Showing papers by "University of York published in 1971"

The tertiary endodermis in barley roots: Fine structure in relation to radial transport of ions and water.

Abstract: The presence of numerous pits containing plasmodesmata in the inner tangential wall of the tertiary endodermis in barley roots is demonstrated by electron microscopy. The pit floor is covered by a thin layer of material which is continuous with and resembles the tertiary wall. The plasmodesmatal pore is constricted at its ends so that the plasmalemma lining the pore is appressed to the desmotubule. The frequency of plasmodesmata and their cross-sectional area is estimated, and phosphate and water fluxes through them are calculated on the assumption that they represent the only communication between the cortex and the vascular tissue. The pressure gradient across the ends of the plasmodesmata necessary to support the observed water flux is calculated for limiting cases of the pore radius and the viscosity of the fluid passing through the pore.

The ultrastructure of plasmodesmata

Abstract: It is suggested that the central strand which traverses plasmodesmata is in open continuity with the endoplasmic reticulum of adjacent cells, and that this strand (desmotubule) represents a modulation of a normal ER membrane so that it comprises solely spherical protein subunits. This concept is used to illustrate how plasmodesmata could form a median nodule or anastomosing central strands. The implications of this model in relation to current theories of symplasmic transport are discussed, and the possibility for further experimental work is outlined.

The structure of calf liver cytochrome b 5 at 2.8 A resolution.

Abstract: CYTOCHROME b5 is a haem-containing protein in the microsomes of liver tissue. It interacts specifically with a flavo-protein, cytochrome b5 reductase, which catalyses the transfer of electrons from NADH to the haem iron of the cytochrome1. The microsomal cytochrome b5 system has been implicated in fatty acid desaturation reactions2 and a similar system in erythrocytes may catalyse the reduction of methaemoglobin3. Calf liver cytochrome b5, solubilized by pancreatic lipase, has a molecular weight of 11,000 and consists of ninety-three amino-acids in the sequence shown in Fig. 1 (refs. 4 and 5). The haem group is non-covalently bound to the protein and can be removed reversibly by acid acetone treatment6.

Two thousand generations of hybridisation in a heliconius butterfly.

Abstract: The neotropical butterfly Heliconius melpomnene (L.) is extremely polymorphic in some parts of its range. About fifty varieties have been named from three localities in the west of Guyane (French Guiana), where perhaps many thousands of specimens have been collected for export by dealers (Joicey and Kaye, 1917; 1919). As the butterfly is mimetic, mutually copying in strict parallel its relative H. erato (L.) and several other species in the genus (Turner, 1970; 1971a), and distasteful to birds (Brower, Brower and Collins, 1963), this polymorphism is extremely interesting as the mimicry is Muellerian rather than Batesian. Its genetics should cast light on the genetic architecture of Muellerian mimicry and, as Muellerian mimics tend to be monomorphic (Ford, 1953) the polymorphism is a little surprising. This paper sets out to show, by genetical and other evidence, that the polymorphism of H. melpomene in the Guianas is produced by the hybridisation of monomorphic subspecies, the importance of this conclusion being that it shows that the polymorphism is not anomalous, and that a correct understanding of the polymorphism is essential to any discussion of the genetic architecture of mimicry. When the hypothesis of hybridisation was first put forward, it produced certain

Direct methods in crystallography

Abstract: The basic problem of structure determination in x ray crystallography is described together with the methods available up to 1948 - primarily the Patterson, isomorphous replacement and heavy-atom methods. In 1948 inequality relationships were derived by Harker and Kasper but these could only give a solution for very simple structures. A breakthrough came in 1952 with the introduction of the triple-product sign relationship, which could only be applied to centrosymmetric structures. In 1955 Cochran showed that the general-valued phases for non-centrosymmetric structures were also related. After several years, during which a number of more-or-less involved methods were proposed for applying sign relationships, it was shown by Karle and Karle that a comparatively simple approach, the symbolic addition method, could be applied even to complex structures. A variant of the method is also applicable to non-centrosymmetric structures. A different technique of applying phase relationships, developed by Germain, Main and Woolfson has been fully automated and quite complex structures can now be solved with computer programmes which require as input only unprocessed observed data. Some future trends in the development of direct methods may be seen in present work in the use of inequalities involving determinants of high rank and also in methods which attempt to derive values of certain types of structure-invariant quantities.

Pollen Tube Growth and Fine Structure

Abstract: Publisher Summary This chapter discusses pollen germination and tube growth. Lilium longiflorum pollen cytoplasm possesses numerous lipid droplets but lacks starch. Water uptake and the activation or synthesis of enzymes are no doubt the basic initiating factors of the germination process. Cytochemistry at the electron microscope level indicates that the cap and the vesicles contain pectin and that the RNA mentioned earlier resides in the smooth membranes. Only a few reports are available on the fine structure of pollen tubes growing in the pistil and not all of these are concerned with the organization of the tube tips. The embayments at the tips of compatible pollen tubes bear a striking resemblance to the wall ingrowths that characterize the cells that Gunning and Pate refer to as transfer cells. The pistil produces chemotropically active substances that are easily demonstrated in vitro and that presumably are active during at least a part of the journey of the tube through the pistil.

