Zur Regulation der RNS-Synthese durch Phytochrom bei der Photomorphogenese des Senfkeimlings (Sinapis alba L.)

Summary P₇₃₀, the active phytochrome, increases the rate of RNA synthesis (Table) and the RNA contents in the cotyledons of the mustard seedling (Sinapis alba L.) (Fig. 1) whereas the RNA contents in the hypocotyl is decreased under the influence of P₇₃₀ (Fig. 2).—It takes about 6 hours until changes in the RNA contents-which must be attributed to the formation of P₇₃₀—can be measured after the onset of light (continuous far-red). Since the lag-phases of positive photoresponses in the cotyledons and negative photoresponses in the hypocotyl (Mohr, 1966) are in general much shorter than 6 hours, the changes of the RNA contents of the organs cannot be regarded as being directly connected with the formation of characteristic positive photoresponses such as anthocyanin synthesis, induced enzyme synthesis, ascorbic acid synthesis, etc., or negative photoresponses such as inhibition of hypocotyl lengthening.We have rather to conclude that the changes of RNA contents are secondary adaptations of the organs which lead to an increase (cotyledons) or decrease (hypocotyl) of protein synthesizing capacity of the cells and tissues. The P₇₃₀-dependent increase of bulk RNA in the cotyledons is probably due to a differential gene activation and the P₇₃₀-dependent decrease of bulk RNA in the hypocotyl is due to a differential gene repression. The causalities of these processes are possibly complex.The hypothesis of differential gene activation or repression by P₇₃₀ (Mohr, 1966; Schopfer, 1967a, b) is not disproved by these results. We have rather to reach a conclusion which has already been suggested by other data (e.g. Karow and Mohr, 1966), namely, that positive as well as negative photoresponses are due to changes in the activity of a limited (possibly small) number of enzymes. Correspondingly changes in only a minute amount of the total RNA are directly involved in the formation of photoresponses. These changes cannot be detected by following RNA contents.—It seems to be of great interest, however, that P₇₃₀ eventually brings about strong tissue specific changes in the RNA contents per cell as described in the present paper.     

Free Ia Eα chain expression in the E α + :E β − : recombinant strain A.TFR5

Molecular and biochemical techniques have been used to explore the reasons behind low E chain expression in the E ⁺ E ^– I-region recombinant strain, A.TFR5. A.TFR5 (A ^f E ^k, ap5), a recombinant between A.CA (A ^f E ^f) and A.TL (A ^k E ^k), carries the E ^k subregion. Previous results have shown that it expresses the E chain, but at reduced levels relative to E ⁺ E ⁺ strains. No E chains were detected, which is consistent with the A.TFR5E gene being derived from the A.CA parent, which carries the null E ^f allele. In this paper, the defect in E-chain expression is explored. Restriction fragment length polymorphism analysis has localized the recombination event in A.TFR5 approximately 30 kb upstream of E, in the region of the large intervening sequence of E. Northern blot analysis of total RNA from A.TFR5 shows normal amounts of the E message, but no E message. Two-dimensional gel analysis of 15 min pulse-labeled A.TFR5, A.CA, and A.TL E immunoprecipitates shows decreased levels of the intracellular E chain in A.TFR5 relative to A.TL. However, analysis of total cell extracts shows normal levels of this protein. A glycoprotein fraction isolated from total cell extracts of 5 h labeled cells contains normal amounts of intracellular E, but decreased amounts of the mature cell-surface protein. These data suggest that in the absence of E, the E chain (1) takes on an altered conformation that is not as efficiently recognized by alloantibodies, and (2) is found in normal levels as the partially glycosylated intracellular precursor, but is not processed and/or transported efficiently to the cell surface.     

Composition and photoinduced biosynthesis of the carotenoids of a protoplast-like Neurospora crass “slime” mutant

A spheroplast-like slime mutant of Neurospora crassa (lacking a rigid cell wall) was found to synthesize an identical spectrum of carotenoids as wild type strains except for -carotene. Furthermore strict photoregulation of the biosynthesis of these pigments as well as the characteristics of photoinduced carotenogenesis were also nearly identical in the mutant and in the wild type.     

