The Royal Society
0080-4622
2054-0280
Cơ quản chủ quản: N/A
Lĩnh vực:
Phân tích ảnh hưởng
Thông tin về tạp chí
Các bài báo tiêu biểu
Agricultural Development and Advisory Service (A. D. A. S. ) recommendations to farmers on manure disposal and recycling Farm animal excreta may fall directly on grazed crops or be collected from housed animals as semi-liquid slurry or more solid manure. Estimates are given of the total amounts of N in animal excreta in the U. K. and in the various forms in which it may be collected from housed animals. The factors affecting the losses of N from stored manures and the availability to crops of the N content of manures are reviewed. The optimum use of the N content of manures as a crop nutrient is discussed in relation to practical farming problems including the costs to farmers of storing manures and slurries, the difficulties of applying these materials to land at particular times of the year, and the risks of causing pollution of water.
Tập 296 Số 1082 - Trang 329-332 - 1982
Nitrogen cycling in United Kingdom forests: the relevance of basic ecological research
Following recent reviews on nutrient cycling in temperate and British forests, two aspects of N cycling are selected for examination because of their fundamental importance and their relevance to management—ecosystem research and mobilization of N during decomposition. Despite the early work by J. D. Ovington and P. J. Rennie, only two major forest ecosystem studies have been developed in Britain. Summaries from these ecosystems show that they span the range of temperate forests in the N cycling characteristics. The
Pinus nigra
plantation on Culbin Sands is functioning on a low capital with low rates of transfer and shows N deficiency after 40 years. In contrast, the mixed deciduous Meathop Wood has a much larger capital of N and higher rates of transfer. The
Pinus
ecosystem retained a high proportion of added fertilizer N in the biomass and forest floor, and increased growth rates can be explained through N recycling within the trees. Basic concepts of N mineralization are reviewed and it is shown that they need to be modified for application to forests. Uptake by saprophytic and mycorrhizal fungi from the pool of soluble organic N can short-circuit the ammonification pathway and N can be released from substrates with a high C:N ratio by lysis of microbial tissues and faunal feeding. Some examples of evidence of these processes are given, stressing the spatial and temporal mosaic of substrates acting as sinks and sources of N. Management practices can have a marked effect, not only on the quantity of N returned in residues, but on its availability as a result of variation in the associated carbon. However, growth experiments have shown that substrates with C:N ratios above 35 can release significant amounts of N to tree seedlings. A final examination of the recycling processes in the four nutritional stages of a forest emphasizes the importance of the effects of management on the short- and long-term balance of mobilization processes in the forest soil.
Tập 296 Số 1082 - Trang 427-444 - 1982
An anomaly in the response of the eye to light of short wavelengths
Adaptation of the human eye to long-wavelength light leaves it insensitive to short-wavelengths: a blue flash that is visible in the presence of a yellow adapting field may remain invisible for several seconds after the field has been turned off (see experiment 1 and Appendix). This ‘transient tritanopia’ occurs for a large range of adapting intensities, but is abolished if the adapting field is very bright (experiment 2). The loss of sensitivity is primarily confined to the blue-sensitive cone mechanism (experiments 2
a
, 3 and 4 ; and Appendix) and can be produced by small attenuations of the adapting field (experiment 5). It occurs in both foveal and parafoveal vision (experiment 6) but is absent when adapting and test stimuli are presented to opposite eyes (experiment 7). It was found in a protanope (experiment 9
a
) and, in a modified form, in a deuteranope (experiment 9
b
). No differences in sensitivity were found for blue flashes presented in the light and dark phases of a field flickering at a rate above the fusion frequency (Appendix). The sensitivity of the blue-sensitive mechanism of the eye appears to be controlled not only by quanta absorbed by the blue receptors but also by a mechanism with a different spectral sensitivity
Tập 278 Số 960 - Trang 207-240 - 1977
Anion-dependent cation transport in erythrocytes A selective survey of the literature reveals at least three major anion-dependent cation transport systems, defined as Na+ + Cl- , K+ + Cl- and Na+ + K+ + Cl- respectively. In human red cells, kinetic data on the fraction of K+ and Na+ influx inhibitable by bumetanide are presented to indicate an Na+ :K+ stoichiometry of 1:2. For LK sheep red cells the large Cl- -dependent K+ leak induced by swelling is shown to share many characteristics with that induced by A-ethyl maleimide (NEM) treatment. NEM has complex effects, both inhibiting and then activating Cl “-dependent K+ fluxes dependent on NEM concentration. The alloantibody anti-L can prevent the action of NEM. In human red cells NEM induces a large Cl- -dependent specific K+ flux, which shows saturation kinetics. Its anion preference is Cl- > Br- > SCN- > I- > NO- >3 MeSO- 4 . This transport pathway is not inhibited by oligomycin or SITS, although phloretin and high concentrations of furosemide and bumetanide (over 0.3 mM) do inhibit. Quinine (0.5 mM) is also an inhibitor. It is concluded that at least two distinct Cl- -dependent transport pathways for K+ are inducible in mammalian red cells, although the evidence for their separation is not absolute.