Isozyme variation under selection in Zea mays.

Abstract: THE extent of polymorphism in genetically determined isozyme variants (allozyme variation) in natural populations raises the issue of its adaptive significance. It was originally thought that this variation is neutral with respect to fitness1, but recent evidence supports the assertion that its maintenance involves a selective component2. If an allelic substitution yields an enzyme with altered properties3, maintenance by selection in natural populations may result in a distinct geographic distribution of allele frequencies. Such a pattern has been described4 in the case of a serum esterase polymorphism in the freshwater fish Catostomus clarkii.

Ligand-free Haemoglobin Dimers

Abstract: HAEMOGLOBIN is a tetramer (two α and two β subunits) in natural conditions in erythrocytes. I have studied the relationship between quaternary structure and haemoglobin function to determine the sequence of molecular events within the tetramer that bring about the conformational change (from constrained to unconstrained structures1,2) responsible for cooperative ligand binding3. In most technically accessible conditions, however, the tetramer exists in a dissociation-association equilibrium with dimer4–10. To elucidate the tetramer mechanism, therefore, it had to be determined whether the free dimer may undergo an equivalent conformational transition; that is, whether the dimer possesses the functional interactions responsible for cooperativity in the tetramer11–13.

Effects of defruiting, scoring, defoliation and shading on the carbohydrate content of ‘Golden Delicious’ apple trees

Abstract: SummaryYoung, bearing ‘Golden Delicious’ apple trees were defruited, scored, partially defoliated and shaded during the growing seasons of 1967 and 1968. The year after treatment there were significant differences in flowering, fruit-set and early growth. Carbohydrate analysis of shoots and spurs collected in early spring, at green tip, full bloom and three weeks after bloom indicated that these responses were not related to carbohydrate levels as measured in the year after treatment.

Studies on populations of Folsomia candida (Insecta: Collembola) : The productivity of populations in relation to food and exploitation.

Abstract: Experiments are described that were designed to investigate the effects of food availability and rate of exploitation on the growth and production of populations of Folsomia candida (Willem) In an initial experiment in which there was excess food it was found that increasing the rate of exploitation resulted in increases in both the numerical and biomass productivity of the populations In a second experiment it was shown that, when there is severe competition for food, the rate of exploitation does not affect either the biomass or the numerical production It is concluded that the effect of overcrowding, in the form of competition for space, does contribute to the growth in numbers of populations, but that the supply of food plays a more important role in regulating the population Anomalous results, showing that exploitation has a lesser effect when there is severe competition for food, are discussed

Gland cells and secretions in the miracidium of Fasciola hepatica.

Abstract: The distribution and fine structure of gland cells in the miracidium of Fasciola hepatica is described. There is a large flask-shaped, multinucleate, apical gland lying ventrally in the anterior. On each side of this are pairs of uninucleate accessory gland cells. Both apical gland and accessory gland cells communicate with the tip of the apical papilla. A third group of vesiculated gland cells opens by ducts at the base of the apical papilla. The miracidium is covered with a surface mucous coat and this appears to be directly extruded from the underlying ciliated epithelial cells. The findings are discussed with relevance to attachment to, and penetration of, the snail intermediate host.

Gene frequencies in the domestic cats of york: Evidence of selection

Abstract: RECENT work on the population genetics of the domestic cat has shown that not only do the frequencies of the various coat colours differ between Europe and the Far East, but also these frequencies differ between Europe and North America; as the North American population was presumably derived from that of Europe about two centuries ago, this indicates rapid evolution (Searle, 1964; Todd, 1966). The present survey gives an additional estimate of frequencies in the cat population of England, the country from which one assumes a large proportion of North American cats, as of North American people, are derived; and makes a comparison between the frequencies of a relatively small town and the large London conurbation, in the same way that certain small French towns have been compared with Paris (Dreux, 1967a, b, c, l968c). The data has also been used to discuss the problems of randomness of mating, randomness of sampling, and selection—natural or otherwise. York is a comparatively small city, based on a medieval walled town, with a human population of about 108,600. It has a small amount of light industry, and is still used as an agricultural market centre. Air pollution from large industrial cities further west and from local domestic fires is fairly heavy. The city is much less urban in character than London, which lies 200 miles to the south. The cats, scored at the R.S.P.C.A. home for animals in York, consisted of adults and kittens brought in by their owners, and stray animals brought in by R.S.P.C.A. inspectors and members of the public, both from York and from surrounding rural areas. We scored the following characters: