Showing papers by "University of York published in 1973"

The structure of barley roots in relation to the transport of ions into the stele

Abstract: Three successive states are recognized in the development of endodermal cells in seminal and nodal axes and primary lateral roots of barley: 1. Casparian bands in the anticlinal walls; 2. suberin lamella around the whole inner face of the wall; 3. unevenly deposited cellulosic wall thickening. These states develop asynchronously, the cells adjacent to the protoxylem pole cells always being last to mature. All cells have progressed to at least the secondary state by 32 cm from the tip in seminal axes, 48 cm from the tip in nodal axes, but only 6 cm from the tip in primary laterals. The asynchronous development gives the appearance of “passage” cells adjacent to the protoxylem pole cells, although all cells eventually attain the same state and degree of wall thickening. Long distance transport of calcium shows a close correlation with the incidence of suberin lamellae in the three types of root examined; it is suggested that formation of a suberin lamella effectively blocks calcium movement into the stele and, therefore, long distance transport. Plasmodesmata are present in comparable frequencies through both tangential and radial endodermal walls; they appear to maintain intercellular continuity until a late stage in development.

Plastid Differentiation, Acyl Lipid, and Fatty Acid Changes in Developing Green Maize Leaves

Abstract: Plastid differentiation, acyl lipid, and fatty acid composition have been followed in successive 2-cm sections from the base (youngest tissue) to the tip (oldest tissue) of green Zea mays (maize) leaves grown under a normal diurnal light regime. Although the youngest cells (0-4 cm from the leaf base) had only proplastids with one or two grana, they contained chlorophylls a and b, monogalactosyldiglyceride, digalactosyldiglyceride, sulfolipid, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol. In the more mature sections, the plastids increased in size 5-fold, and differentiation into mesophyll and bundle-shealth chloroplasts had occurred. Concomitantly, the levels of all the lipids increased with the exception of phosphatidylcholine and phosphatidylethanolamine which decreased. With increasing cell maturity, the percentage of linolenic acid increased in all the individual acyl lipids, but palmitic acid remained constant in phosphatidylcholine, phosphatidylethanolamine, and sulfolipid. The Δ3t-hexadecenoic acid was only detectable in the phosphatidylglycerol of the most mature maize tissue.

Ultrastructural Study of Poly-β-Hydroxybutyrate Granules from Bacillus cereus

Abstract: The freeze-etching technique was used to examine the effects of fracturing and etching on the appearance of poly-β-hydroxybutyrate granules from Bacillus cereus. These granules were examined in extracts isolated by hypochlorite or by sonic treatment, and also in fixed and unfixed intact cells; in the latter case they were compared with granules in thin sections of intact cells. After freeze-fracturing, the diameter of the granules in intact cells was between 240 and 720 nm. The granules consisted of a central core, of diameter between 140 and 370 nm, which occupied less than 50% of the volume of the granule and which was either stretched or removed on fracturing; the core was surrounded by an outer coat which may be bounded by a membrane. Images

The Management Plan

Abstract: In Part One, ‘Ecological theory’, of this book we examined the dynamic nature of the plant and animal species which make up the ecosystem. Although at first glance many of the processes being discussed seemed to be precise, yet one becomes increasingly aware of the random influences acting to ‘blur’ this precision. In Chapter 2, for example, we discussed ecological succession and defined clear-cut successful stages, nine for the shingle succession at Dungeness. Such a process is in reality an oversimplification, since succession is far more complex and less precise. On any piece of land, arbitrarily chosen, it is more than likely that there will be some abnormality in the succession, either by one stage being missed out, or by a reversal to a previous stage, or by the chance introduction of another species causing a temporary variation of the stage. The clear-cut stages that are put forward are in reality averages or means in the development of a community, forming part of a continuum of variation both lengthwise from the initial to the final form of the community and breadthwise in that each stage has its own variation. In ecological studies we usually attempt to define the mean, whereas it might be that the variance attached to the mean is of as much importance.

The effects of pregnancy and lactation on some mechanical properties of the femora of the rat.