Tập 299 Số 1097 - Trang 483-495 - 1982
The temporal context of spatial memory Foraging strategies provide an effective and convenient means of investigating some characteristics of animal intelligence. The temporal and spatial distributions of food in the environment are not entirely random, and animals that use information about these patterns can gain an adaptive advantage over animals that do not. For some types of strategies to be effective, the animal must remember the temporal context of a visit to a spatial location and use this memory to make decisions about the current distribution of food in the environment. Experiments with different species (rat, Hawaiian honeycreeper, Marsh tit, Clark’s nutcracker) have used variations of a delayed conditional discrimination to examine the cognitive processes influencing this type of memory. These include: primacy, recency, proactive interference, retroactive interference, decay, consolidation, and chunking. The results of these experiments provide information about the types of memory processes that animals use when searching for food, and illustrate the usefulness of combining psychological and ethological approaches when studying animal intelligence.
Tập 308 Số 1135 - Trang 79-86 - 1985
The existence during gestation of an immunological buffer zone at the interface between maternal and foetal tissues In mammalian pregnancy the trophoblast normally constitutes an uninterrupted boundary of foetal tissue in immediate contact with maternal tissue, including blood in some species, and is the decisive immunological barrier to rejection of the foetus as an allograft. The ability of the trophoblast to function as a barrier evidently results from its capacity to resist immunological attack by either alloantibody or alloimmune cells and to prevent immunocompetent cells from reaching and damaging the foetus but, as yet, there is no general agreement regarding the means by which it exercises these functions. In view of the dramatic hormonal changes that occur during pregnancy and the undisputed involvement of trophoblast in these endocrine events, the possibility exists of an interaction between the hormones of pregnancy and the immunological phenomena. The present account furnishes evidence that endocrine activity at the maternal surface of the trophoblast, the presumptive site of the immunological frontier between foetus and mother, may be a factor in its local survival at implantation. The placental hormones so far known that are capable of blocking the antigen receptor sites of the mother’s lymphocytes and thus preventing the latter from reacting with the foetal antigens are the glycoprotein, human chorionic gonadotrophin (HCG) and the polypeptide hormone, human chorionic somatomammotrophin (HCS) or human placental lactogen (HPL), both of which are specific to the human placenta. The origin of these hormones, their spatial distribution and their probable interaction with placental steroid hormones are discussed. It is argued that the place of highest concentration of these hormones is on the surface of the syncytial microvilli and the adjacent caviolae of the apical plasma membrane, as well as on the surfaces of the persisting cytotrophoblastic cells of the basal plate (cytotrophoblastic shell), the cell islands and the septa - precisely where the immunological challenge of the foetal allograft to the maternal host occurs. An explanation is offered for the continuing production of the voluminous quantities of these hormones during human pregnancy.
