Showing papers by "University of Melbourne published in 1991"

Digital terrain modelling: A review of hydrological, geomorphological, and biological applications

Abstract: The topography of a catchment has a major impact on the hydrological, geomorphological. and biological processes active in the landscape. The spatial distribution of topographic attributes can often be used as an indirect measure of the spatial variability of these processes and allows them to be mapped using relatively simple techniques. Many geographic information systems are being developed that store topographic information as the primary data for analysing water resource and biological problems. Furthermore, topography can be used to develop more physically realistic structures for hydrologic and water quality models that directly account for the impact of topography on the hydrology. Digital elevation models are the primary data used in the analysis of catchment topography. We describe elevation data sources, digital elevation model structures, and the analysis of digital elevation data for hydrological, geomorphological, and biological applications. Some hydrologic models that make use of digital representations of topography are also considered.

A performance indicator of teaching quality in higher education: The Course Experience Questionnaire

Abstract: Performance indicators (PIs) in higher education have focused chiefly on research outputs. They have largely ignored the teaching function of universities and colleges. This article outlines the development of a student evaluation instrument designed to measure the teaching performance of academic organisational units. The theory of teaching and learning that underlies the Course Experience Questionnaire (CEQ) is described. The instrument's statistical qualities and its ability to discriminate intelligibly between different courses are discussed in the context of results from national trials in Australian higher education. The principal conclusion reached is that the CEQ offers a reliable, verifiable and useful means of determining the perceived teaching quality of academic units in systems of higher education that are based on British models. Several technical and political issues remain unresolved in its application as a PI.

Protein kinase phosphorylation site sequences and consensus specificity motifs: tabulations.

Abstract: Publisher Summary This chapter presents a table of phosphorylation site sequences for protein-serine/threonine and protein-tyrosine kinases. Over 240 phosphorylation site sequences along with the phosphorylated residue(s) are included in this table. The chapter presents another table containing consensus phosphorylation site motifs for each enzyme. The frequencies listed in this table are derived from the first table, unless indicated otherwise. The S : T ratio is for the total number of phosphorylation sites.

On the mechanics of earthquake afterslip

Abstract: We propose a model for earthquake afterslip based on rate and state variable friction laws. In the model, afterslip is attributed to the interaction of a velocity-weakening region at depth (within which earthquakes nucleate) with an upper region of velocity-strengthening frictional behavior. The existence of this upper region is supported by independent seismologic observations and the results of laboratory friction experiments. In our model, afterslip is the result of relaxation of a stress perturbation within the velocity-strengthening region, which arises when an earthquake propagates into that region from below. We derive the stress perturbation and its decay from the friction constitutive law using a simple, 1 degree-of-freedom approximation for the elastic interaction between the fault and its surroundings. This approximation is based on thickness-averaged displacements and slip velocities within the velocity-strengthening region, which is assumed to slip as a rigid block. Coseismic and postseismic slip are coupled through the thickness-averaged stiffness k of the velocity-strengthening region. We assume k to be inversely proportional to the thickness of this region, which means that thicker velocity strengthening regions have a greater tendency to arrest coseismic slip. We model the afterslip-time histories of the 1966 Parkfield and 1987 Superstition Hills earthquakes and relate the model parameters to physical parameters which may govern the rheologic behavior of the faults. In accord with field observations, our model predicts (1) that afterslip on some faults scales with the thickness of the (unconsolidated) sedimentary cover and (2) that proportionally more afterslip occurs for earthquakes in which coseismic surface slip is small compared with coseismic slip at depth. Velocity-strengthening frictional behavior is to be expected for faults within poorly consolidated sediments and for those that contain significant gouge zones (about >500 m) within their shallow regions (<3–5 km). Combining our results with those of recent laboratory friction studies indicates that relatively young faults with little accumulated fault gouge should exhibit little afterslip.

