolic species for the three flames, (2) global chemistry using a 12-species mechanism for the above three flames, and (3) a 72-step reaction mechanism with 39-species representing all the flames. The 12-species, 3-step global reaction mechanism provides a temperature field similar to the 72-step mechanism unlike the global reaction mechanism involving symbolic species. Further, the triple flame structure of the primary diffusion flame and the ‘canopy’ premixed flame over the middle lamina (for a self-deflagrating mixture of fine AP and binder) are well captured by the 12-species mechanism unlike the other two considered.

Expression of human papillomavirus DNA in keratoacanthoma tissue

Objective To detect the infection rate of human papillomavirus (HPV) types in keratoacanthoma (KA) tissue, and to study the distribution of HPV DNA-positive cells. Methods In situ hybridization was performed to detect HPV subtypes 6/11, 16/18, and 31/33 in paraffin-embeded tissue samples from 46 patients with KA and 34 nomal human controls. Results Mucosal HPV DNA was positive in 31 (67.39%)of the 46 KA samples, and mixed infection rate of HPV amounted to 83.87% (26/31) in these positive samples.The DNA of tested mucosal HPV types existed mainly in episomal status. Positive reactions were observed within or at the edge of nuclei. HPV infected cells showed a stripe-, slice-like or focal distribution mainly in superficial middle lamina of spinous layer at the bottom or lateral margin of crater-shaped epidermal depression. No mucosal HPV DNA was noted in normal control samples. Conclusions Compared with normal controls, patients with KA are infected with mucosal HPV at a higher frequency, and mucosal HPV DNA mainly exists in episomal status in KA tissue, hinting that HPV infection play a certain role in the pathogenesis of KA. Key words: Keratoacanthoma; Papillomavirus, human; In situ hybridization

Accumulation of nano-aluminium during combustion of composite solid propellant mixtures

Nano-aluminium particles are produced using the electric wire explosion process in an inert atmosphere at our institute. This paper reports the characterization of nanoaluminium particles in combination with other solid propellant ingredients for their thermal and combustion behaviour. High-heating-rate hot-stage microscopic experiments are performed with different mixtures of propellant ingredients. The effects of addition of nano-aluminium versus micron-sized aluminium in the middle lamina of sandwiches are analyzed for burning rates and by means of scanning electron microscope and transmission electron microscope micrographs of quench-collected aluminium agglomerates. Nano-aluminized sandwiches with thinner middle lamina show slightly higher burning rates than micron-sized aluminized ones. The quenched surface of nano-aluminized sandwiches shows relatively smaller aluminium agglomerates than with micron-sized aluminium. The resultant particle sizes of nano-aluminium agglomerates is in the range of 1–5 µm, which indicates a higher rate of agglomeration than with micro-aluminium, but these sizes are small relative to the agglomerates of the latter. This is also confirmed by quench-collected agglomerates of nano-aluminium emerging from the burning surface of a composite propellant.

Simultaneous determination of shear strength and shear modulus in glued-laminated timber using a full-scale shear block specimen

A newly developed full-scale shear block specimen was used to simultaneously determine the shear strength and shear modulus of glued-laminated timber. The shear modulus was calculated using the shear strain distribution measured by means of digital image correlation. To obtain the exact relationship between shear modulus and shear strength, the shear strain in the intended shear plane was measured. A relatively high correlation coefficient of 0.75 was found between the shear modulus and nominal shear strength in two types of glued-laminated timber tested: sugi (Cryptomeria japonica) and Douglas-fir (Pseudotsuga menziesii). Good correlation was also found between the density of middle lamina and shear strength. However, the relationships to density showed species dependence, which was not observed in the relationship between shear modulus and shear strength.