Abstract: Changes in the mechanical properties of the femoral diaphyses of the rat in pregnancy and lactation were studied. Virgins bore higher loads than post-partums and lactators, and had higher moduli of elasticity and rupture. The amount of work needed to fracture the bones did not, however, vary significantly between the three classes. The lactators had a significantly lower modulus of rupture than the post-partums but otherwise did not differ significantly in mechanical properties. The structure of the bone, the amount of erosional activity and the ash content were studied. Some of the alteration in mechanical properties could be ascribed to a thinning of the shaft wall in lactators. More than 35% of the variation in the modulus of rupture, and more than 20% of the variation in the modulus of elasticity could be explained by variation in ash content. Rats seem to lose most bone from the end of the bones. The bone lost from the shaft is lost from the endosteal surface. Mechanically, this is the best strategy.

The genetics of Cepaea esterases I. Cepaea nemoralis

Abstract: Using Polyacrylamide disk electrophoresis, at least 23 zones of general esterase activity can be distinguished in extracts of Cepaea nemoralis hepatopancreas. Breeding experiments have thrown light on the inheritance of three esterase systems. It has been suggested that the four most cathodally migrating esterase zones may be controlled by four linked loci, forming an esterase supergene. Zone Est. 5 also seems to be part of the same system and might be coded for by another linked locus. At each of these loci three alleles have been postulated, one producing no detectable enzyme, another producing a weak enzyme and the third coding for an active enzyme. Each locus seems to control enzyme activity at one level in the gel. Another esterase system is controlled by three alleles at one locus. The enzyme produced is a dimer, heterozygotes having three zones of activity. Attemps to hybridise “parental” enzymes so as to give a hybrid zone on electrophoresis failed. The third esterase system examined is one of presence and absence of the enzyme. It is controlled by two alleles at one locus with the presence of enzyme being dominant to its absence. There is no linkage detectable between any of the (effective) esterase loci or between the esterase loci and three loci determining the shell colour and banding morphs.

The Concept of the Ecosystem

Abstract: In the previous two chapters we have investigated the spatial and temporal heterogeneity of organisms, using from time to time the terms ‘population’ and ‘community’. A ‘population’ was assumed to be all of the individuals of one of the species in a multi-species ‘community’, or to be a community of only one species. However, this does not define either term rigorously. We must either make a precise definition or investigate why a more casual definition is the more useful.

Gas-phase oxidation of butene-2: The role of acetaldehyde in the reaction

Abstract: The gas-phase oxidation of butene-2 has been studied under two different sets of conditions. At 184°C, the principal products, when butene-2 is co-oxidized with acetaldehyde, are the isomeric epoxybutanes. Between 277° and 400°C, butene-2-molecular oxygen mixtures yield acetaldehyde and oxides of carbon as the main products, although the eposides are still formed in significant yield. The proportion of acetaldehyde to epoxides does not change on increasing the temperature, while the proportion of carbon monoxide to carbon dioxide increases significantly. The results are explained in terms of addition reactions of oxygenated radicals to the alkene. Acetaldehyde is considered to be formed by addition of hydroxy, followed by oxygen, while, below 300°C, the principal source of epoxide is thought to be peracetyl attack on the alkene. Rate data for the addition of peracetyl to butene-2 at 184°C suggests that this reaction is as facile as the abstraction reaction between peracetyl and acetaldehyde.

Long-term prereward suppression in monkeys unaccompanied by cardiovascular conditioning

Abstract: In Experiment I, a 3-min tone that preceded a free pellet of food suppressed variable-interval performances maintained by the same type of pellets, but failed to elicit conditioned changes in the heart rates and blood pressures of two rhesus monkeys. Initially severe, the prereward suppression became temporally discriminated to progressively later portions of the tone, and was maintained at an attenuated level for over four months. The suppression was apparently not caused by interfering autonomic respondents, nor was it superstitiously conditioned, since 21 of the initial 25 tone-food pairings took place outside of baseline sessions. In Experiment II, a 1-min light, paired with four free pellets of food, suppressed the variable-interval responding of a second pair of similarly trained monkeys. An interresponse-time analysis showed that in one subject, mild prereward suppression of responding developed through two stages. On early trials, response rate slowed by 10% throughout the prefood interval. On later trials, the animal suppressed by pausing for a like portion of the interval, most often near the end, but otherwise responded normally during the prefood signal.