Canopy seed storage in woody plants

Abstract: The retention of seeds in the plant canopy for one to 30 years or more is termed serotiny. It is well represented floristically and physiognomically in fire-prone, nutrient-poor and seasonally-dry sclerophyll vegetation in Australia, and to a lesser extent, South Africa followed by North America. While the seed-storing structures vary greatly, all will release their propagules following exposure to the heat of a fire (pyriscence). This phenomenon can be contrasted with seed release at maturity (non-storage) and soil storage of seeds. Although the evolutionary requirements for serotiny are clear, its adaptive advantages over other seed storage syndromes are largely the subject of conjecture in the absence of comparative experiments. Nine hypotheses were assessed here. Canopy storage maximises the quantity of seeds available for the next post-fire generation (unlike non-storage). Synchronized post-fire release satiates post-dispersal granivores (unlike non-storage and soil storage) and ensures arrival on a seed bed conducive to seedling recruitment (unlike non-storage). Canopy stored seeds are better insulated from the heat of a fire than non-stored, and probably soil-stored, seeds. Fluctuating annual seed crops, the opportunity for post-fire wind-dispersal, the possible advantages of dense stands of adults, short lifespan of the dispersed seeds and their optimal location in the soil for germination have only a limited role in explaining the advantages of serotiny. It is concluded that canopy seed storage is favoured in regions where seed production is restricted and inter-fire establishment and maturation are unlikely. In addition, these regions have a reliable seasonal rainfall and are subjected to intense fires at intervals occurring within the reproductive lifespan of the species.

The complex action of phosphonates as antifungal agents

Abstract: C O N T E S T S

Localization of Parathyroid Hormone-related Protein in Breast Cancer Metastases: Increased Incidence in Bone Compared with Other Sites

Abstract: Parathyroid hormone-related protein (PTHrP) has recently been identified in 60% of a series of primary breast cancers. The detection of a bone-resorbing factor in tumors with a propensity to metastasize to bone prompted study of PTHrP in breast cancer metastasis. PTHrP was localized by immunohistology in 12 of 13 (92%) breast cancer metastases in bone and in 3 of 18 (17%) metastases in non-bone sites. The statistical difference was highly significant (P less than 0.0001). Production of PTHrP as a bone-resorbing agent may contribute to the ability of breast cancers to grow as bone metastases.

Cytotoxic T-lymphocytes derived from patients with breast adenocarcinoma recognize an epitope present on the protein core of a mucin molecule preferentially expressed by malignant cells.

Abstract: A population of tumor-reactive cytotoxic T-cells can be propagated from tumor-draining lymph nodes of patients with breast adenocarcinoma. These T-cells specifically recognize breast and pancreatic tumor cells in a major histocompatibility complex (MHC)-unrestricted fashion but not other tumors of epithelial origin or the natural killer target K562. The tumor-specific but MHC-unrestricted lytic activity of these cytotoxic T-lymphocytes (CTLs) is mediated through the alpha/beta T-cell receptor. The molecule recognized by these CTLs is ductal epithelial mucin produced by breast and pancreatic adenocarcinomas. The protein core of the mucin consists of multiple tandem repeats of a 20-amino acid sequence. Antibody SM3, directed against a determinant on the mucin protein core preferentially expressed on malignant cells is able to significantly inhibit lysis of tumor cells by the CTL, while other antibodies binding to different core epitopes are not. Normal breast epithelial lines, which also express mucin but not the SM3 epitope, are not lysed by these tumor-reactive CTLs or act as cold target inhibitors of lysis of tumor lines. The data suggest that the highly repetitive nature of the mucin allows cross-linking of the T-cell receptor on mucin-specific T-cells and therefore accounts for the lack of MHC restriction seen in this system. They further suggest that the mucin core epitope recognized on tumor cells is not expressed on normal epithelial cells in a manner that can be recognized by tumor-reactive CTLs. These findings support the role of mucins as important tumor-associated antigens mediating the cellular response to certain human cancers and suggest that epithelial mucin core sequences might form the basis for an effective vaccine to augment the antitumor immune response.

Top-Down Versus Bottom-Up Theories of Subjective Well-Being

Abstract: This paper addresses issues of causal direction in research on subjective well being (SWB). Previous researchers have generally assumed that such variables as domain satisfactions, social support, life events, and levels of expectation and aspiration are causes of SWB. Critics have pointed out that they could just as well be consequences (Costa and McCrae, 1980, Veenhoven, 1988). In some contexts this has been referred to as the top-down versus bottom-up controversy (Diener, 1984). The main purpose is to propose a general statistical model which holds promise of resolving this controversy. The model can be used when three or more waves of panel data are available. It is used here to assess causal direction between six domain satisfactions (marriage, work, leisure, standard of living, friendship and health) and SWB. Data are drawn from four waves of an Australian Quality of Life panel survey (1981-1987) with an initial sample size of 942.