Coupling of leading edge flames in the combustion zone of composite solid propellants

The burning rate trends of both pure binder sandwiches and those containing a mixture of binder with fine ammonium perchlorate particles are investigated over appreciable ranges of pressure and thickness of the middle lamina in the sandwich. The crucial aspect of this study is the wide variation of the fine ammonium perchlorate/binder ratio in the middle lamina of the sandwiches. The sandwiches with fine ammonium perchlorate particles have been quenched by rapid depressurization and their surface profiles have been carefully mapped for different conditions. The gas phase flame structure is estimated through numerical simulation, with the observed surface profiles and burning rates taken as inputs. It is found that, as the fine ammonium perchlorate particles are added to the binder in small quantities, they tend to dilute the fuel and increase the lamina thickness for maximum interaction between the leading edge flames, but the trend in the variation of the optimum thickness for maximum burning rate with increase in pressure is reversed when compared to the case of pure binder sandwiches reported earlier. The burning rates of these sandwiches decrease slightly relative to those of the pure binder sandwiches. With higher fine AP content, there is greater interaction between the leading edge flames due to the inward shift of the stoichiometric surface and the extension of the fuel-rich sides of the leading edge flames over the middle lamina, ultimately resulting in the establishment of the canopy premixed flame between them. This restores the optimum thickness trends with pressure as in the case of pure binder sandwiches.

Histochemical correlations between egg capsule laminae and the female gonoduct reveal the process of capsule formation in the Muricidae (Neogastropoda: Mollusca)

Summary Limited information is currently available on the process of encapsulation and degree of variation in the micromorphology of egg capsules deposited by muricid gastropods. In this study, a histochemical approach was employed to determine the origin of egg capsule laminae within the female gonoduct of Dicathais orbita. Comparisons of capsule micromorphology from this Australian Rapaninae, with species from the Muricinae and Ocenebrinae, suggest that whole capsule and lamina 1 (L1) thickness, L2 fiber orientation and intracapsular fluid biochemistry display interspecific variability. Furthermore, the method of capsule laminae deposition was found to be substantially different to previous descriptions of muricid capsule formation. Simultaneous examination of capsule and gonoduct biochemistry in D. orbita revealed that the intracapsular fluid nd inner capsule wall (L3) contain secretions from the posterior lobe of the capsule gland; the thick middle lamina (L2) is derived from the lateral capsule gland lobes; and the outer layers (L1 and L0) include dorsal capsule lobe, albumen gland and pedal gland components. The insight gained into the processes involved in capsule formation imply that interspecific differences in capsule micromorphology are dependent on the timing of egg entry during capsule formation, and the protein content of posterior lobe secretory products coupled with the presence or absence of albumen gland secretions. This investigation supplements existing evidence on the origin of capsule components and variability in capsule micromorphology in the Muricidae by providing a simple method for deciphering the complex process of encapsulation in neogastropods.

Utilization of fresh human tympanic membranes for structural analysis and cytokeratin immunocytochemistry implementing resin techniques

Conclusions. The results of this study have demonstrated for the first time that tympanic membrane (TM) structure is preserved following removal of fresh, normal tissue from patients undergoing surgery. Greater clarity has been demonstrated using resin sections than in previous studies on paraffin sections. Of particular note, cytokeratin (CK) immunocytochemistry was successfully performed on resin sections, which has not been previously reported. This may have potential applications for future work involving tissues that express CKs. Objectives. To analyse the structure of normal, fresh human TM specimens after surgical removal and to evaluate their CK immunocytochemistry using resin techniques, neither of which have been demonstrated previously. Material and methods. Seven TM specimens were removed during surgery and then preserved in a modified Karnovsky's fixative. Semi-thin and thin sections were examined by means of light and electron microscopy, respectively. For comparison purposes, paraffin block-embedded specimens were also sectioned. CK immunocytochemistry was performed on semi-thin sections using standard immunoperoxidase techniques, with expression being demonstrated using light microscopy. Results. The three-layer architecture of the TM was preserved. The morphology of the TM was vastly superior in the semi-thin resin sections than in the thicker paraffin sections. The outer, middle and inner layers were clearly demonstrated. The integrity of the outer epithelial layer was maintained, with an outer keratinizing stratum corneum and underlying stratum granulosum, stratum spinosum and stratum basale layers resting on the basal lamina. The thin inner mucosal layer was also viable, consisting of simple squamous or cuboidal cells. Preservation of the middle lamina propria was achieved, with demonstration of the outer radial and inner circular fibres. CK immunocytochemistry utilizing resin techniques provided excellent staining of CK 7 and 8 in the inner layer, with positive staining of CK 5 and 10 in the outer layer.