The quantitative analysis of chromosome pairing and chiasma formation based on the relative frequencies of M I configurations

Whilst reliable estimates of chiasma frequencies can usually not be obtained, the probability (b) of a chromosome arm to be bound by at least one chiasma can often be determined. In the absence of interference this probability equals (1–e ^–2), where 2 is the average chiasma frequency of the chromosome arm and the average crossover frequency or map length. In the presence of interference is shown to retain its genetic meaning as an additive metric that may describe the chromosome arm or other distinctive chromosome segment in terms of genetic recombination. It is a form of potential map length, comparable to, but numerically different from the regular map length. It is termed provisionally crossing-over potential.A chromosome with armsm andn with crossing-over potentials and will form ring bivalents with a frequency (1–e ^–2).(1–e ^–2); open bivalents with a frequency (1–e ^–2).e ^–2+(1–e ^–2).e ^–2; univalent pairs with a frequencye ^–2.e ^–2. Estimates of these frequencies yield equations from which and may be solved. In rye (Secale cereale) their ratio (q) is approximately two and differs from the mitotic arm length ratio of 1.4, indicating localization of chiasmata in the long arms.Graphs are given to show how, with constantq, the relation between the probabilitiesb _m andb _n of the two arms being bound changes with changing averageb.Data are presented on chiasma frequencies in M I, and compared with the frequencies expected in the absence of interference to give an impression of the degree of interference. Apparent fusion of chiasmata simulates interference.     

The effect of ionic stress on photosynthesis in Dunaliella tertiolecta

A comparison of the effects of ionic stress and an uncoupler on long-term fluorescence transients (the Kautsky effect) in the green alga Dunaliella tertiolecta indicated that the large quenching induced by ionic stress was caused by a pH gradient across the thylakoid membrane. This possiblity was given support by the increase in the slow phase of 3-(3,4-dichlorophenyl)-1,1-dimethylurea-induced fluorescence relaxation in algae subjected to ionic stress. Low-temperature fluorescence emission spectra indicated that salt stress enhanced photosystem-I emission in the dark, and a comparison of simultaneous emissions at 695 and 720 nm at room temperature indicated a further increase in photosystem-I emission during the fluorescence transients. Taken together with the decrease in the fast phase of 3-(3,4-dichlorophenyl)-1,1-dimethylurea-induced fluorescence relaxation in stressed algae, our results indicate that ionic stress stimulates cyclic electron flow, and that non-cyclic flow is inhibited. The effect of sucrose-induced osmotic stress was similar to, but less marked than, the effects of NaCl and KCl; the effect of decreasing the external salinity was small.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - FCCP carbonylcyanide p-trifluoromethoxyphenylhydrazone - PSI, II photosystem I, II     

Zur Ei- und Embryonalentwicklung des HydroidpolypenEudendrium armatum

Zusammenfassung 1. Die Oocyten-, Blastostyl- und Embryonalentwicklung vonEudendrium armatum Tichomirov wurde licht- und elektronenmikroskopisch untersucht.2. Oocyten entstehen einzeln oder in dichter Lagerung aus undifferenzierten Zellen des Ektoderms in jüngeren sowie älteren Hydrocaulusabschnitten. Bereits vor der Blastostylknospung sind im Hydrocaulus zahlreiche Oocyten vorhanden. Gesetzmäßige Lagebeziehungen zwischen den Orten der Oocytenentstehung (Keimzonen) und dem Verzweigungssystem lassen sich nicht feststellen. Das Wandern der Oocyten im Hydrocaulus kann am lebenden Stöckchen verfolgt werden.3. Blastostyle sind von Nährpolypen im Knospenzustand durch in ihren Gastralraum eingewanderte Oocyten und später durch ihre Spadixbildung unterscheidbar. Die möglichen Wechselbeziehungen zwischen Oogenese und Oocytenwanderung einerseits und Blastostylen andererseits werden diskutiert.4. Während der Vitellogenese wird vom Spadixentoderm granulöses Material — möglicherweise Glykogen — an die Oocyte abgegeben. Das Spadixentoderm hat durch Zellausläufer direkten Kontakt mit der Oocyte.5. Nach der Befruchtung bildet die Oocyte eine Eihülle. Das Material dieser Eihülle entspricht wahrscheinlich der Peridermsubstanz.6. Die Eihülle wird gleichzeitig mit dem Periderm unterhalb des Blastostyls verlötet. Dies geschieht in Wechselbeziehung zu einer Abhebung und Retraktion des Spadix vom Ei. Danach bleibt die Stützlamella aus dem Spadixbereich als gefaltetes Paket in der Gastralwand des Blastostyls liegen. Der fibrilläre Randsaum ist weitgehend ungestört. Es wird diskutiert, ob die Myoepithelzellen ihre Bindung an die Stützlamelle lösen und gegebenenfalls wieder knüpfen können.7. Die Furchung verläuft — zumindest vom 8-Kern-Stadium ab — total. Der Beginn der Durchfurchung wurde stets in zeitlicher Verzögerung zu den ersten Kernteilungen beobachtet. Zellgrenzen wurden frühestens im 4-Kern-, spätestens im 8-Kern-Stadium gefunden. Die widersprüchlichen Angaben über totale, syncytiale und superfizielle Furchung in der GattungEudendrium werden an Hand der Befunde diskutiert.8. Am Ende der Vitellogenese und zu Beginn der Embryonalentwicklung werden homogene Dotterbereiche in — je nach Fixierung unterschiedlicher — Verbreitung gefunden. In solchen Verflüssigungsbereichen liegt der Komplexdotter nicht in von Membranen umgrenzten Tröpfchen (Vesikeln) vor. Diese Bildung wird als Fixierungsartefakt gedeutet.9. Die histologische Differenzierung beginnt bereits in der späten Furchung parallel zur Anlage der Körperschichten. Der beschriebene Entstehungsmodus der Zweischichtigkeit kann als Moruladelamination bezeichnet werden.

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On egg and embryonic development of the hydroid polypEudendrium armatum. A light and electron microscopic study

Oocytes ofEudendrium armatum originate in the branching stalks from undifferentiated ectoderm cells, in younger as well as in older parts, individually or in groups — prior to the development of blastostyles. Oocyte migration is caused by an autonomous activity. Possible interrelationships between oogenesis and the migration of oocytes on the one hand, and the development of gonozoids on the other are discussed. Cleavage is complete, at least from the eighth nucleus stage onwards. Controversial opinions about cleavage in various species ofEudendrium are discussed, with special reference to the problem of the syncytial cleavage and areas of liquefied yolk. InE. armatum, the latter is regarded as an artefact of fixation. Egg shell formation, function and retraction of the spadix and embryogenesis are described.

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Abkürzungen in den Abbildungen B Bakterium - Bl Blastostyl - Cb Cnidoblast - Ci Cilium - Ch Chromatin - dEhG dunkle Eihüllen-Grana - Dm Doppelmembran - Do Komplexdotter - Drg Drüsenring - Drz Grana-haltige Drüsenzellen - Eh Eihüllenschicht I - Eh Eihüllenschicht II - Ek Ektoderm - Em Extrusionsmaterial - Em vermutliches Em, von Doppelmembran umgeben - En Entoderm - F Filamente - Fa Fixierung nachFahrenbach - f.Ch fibrilläres Chromatin - Fp Freßpolyp - F.Stl Fibrillen der Stützlamelle - Fs.Stl Fibrillensaum der Stützlamelle - -C glatte Membranen - G Gastralraum - Gl vermutliches Glycogen - Go Golgi-Apparat - Gr strukturiertes Granum - hEhG helle Eihüllen-Grana - I-Z I-Zelle - k.Ch kondensiertes Material innerhalb des fibrillären Chromatin - Mf Myofibrillen - Mi OsO₄ nachMillonig - Mit Mitochondrium - N Nucleus - Nl Nucleolus - Nm Kernmembran - Np Kernporen - O Oocyte - Pa OsO₄ nachPalade - Pd' äußeres Periderm - Pdb Peridermbildungszone - PdG Peridermbildungs-Grana - hPdG helle Peridermbildungs-Grana - dPdG dunkle Peridermbildungs-Grana - p.f pars fibrosa des Nucleolus - p.g pars granulosa des Nucleolus - Psp pseudopodienartiger Ausläufer einer Oocyte - R Ribosomen - Sp Spadix - Sp.R Spadix in Retraktion - Stl Stützlamelle - StM Styrol Methacrylat - Sz Schleimzellen - V Vesikel und V-reihen - /V Vestopal - Verfl Dotter-Verflüssigungszone - Wp Wehrpolyp - X peripherer Bereich ohne Zellorganelle - Zm Zellmembran     

A xyloglucan-oligosaccharide-specific α-d-xylosidase or exo-oligoxyloglucan-α-xylohydrolase from germinated nasturtium (Tropaeolum majus L.) seeds

The -xylosidase which is involved in the postgerminative mobilisation of xyloglucan in nasturtium seed cotyledons has now been purified to apparent homogeneity by a facile procedure involving lectin affinity chromatography. The purified enzyme, a glycoprotein, moved as a single band (apparent molecular weight 85000) on sodium dodecyl sulphate-gel electrophoresis, whilst isoelectric focusing gave a number of enzymatically active protein bands spanning the range pI = 5.0 to 7.1 (maximum activity at pI = 6.1). The enzyme did not hydrolyse the simple -xylosides p-nitrophenyl--d-xylopyranoside and woprimeverose (-d-Xyl(16)-d-Glc), or polymeric tamarind-seed xyloglucan. It released xylose from a complex mixture of oligosaccharides produced by exhaustive hydrolysis of tamarind seed xyloglucan using the xyloglucan-specific endo-(14)--d-glucanase from germinated nasturtium seeds (M. Edwards et al. 1986, J. Biol. Chem., 261. 9489–9494). The three xyloglucan oligosaccharides of lowest molecular size were purified from this mixture and were shown by ¹H-nuclear magnetic resonance (¹H-NMR) and enzymatic analysis to have the structures:Abbreviations Con A Concanavalin A - DEAE diethylaminoethyl - Gal galactose - Glc glucose - HPLC high-performance liquid chromatography - M _r apparent molecular mass - NMR nuclear magnetic resonance - pI isoelectric point - SDS-PAGE sodium dodecyl sulphate-polyacrylamide gel electrophoresis - Xyl xylose Much of the work reported in this paper was carried out with the aid of the European Community's Science Stimulation Action (Contract No. ST2P-0250-UK), and we wish to record our appreciation of this support.     

Biological chromatology. The laws of colour and design in nature

Summary Biochromatology can be defined as the study of the relationships between colours, and between colours and design in nature. Certain laws concerning biochromatics can be formulated in the terminology of the Colour Circle. One of them is that planes in warm colours and planes in cold colours never adjoin. Another law is that design is never polychromatic. It seems that the observed biochromatical facts can be explained with the help of the principle optimal manifestation of colour, which in turn is based on the formative principles of matter itself.     

Production of a thermostable alkali-tolerant xylanase from Bacillus circulans AB 16 grown on wheat straw

Bacillus circulans AB 16 was able to produce 50 IU/ml of xylanase, with negligible cellulase activity when grown on untreated wheat straw. The pH optimum of the crude enzyme was 6–7 with a temperature optimum of 80 C. The enzyme showed high pH and thermal stability retaining 100% activity at 60 C, pH 8 and 9 after 2.5 h of incubation. The residual activity at 70 C after 2.5 h was 62% and 45% at pH 8 and 9, respectively. At 75 C only 22.2% activity remained at pH 8 after 1 h incubation. Since Kraft pulp is alkaline this enzyme could be used for prebleaching of pulp at temperatures up to 70 C without pH adjustment.     

Socio-Ecological Webs and Sites of Sociality:Levins' Strategy of Model Building Revisited

This essay extends Levins' 1966 analysis of modelbuilding in ecology and evolutionary biology. Amodel, as the product of modeling, might bevalued according to its correspondence to reality. Yet Levins' emphasis on provisionality and changeredirects attention to the processes ofmodeling, through which scientists select and generatetheir problems, define their categories, collect theirdata, compare competing models, and present theirfindings. I identify several points where decisionsare required that are not determined by nature. Thisinvites examination of the social considerationsmodelers are reacting to at the sites of sociality.Modelers must weave socio-ecological webs so thatthe models can be seen to represent their subjectmatter at the same time as the modelers secure thesupport of colleagues, collaborators and institutions,and enjoin others to act upon their conclusions. Notonly do theory justification and theory generationmerge, but the joint project becomes simultaneouslyphilosophical and sociological.     

Ultrastructural localization of corticotropin, β-lipotropin,and αand β-endorphin in the porcine anterior pituitary

Summary The peroxidase-antiperoxidase immunocytochemical technique was used to identify the ACTH/endorphin cells in the porcine pituitary at the ultrastructural level and to determine the precise subcellular localization of the pro-ACTH/endorphin fragments. The cells display different aspects: 1) large, regular shapes with numerous and large secretory granules; 2) small, irregular and angular shapes with small granules aligned along the periphery of the cell; and 3) intermediate forms. The presence of and -endorphin not only in the same cells but also in the same secretory granules that contain ACTH and -LPH clearly indicates that both the precursor or its fragments and the abovementioned peptides are stored in the same granules and released simultaneously by the corticotropic cells. The presence of FSH in some corticotropic cells is also discussed.Abbreviations used in this Article ACTH corticotropin - -MSH -melanotropin (ACTH I–I3) - CLIP corticotropin-like intermediate lobe peptide (ACTH 18–39) - -LPH -lipotropin - -MSH -melanotropin (-LPH 41–58); -endorphin (-LPH 61–91); -endorphin (-LPH 61–76)     

Patterns and conformations of commonly occurring supersecondary structures (basic motifs) in protein data bank

Short peptides connecting-helices and-strands have been analyzed in 240 proteins refined at resolutions of 0.25 nm or better. Connecting peptides of lengths between one and five residues have been classified as part of supersecondary motifs of four types:, , , and. Careful consideration has been given to the definition of secondary structures on the basis of hydrogen bonds and main-chain conformational angles. Using five classes of residue conformation—a, b, e, l, t—in the nonregular structure regions of, space, 34 classes of supersecondary motifs occurring at least five times have been identified. Among these 34 classes, 11 classes that occur more than 25 times are commonly occurring supersecondary structure motifs. The patterns and conformations of the 11 commonly occurring supersecondary structure motifs have been characterized, demonstrating that patterns and conformations adopted by supersecondary structure motifs are limited. The results have relevance to structure prediction, comparative modeling, and protein folding.     

Mode of uptake of sterol by Arthrobacter simplex

The mechanism of uptake of water-insoluble -sitosterol by a newly isolated strain of Arthrobacter simplex SS-7 was studied. The production of an extracellular sterol-pseudosolubilizing protein during growth of A. simplex on -sitosterol was demonstrated by isolating the factor from the cell-free supernatant and its subsequent purification by Sephadex G-150 column chromatography. The M _r of the purified sterol-pseudosolubilizing protein determined by SDS–PAGE was 19kDa. The rate of sterol pseudosolubilization (5.2×10^–3g l^–1h^–1) could not adequately account for the rate of sterol uptake (72×10^–3g l^–1h^–1) and the specific growth rate (56×10^–3 h^–1). However in the unfavourable growth condition, when the cells were treated with sodium azide at the level of 30–60% of MIC, the sterol pseudosolubilization accounted for nearly 74% of the total growth containing 96% free cells. Cellular adherence to substrate particles was found to play an active role in the normal growth of the strain on -sitosterol. Unlike sodium acetate-grown cells, whose surface activity was negligible (60mNm^–1), the sterol-grown cells had strong surface activity (40mNm^–1). The high lipid content and long chain fatty acids in the cell-wall of -sitosterol-grown cells probably contribute to the high sterol adherence activity of the cells.     

A gene encoding Achlya bisexualis β-amylase and its expression in Saccharomyces cerevisiae

Part of a -amylase genomic DNA sequence from the oomycete, Achlya bisexualis was cloned by polymerase chain reaction (PCR) using degenerate oligonucleotide primers derived from the conserved regions of other known -amylase sequences. The 5- and 3-regions of the -amylase gene were amplified by genome walking method. The Ach. bisexualis -amylase gene consisted of a 1338bp open reading frame, encoding a protein of 446 amino acids with a molecular weight of 49 381Da, and was not interrupted by any intron. The deduced amino acid sequence of the -amylase gene had 67% similarity to the -amylase of Saprolegnia ferax, followed by 40% similarity to that ofArabidopsis thaliana. The -amylase gene was expressed in Saccharomyces cerevisiae placing it under the control of the alcohol dehydrogenase gene (ADC1) promoter.     

Evidence of regional differences in the lectin histochemistry along the ductus epididymis of the lizard,Podarcis sicula Raf

The regional difference in the carbohydrate components of the ductus epididymis epithelium of a lizard was delineated by means of 13 lectins. Basal cells expressed only N-acetylglucosamine (GlcNAc). Throughout the ductus, the secretory cells showed oligosaccharides with terminal N-acetylneuraminic acid (Neu5Ac)(2,6)galactose (Gal)/N-acetylgalactosamine (GalNAc) and internal mannose (Man) and/or glucose (Glc) in the whole cytoplasm, oligosaccharides terminating in Neu5Ac(2,6)Gal(1,3)GalNAc, Neu5Ac(2,6)Gal (1,4)GlcNAc, GalNAc, GlcNAc, and fucose (Fuc) in the supra-nuclear zone, and also glycans terminating in Neu5Ac(2,3)Gal (1,4)GlcNAc, Neu5Ac(2,6)Gal(1,3)GalNAc, Gal (1,4)GlcNAc on the luminal surface. In the caput and corpus regions, the supra-nuclear cytoplasm was characterized by terminal Gal(1,4)GlcNAc and GalNAc, the luminal surface by GalNAc and Gal. The Golgi zone, showing oligosaccharides with terminal Neu5Ac(2,3)Gal (1,4)GlcNAc, Neu5Ac(2,6)Gal (1,3)GalNAc, Neu5Ac(2,6)Gal (1,4)GlcNAc, and internal GlcNAc, expressed terminal Gal (1,4)GlcNAc and GalNAc in the caput, and terminal GalNAc in the corpus. The granules showed all the investigated carbohydrates in their peripheral zone except terminal GalNAc and Fuc, whereas internal Man/Glc and terminal Gal were expressed in the central core, and Fuc throughout the ductus, terminal GlcNAc in the caput and corpus, and terminal GalNAc only in the corpus.     

Photoreversion und Photoprotektion bei UV-induzierten Mutationen des nichtphotoreaktivierbaren Bacteriums E.coli phr?

Summary Stationary phase cells of strain phr^–/MC 2 ofE. coli are not photoreactivable but the frequency of UV-induced mutations to low Streptomycine-resistance (S 3, 3/ml) is decreased strongly by illumination with light of fluorescence tubes (310 to 500 nm) after UV-irradiation. Also dark-reversion (DRM) of these mutations due to keeping UV-irradiated cells in saline is observed. Illumination before UV-irradiation decreases the frequency of the mutations (photoprotection against mutation=PPM) to the same extent as the combined action of photoreversion (PRM) and DRM. The lag-phase of cell division is prolonged strongly by illumination from 80 min without light to 150 min by the light-dose of highest activity. The additional lag is nearly the same if the illumination is done before, after or without UV-irradiation; this lag is about additive to the small lag caused by UV. Pre-illumination of the stationary-phase cells does not cause photoprotection against killing (PP), it even decreases the survival after high UV-doses. The observations support the hypothesis that PRM in this strain may be indirect, i.e. caused by the light-induced additional division lag which enhances the dark repair of UV-premutations. Also spontaneous premutations which are apparently present in the stationary-phase cells seem to be influenced by the light in this way.     

The low-light reduction in the quantum yield of photosynthesis: potential errors and biases when calculating the maximum quantum yield

Photosynthesis-irradiance (P-E) curves are widely used to describe photosynthetic efficiency and potential. Contemporary models assume maximal photosynthetic quantum yield () at low irradiances. But P-E observations made with both oxygen evolution and carbon uptake techniques show that this is not always the case. Using new and published data in conjunction with modeling exercises, we demonstrate that regardless of the mechanism there can be reductions in at low irradiances that are not readily observable using conventional P-E analyses. We also show that analytical errors, such as inaccurate estimation of dark oxygen consumption or carbon uptake, can markedly affect the structure of -E curves with negligible effect on P-E curve structure. Whether from respiration `corrections' or other mechanisms, these deviations in at low light levels from the maximum quantum yield of photosynthesis (_max) can lead to significant errors (> 50%) in the estimation of the linear portion of the P-E curve and ultimately _max. Non-linear models of P-E, such as the rectangular hyperbola, quadratic, exponential and hyperbolic tangent that are commonly used to estimate the initial slope () of the P-E curve assume that is maximal at low light levels and therefore can err in the estimation of _max when is reduced at low light levels. Using a diverse data set of 622 P-E curves with a total of 7623 points, we show that although model skills are high (r ² = 0.96 ± 0.05, 0.97 ± 0.04, 0.97 ± 0.04 and 0.97 ± 0.04, respectively), a large fraction of the model-predicted _max differ by greater than 10% from true _max values (91%, 50%, 82% and 46%, respectively). Data from these observations and modeling exercises lead us to suggest that _max be determined by directly estimating the true maximum of a -E curve rather than using the more conventional methodology employing the initial slope of the P-E curve.This revised version was published online in October 2005 with corrections to the Cover Date.     

Hexagonally ordered “rosettes” of particles in the plasma membrane ofMicrasterias denticulata Bréb. and their significance for microfibril formation and orientation

Summary Cells ofMicrasterias denticulata Bréb. at the stage of secondary wall formation have been studied by freeze-etching. It was found that the plasma membrane exhibits oval areas in which arrays of membrane particles occur. These particles form rosettes which are arranged in a hexagonally ordered lattice with a center to center spacing of 25 nm. Nearly the same periodicities can be found between microfibrils. It is concluded that the rosettes probably together with the thickened area of the plasma membrane below them represent the apparatus for the production and orientation of microfibrils. The hypothesis suggesting the incorporation of membrane templates functional in microfibril formation, originally advanced byKiermayer andDobberstein (1973) has received further support.