Simplest Z' model.

Abstract: The differences in family-lepton numbers are anomaly-free in the minimal standard model (MSM), and can therefore be gauged. For three generations of quarks and leptons, three models emerge depending on whether (i) ${L}_{e}\ensuremath{-}{L}_{\ensuremath{\mu}}$, (ii) ${L}_{e}\ensuremath{-}{L}_{\ensuremath{\tau}}$, or (iii) ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ are gauged. These are the simplest models to feature a ${Z}^{\ensuremath{'}}$ boson because no fermions beyond those already present in the MSM are required to cancel gauge anomalies. We analyze the phenomenology of models (i) and (ii) in detail, and present constraints derived from low-energy neutral-current data and CERN LEP data. We find that these ${Z}^{\ensuremath{'}}$ bosons may have a relatively low mass yet still evade present experimental bounds, while remaining detectable in current accelerators. The introduction of neutrino masses into the models is then considered. We discuss how one may incorporate both the reported 17-keV neutrino, and the Mikheyev-Smirnov-Wolfenstein effect solution of the solar-neutrino problem. We then describe how to embed the extra U(1) gauge group into a horizontal SU(2)-symmetry group acting on leptons.

Predictive value of CD4 lymphocyte numbers for the development of opportunistic infections and malignancies in HIV-infected persons.

Abstract: Infection with the human immunodeficiency virus (HIV) results in progressive depletion of the CD4 subset T-lymphocytes and the development of opportunistic infections and certain malignancies. Charts were reviewed for 185 HIV-infected individuals with 265 AIDS-defining illnesses (ADIs) who had T-lymphocyte subset analyses performed within 2 months prior to or 1 month following the diagnosis. Also included were 22 HIV-infected patients with oral candidiasis and 20 with asymptomatic infection. Significant differences in CD4 lymphocyte numbers were observed between the 12 ADIs, oral candidiasis, and asymptomatic infection, allowing them to be grouped into five general categories, based on mean CD4 count: (a) asymptomatic infection, CD4 greater than 500/mm3; (b) oral candidiasis and tuberculosis, range 250-500/mm3; (c) Kaposi's sarcoma, lymphoma, and cryptosporidiosis, range 150-200/mm3; (d) Pneumocystis carinii pneumonitis, disseminated Mycobacterium avium complex, herpes simplex ulceration, toxoplasmosis, cryptococcosis, and esophageal candidiasis, range 75-125/mm3; (e) cytomegalovirus retinitis, less than 50/mm3. Our data concur with clinical impressions and provide a basis for interim treatment and prophylaxis recommendations.

A Compensated-Redlich-Kwong (CORK) equation for volumes and fugacities of CO 2 and H 2 O in the range 1 bar to 50 kbar and 100 1600°C

Abstract: We present a simple virial-type extension to the modified Redlich-Kwong (MRK) equation for calculation of the volumes and fugacities of H2O and CO2 over the pressure range 0.001–50 kbar and 100 to 1400°C (H2O) and 100 to 1600°C (CO2). This extension has been designed to: (a) compensate for the tendency of the MRK equation to overestimate volumes at high pressures, and (b) accommodate the volume behaviour of coexisting gas and liquid phases along the saturation curve. The equation developed for CO2 may be used to derive volumes and fugacities of CO, H2, CH4, N2, O2 and other gases which conform to the corresponding states principle. For H2O the measured volumes of Burnham et al. are significantly higher in the range 4–10 kbar than those presented by other workers. For CO2 the volume behaviour at high pressures derived from published MRK equations are very different (larger volumes, steeper (∂P/∂T)V, and hence larger fugacities) from the virial-type equations of Saxena and Fei. Our CORK equation for CO2 yields fugacities which are in closer agreement with the available high pressure experimental decarbonation reactions.

A comparison of steady-state evoked potentials to modulated tones in awake and sleeping humans.