Laminar organization of the developing lateral olfactory tract revealed by differential expression of cell recognition molecules.

The projection neurons in the olfactory bulb (mitral and tufted cells) send axons through the lateral olfactory tract (LOT) onto several structures of the olfactory cortex. However, little is known of the molecular and cellular mechanisms underlying establishment of functional connectivity from the bulb to the cortex. Here, we investigated the developmental process of LOT formation by observing expression patterns of cell recognition molecules in embryonic mice. We immunohistochemically identified a dozen molecules expressed in the developing LOT and some of them were localized to subsets of mitral cell axons. Combinatorial immunostaining for these molecules revealed that the developing LOT consists of three laminas: superficial, middle, and deep. Detailed immunohistochemical, in situ hybridization, and 5-bromodeoxyuridine labeling analyses suggested that the laminar organization reflects: 1) the segregated pathways from the accessory and main olfactory bulbs, and 2) the different maturity of mitral cell axons. Mitral cell axons of the accessory olfactory bulb were localized to the deep lamina, segregated from those of the main olfactory bulb. In the main olfactory pathway, axons of mature mitral cells, whose somata is located in the apical sublayer of the mitral cell layer, were localized to the middle lamina within LOT, while those of immature mitral cells that located in the basal sublayer were complementarily localized to the superficial lamina. These results suggest that newly generated immature axons are added to the most superficial lamina of LOT successively, leading to the formation of piled laminas with different maturational stages of the mitral cell axons.

The shaking B gene in Drosophila regulates the number of gap junctions between photoreceptor terminals in the lamina.

The molecular structure of insect gap junctions differs from that in vertebrates, and in Drosophila is possibly encoded by the shaking B (= Passover) locus. shaking B2 is a null allele that acts in the nervous system. In the shakB2 mutant, one site of action are gap junctions between photoreceptor terminals in the cartridges of the lamina, beneath the compound eye, which we assayed from the number of close-apposition profiles in thin-section EM. The number of profiles in the Canton-S (C-S) wild type is about 0.5 per cartridge per section in distal and mid-lamina depths, and significantly less, about one quarter this value, closer to the brain, in the proximal lamina. In shakB2, there are fewer profiles, approximately one quarter the number of appositions in distal and mid-lamina depths as in C-S, and their number does not differ significantly from those at the proximal depth in either the mutant or wild type. Thus mutant action is associated with a reduced number of appositions at distal and mid-lamina depths. We propose that R1-R6 gap junctions are partitioned into at least two strata, proximal and distal, and that two populations of gap junctions exist, one extending throughout the lamina that does not require shakB, and a second at distal and mid-depth levels, which does. The number of gap junctions is reduced in mutant shakB2, and surviving appositions at distal and middle lamina depths possibly have wider clefts than in C-S. Gap junctions are reduced equally between all R1-R6 terminals, so the two different types of junction proposed, shakB2- and non-shakB2-dependent, can apparently express in a single receptor terminal.

Taxonomy of Coscinodon arctolimnius (Bryopsida, Grimmiaceae) including a New Subspecies from Asia

Taxonomy of Coscinodon arctolimnius (Bryopsida, Grimmiaceae) including a New Subspecies from Asia

The fine structure of the guinea pig tympanic membrane with special reference to the fiber arrangement.

The tympanic membrane of the guinea pig was studied by electron microscopy. The pars tensa was composed of three layers, i.e., the outer epidermis, the middle lamina propria and the inner mucosal membrane. The pars flaccida was negligible in size. The fibers in the lamina propria of the tympanic membrane were not clearly observable by conventional electron microscopy. With the silver methenamine method the fibers were more intensely stained than the epithelial cells and could thus be distinctly demonstrated by this method. In the inferior larger portion of the pars tensa, the lamina propria contained the outer radial and the inner circular fiber bundles. The radial fiber bundles were thicker and more densely packed than the circular ones. This fact indicates that the radial fibers form the major skeleton of this portion. The superior smaller portion of the pars tensa contained several layers of irregularly arranged fiber bundles in the lamina propria and was considerably thicker than the inferior larger portion.

Electrode for selective recording of electromyograms from intercostal muscles

An electromyogram-recording electrode is described that makes it possible to record separately the electrical signals generated within two closely approximated muscle layers. The device consists of two bipolar wire hook electrodes embedded in opposite faces of a thin laminated plastic wafer. The middle lamina of the wafer is a sheet of metal foil that shields the electrical field on one side of the wafer from the bipolar electrode on the other side. The device was tested by inserting it from the inside of the chest wall between the internal and external intercostal muscle layers. Signals from the two muscle layers were clearly separated. Single motor unit spikes were attenuated by factors ranging from 41 to 2.4. The device can be implanted with minimal trauma to surrounding muscles and is suitable for chronic animal experiments.

A Golgi study of the neuronal population of the marginal zone (lamina I) of the rat spinal cord.

On the basis of dendroarchitecture and cell body shape, complemented with morphometry of dendritic ramification, four major neuronal types were distinguished in lamina I of the spinal cord of the rat. (I) Fusiform spiny cells (39% of impregnated neurons) have longitudinal spindle-shaped perikarya with bipolar, less frequently unipolar, dendritic trees rich in pedicled spines and a thin, beaded longitudinal axon; such neurons occur mainly in the lateral marginal zone. In type IA cells (33% of the total), the dendritic domain occupies a narrow longitudinal area, while in type Ib cells (6%) the polar dendritic trees partly arborize ventrally. Fusiform neurons are considered intrinsic cells connected with the longitudinal afferent plexus in lateral lamina I, and in type IB cells also receiving primary input in the substantia gelatinosa. (II) Multipolar cells (23%) have a dense dendritic arbor originating from numerous primary trunks and they predominate in the medial marginal zone. The dendritic arbor is moderately extended dorsoventrally in type IIA cells and reaches lamina III in the larger type IIB cells. The former possess a variety of spines, axonlike processes and sometimes an unmyelinated axon, and are presumably interneurons, while type IIB cells show a thick tapering axon that is probably myelinated. (III) Flattened aspiny neurons (13%) with a polygonal body flattened in the horizontal plane, and a horizontal dendritic arbor confined to lamina I; these cells predominate in middle lamina I. (IV) Pyramidal neurons (25%) have longitudinally elongated perikarya that bulge into the white matter. The arbor has a large longitudinal and lateromedial spread and includes branches which ramify in the white matter. Types III and IV show the classical lateromedially elongated orientation of the marginal cells of the old literature; they show thick tapering axons and probably make up the bulk of the projection neurons of rat lamina I.

The mechanism of plication inception in palm leaves: histogenetic observations on the pinnate leaf of Chrysalidocarpus lutescens

The reduplicate, pinnately compound leaf of Chrysalidocarpus lutescens begins development as a hood-shaped primordium with a smooth lamina. By the time the leaf is 5.5 mm in length, 40–42 pleats consisting of alternating ridges and furrows have been formed in a submarginal position on each half of the lamina. Serial sections of plastic-embedded leaf primordia were studied to distinguish between two alternative mechanisms of plication inception proposed in the literature: differential growth or tissue separation. Counts of the number of cell layers in the lamina during the period of plication initiation support the concept of differential growth, i.e., an increase in the number of tissue layers is observed at the site of furrow formation rather than a decrease in cell layers which would be expected if tissue separation were occurring. Ridges appear first on the adaxial side of the lamina, accompanied by periclinal and oblique divisions in the middle lamina layers. Further growth is associated with zones of anticlinal divisions alternating with the ridges. This plate meristem growth increases the surface area of a portion of the lamina bounded by the leaf apex, the unplicate margin, the petiole, and the rachis, and therefore tends to be accommodated by a buckling of the lamina between adaxial ridges. Intercalary growth of the intercostal sector causes the adaxial and abaxial ridges to be displaced from each other, deepening the plications. Examination of early stages of leaf development with the transmission electron microscope corroborates these results; there is no indication of schizogenous separation of cells within the ground tissue of the lamina or between protodermal cells. The protoderm and its cuticle are continuous at all stages of plication initiation, supporting the concept of differential growth.