Abstract: Steady-state evoked potential responses were measured to binaural amplitude-modulated (AM) and combined amplitude- and frequency-modulated (AM/FM) tones. For awake subjects, AM/FM tones produced larger amplitude responses than did AM tones. Awake and sleeping responses to 30-dB HL AM/FM tones were compared. Response amplitudes were lower during sleep and the extent to which they differed from awake amplitudes was dependent on both carrier and modulation frequencies. Background EEG noise at the stimulus modulation frequency was also reduced during sleep and varied with modulation frequency. A detection efficiency function was used to indicate the modulation frequencies likely to be most suitable for electrical estimation of behavioral threshold. In awake subjects, for all carrier frequencies tested, detection efficiency was highest at a modulation frequency of 45 Hz. In sleeping subjects, the modulation frequency regions of highest efficiency varied with carrier frequency. For carrier frequencies of 250 Hz, 500 Hz, and 1 kHz, the highest efficiencies were found in two modulation frequency regions centered on 45 and 90 Hz. For 2 and 4 kHz, the highest efficiencies were at modulation frequencies above 70 Hz. Sleep stage affected both response amplitude and background EEG noise in a manner that depended on modulation frequency. The results of this study suggest that, for sleeping subjects, modulation frequencies above 70 Hz may be best when using steady-state potentials for hearing threshold estimation.

New-Z' phenomenology.

Abstract: We show that {ital L}{sub 1}={ital L}{sub {ital e}}{minus}{ital L}{sub {mu}}, {ital L}{sub 2}={ital L}{sub {ital e}}{minus}{ital L}{sub {tau}}, and {ital L}{sub 3}={ital L}{sub {mu}}{minus}{ital L}{sub {tau}} (where {ital L}{sub {ital e},{mu},{tau}} are the family lepton numbers) are anomaly-free and can thus be gauged. Three simple theories featuring a flavor-conserving second {ital Z} boson ({ital Z}{sub 1,2,3}{sup {prime}}) result. Bounds on the {ital Z}{sub {ital i}}{sup {prime}} coupling constant ({ital g}{sub {ital i}}{sup {prime}}) and mass are derived. The mass of {ital Z}{sub 1}{sup {prime}} has a lower bound of 60--150 GeV at 90% confidence level for reasonable values of {ital g}{sub 1}{sup {prime}}. The mass of {ital Z}{sub 2}{sup {prime}} has a best-fit value of about 60 GeV. The physics of these bosons in the KEK TRISTAN and CERN LEP window of 63--87 GeV is discussed. Constraints on {ital Z}{sub 3}{sup {prime}} interactions are derived from ({ital g}{minus}2){sub {mu}}.

Evidence that part of the NANC relaxant response of guinea-pig trachea to electrical field stimulation is mediated by nitric oxide.

Abstract: 1. The nitric oxide (NO) synthesis inhibitors NG-monomethyl L-arginine (L-NMMA) and L-nitroarginine methyl ester (L-NAME) reduced relaxations of guinea-pig tracheal smooth muscle elicited by stimulation of intramural non-adrenergic, non-cholinergic (NANC) nerves, but D-NMMA had no effect. L-NAME was 10-30 times more potent than L-NMMA. Relaxations produced by sodium nitroprusside and vasoactive intestinal polypeptide (VIP) were not affected by L-NMMA or L-NAME. 2. The inhibitory effect of L-NMMA on NANC-mediated relaxations was partially reversed by L-arginine but was not affected by D-arginine. 3. VIP antibody and alpha-chymotrypsin abolished or greatly reduced the relaxant action of VIP and reduced relaxations elicited by stimulation of NANC nerves; the residual NANC relaxation was further reduced by L-NAME. 4. The results suggest that NO and VIP are mediators of NANC-induced relaxations of guinea-pig tracheal smooth muscle. We propose the term 'nitrergic' to describe transmission processes which are mediated by NO.

Task uncertainty and its interaction with budgetary participation and budget emphasis: Some methodological issues and empirical investigation

Abstract: This paper reports the results of a study which re-examines Brownell & Hirst ( Journal of Accounting Research , Autumn 1986). In that study, it was hypothesized that Brownell's result ( Journal of Accounting Research , Spring 1982), showing that budget participation and budget emphasis in evaluation should be aligned, would hold only where task uncertainty was low. With managerial performance as the criterion, Brownell & Hirst were unable to find support for the hypothesis. This study incorporates variations in measurement and sampling and finds results which provide strong support for Brownell & Hirst's hypothesis.

Low-Reynolds-number turbulent boundary layers

Abstract: An investigation was undertaken to improve our understanding of low-Reynolds-number turbulent boundary layers flowing over a smooth flat surface in nominally zero pressure gradients. In practice, such flows generally occur in close proximity to a tripping device and, though it was known that the flows are affected by the actual low value of the Reynolds number, it was realized that they may also be affected by the type of tripping device used and variations in free-stream velocity for a given device. Consequently, the experimental programme was devised to investigate systematically the effects of each of these three factors independently. Three different types of device were chosen: a wire, distributed grit and cylindrical pins. Mean-flow, broadband-turbulence and spectral measurements were taken, mostly for values of Rθ varying between about 715 and about 2810. It was found that the mean-flow and broadband-turbulence data showed variations with Rθ, as expected. Spectra were plotted using scaling given by Perry, Henbest & Chong (1986) and were compared with their models which were developed for high-Reynolds-number flows. For the turbulent wall region, spectra showed reasonably good agreement with their model. For the fully turbulent region, spectra did show some appreciable deviations from their model, owing to low-Reynolds-number effects. Mean-flow profiles, broadband-turbulence profiles and spectra were found to be affected very little by the type of device used for Rθ ≈ 1020 and above, indicating an absence of dependence on flow history for this Rθ range. These types of measurements were also compared at both Rθ ≈ 1020 and Rθ ≈ 2175 to see if they were dependent on how Rθ was formed (i.e. the combination of velocity and momentum thickness used to determine Rθ). There were noticeable differences for Rθ ≈ 1020, but these differences were only convincing for the pins, and there was a general overall improvement in agreement for Rθ ≈ 2175.

On SVD for estimating generalized eigenvalues of singular matrix pencil in noise

Abstract: Several algorithms for estimating generalized eigenvalues (GEs) of singular matrix pencils perturbed by noise are reviewed. The singular value decomposition (SVD) is explored as the common structure in the three basic algorithms: direct matrix pencil algorithm, pro-ESPRIT, and TLS-ESPRIT. It is shown that several SVD-based steps inherent in the algorithms are equivalent to the first-order approximation. In particular, the Pro-ESPRIT and its variant TLS-Pro-ESPRIT are shown to be equivalent, and the TLS-ESPRIT and its earlier version LS-ESPRIT are shown to be asymptotically equivalent to the first-order approximation. For the problem of estimating superimposed complex exponential signals, the state-space algorithm is shown to be also equivalent to the previous matrix pencil algorithms to the first-order approximation. The second-order perturbation and the threshold phenomenon are illustrated by simulation results based on a damped sinusoidal signal. An improved state-space algorithm is found to be the most robust to noise. >

Adolescent self‐efficacy, self‐esteem and sexual risk‐taking

Abstract: Self efficacy self esteem and sexual risk taking are examined among 1788 post secondary school students in victoria Australia in the classroom and a mail in questionnaire on knowledge attitudes and behavior related to AIDs. The mean age was 18.3 years; 73% were female. Volunteers came from rural urban vocational and liberal arts schools. 98% were heterosexual and only nonvirgins were selected. Self efficacy was based on a 20 item scale by Libman et al. Self esteem was adapted from scales by Offer and Marsh and tested in a pilot study for consistency. Sexual behavior and risk questions were developed by the project. The results indicated that respondents felt confident in their ability to engage in a range of sex behaviors and to obtain condoms discuss their use and delay sex until contraception was available. However there were low levels of reported condom use. There is a disparity between what respondents say they do and what they believe they can do. Discussing pregnancy prevention may be easier that discussing safe sex. The self efficacy scale wasnt very discriminatory. Care must be taken in linking self efficacy with behavior. Mastery of contraception may not imply the same regard for safe sex precautions. Sexual risktaking was accounted for very little by the set of sexual self perceptions. There were differences in the sense of mastery and in sexual self esteem between men and women however the patterns of relationships which predicted risktaking were similiar. Also factors predicting risk with a casual partner were different from those with a regular partner. Saying No to sex reduces the incidence of unsafe sex among casual partners but in a long term relationship trust of ones partner and communication skills are more salient. AIDs is not perceived to be a risk factor. Assertive skills and feelings of self worth lead to sexual satisfaction but not necessarily protection from AIDs. The reality is that many youth engage in regular relationships that are short term and not monogamous and this is perceived as sexually safe. Spontaneity of sexual behavior may be a critical factor in ones taking precautions. Control of immediate encounters is what requires mastery. Self reports of sexual behavior studies are all limited by accuracy honesty of recall and willingness to report socially disapproved behavior but there is no alternative. The decision to engage in safe sex is governed by multiple factors only 1 of which is confidence in ones ability.

Asymptotic enumeration by degree sequence of graphs with degrees o(n1/2)

Abstract: We determine the asymptotic number of labelled graphs with a given degree sequence for the case where the maximum degree iso(|E(G)|1/3). The previously best enumeration, by the first author, required maximum degreeo(|E(G)|1/4). In particular, ifk=o(n 1/2), the number of regular graphs of degreek and ordern is asymptotically $$\frac{{(nk)!}}{{(nk/2)!2^{nk/2} (k!)^n }}\exp \left( { - \frac{{k^2 - 1}}{4} - \frac{{k^3 }}{{12n}} + 0\left( {k^2 /n} \right)} \right).$$ Under slightly stronger conditions, we also determine the asymptotic number of unlabelled graphs with a given degree sequence. The method used is a switching argument recently used by us to uniformly generate random graphs with given degree sequences.

Risk Indicators: Assessment of Infancy Predictors of Pre-School Behavioural Maladjustment

Abstract: The prediction of later outcome from factors present in infancy has been an ongoing concern, with difficult temperament frequently being posited as one important risk factor. Using data from a longitudinal study of a large representative sample of children, and a categorical approach to analysis, a set of infancy risk factors covering within-child, environmental and relationship variables was related to behavioural and emotional adjustment at 4-5 years. Single risk factors, including difficult temperament, resulted in only modest increases in the prevalence of later maladjustment. However, certain combinations of risk factors were associated with markedly increased prevalence rates. The results indicate the cumulative effects of risk factors, and the need to consider temperament within a contextual framework.

An L-shaped array for estimating 2-D directions of wave arrival

Abstract: A simple structured 2-D array, the L-shaped array, is presented. The L-shaped array consists of two uniform linear arrays (ULA) connected orthogonally at one end of each ULA. It is shown that the Cramer-Rao bounds (CRB) of the estimated wave directions based on the L-shaped array are about 37% smaller than those for the cross array. The CRB indicates the accuracy potential because it is the (reachable) lower bound on the variance of any unbiased estimate. An efficient maximum likelihood algorithm is developed utilizing the ULA structure inherent in the L-shaped array. >

Commitment and effectiveness of situated agents

Abstract: Recent research in real-time Artificial Intelligence has focussed upon the design of situated agents and, in particular, how to achieve effective and robust behaviour with limited computational resources. A range of architectures and design principles has been proposed to solve this problem. This has led to the development of simulated worlds that can serve as testbeds in which the effectiveness of different agents can be evaluated. We report here an experimental program that aimed to investigate how commitment to goals contributes to effective behaviour and to compare the properties of different strategies for reacting to change. Our results demonstrate the feasibility of developing systems for empirical measurement of agent performance that are stable, sensitive, and capable of revealing the effect of "high-level" agent characteristics such as commitment.

The location of (1-3)-β-glucans in the walls of pollen tubes of Nicotiana alata using a (1-3)-β-glucan-specific monoclonal antibody

Abstract: The location of the (1→3)-β-glucan, callose, in the walls of pollen tubes in the style of Nicotiana alata Link et Otto was studied using specific monoclonal antibodies. The antibodies were raised against a laminarinhaemocyanin conjugate. One antibody selected for further characterization was specific for (1→3)-β-glucans and showed no binding activity against either a cellopentaose-bovine serum albumin (BSA) conjugate or a (1→3, 1→4)-β-glucan-BSA conjugate. Binding was inhibited by (1→3)-β-oligoglucosides (DP, 3–6) with maximum competition being shown by laminaripentaose and laminarihexaose, indicating that the epitope included at least five (1→3)-β-linked glucopyranose residues. The monoclonal antibody was determined to have an affinity constant for laminarihexaose of 2.7. 104M−1. When used with a second-stage gold-labelled, rabbit anti-mouse antibody, the monoclonal antibody probe specifically located the (1→3)-β-glucan in the inner wall layer of thin sections of the N. alata pollen tubes.