Tập 271 Số 912 - Trang 343-361 - 1975
Macromolecules in mollusc shells and their functions in biomineralization
Mollusc shells are used as a model for studying ‘organic-matrix -mediated’ biomineralization, in which crystals are nucleated and grow in a pre-formed structural framework composed of proteins and polysaccharides. In particular, the possibility that the organic matrix functions as a template for crystal formation by epitaxial growth, is examined. In general, individual organic matrix sheets are composed of a thin layer of β-chitin sandwiched between layers of proteins adopting the antiparallel β-sheet conformation. The protein polypeptide chains are oriented perpendicular to the chitin fibrils. The matrix surfaces contain acidic proteins and polysaccharides. X -ray and electron diffraction patterns of matrices and mineral crystals from the nacreous layers of a bivalve, a gastropod and the cephalopod,
Nautilus
, show that the chitin fibres and the protein polypeptide chains are aligned with the
a
and
b
aragonite crystallographic axes, respectively. This strongly suggests that the mineral formed epitaxially upon the matrix surface. However, as the degree of orientation of the organic constituents is much less than the mineral constituents, it is postulated that the site of nucleation of the mineral crystals comprises only a small part of the matrix structure and is itself composed of well oriented macromolecules, probably acidic proteins. Two acidic proteins, which may constitute part of the nucleation site for calcite in a bivalve, were identified by comparing all the acidic proteins in the calcite layer with those in the aragonite layer. The two proteins were present in the calcite layer only, and in addition were found to have an aspartic-acid -containing amino acid sequence, not present in any of the other soluble matrix proteins. The concept of a matrix composed of a structural core coated with acidic macromolecules, some of which constitute a nucleation site, may well be applicable to other mineralized tissues.
Tập 304 Số 1121 - Trang 425-434 - 1984
Transforming growth factor-β: multifunctional regulator of differentiation and development Transforming growth factors-β (TGF-β) are 25 kilodalton (kDa) homodimeric peptides with multifunctional actions controlling the growth, differentiation and function of a broad range of target cells of both epithelial and mesenchymal derivation. They are expressed early in embryogenesis and their tissue-specific and developmentally dependent expression is strongly suggestive of an essential role in particular morphogenetic and histogenetic events. Five distinct TGF-βs have been characterized so far, with 65-80 % homology to each other. By using both molecular biological and immunohistochemical techniques, we are currently attempting to define specific sites of expression of the different TGF-βs and to determine whether TGF-βs 1-5 might have unique functions in development and in the mature organism. Comparative study of the promoter regions for the different TGF-βs and for any particular TGF-β in different species is also underway. Mechanistically, TGF-βs act to control gene expression of their target cells, many of their actions converging on a complex, multifaceted scheme of control of matrix proteins and their interactions with cells; these effects on matrix are thought to mediate many of the effects of TGF-β on development.
Tập 327 Số 1239 - Trang 145-154 - 1990
A model of cardiac electrical activity incorporating ionic pumps and concentration changes
Equations have been developed to describe cardiac action potentials and pacemaker activity. The model takes account of extensive developments in experimental work since the formulation of the M.N.T. (R. E. McAllister, D. Noble and R. W. Tsien, J. Physiol., Lond. 251, 1-59 (1975)) and B.R. (G. W. Beeler and H. Reuter,
J. Physiol., Lond
. 268, 177-210 (1977)) equations. The current mechanism
i
K2
has been replaced by the hyperpolarizing-activated current,
i
f
. Depletion and accumulation of potassium ions in the extracellular space are represented either by partial differential equations for diffusion in cylindrical or spherical preparations or, when such accuracy is not essential, by a three-compartment model in which the extracellular concentration in the intercellular space is uniform. The description of the delayed K current,
i
K
, remains based on the work of D. Noble and R. W. Tsien (
J. Physiol., Lond
. 200, 205-231 (1969
a
)). The instantaneous inward-rectifier,
i
K1
, is based on S. Hagiwara and K. Takahashi’s equation (
J. Membrane Biol
. 18, 61-80 (1974)) and on the patch clamp studies ofB. Sakmann and G. Trube (
J. Physiol., Lond
. 347, 641-658 (1984)) and of Y. Momose, G. Szabo and W. R. Giles (
Biophys. J
. 41, 311a (1983)). The equations successfully account for all the properties formerly attributed to
i
K2
, as well as giving more complete descriptions of
i
K1
and
i
K
. The sodium current equations are based on experimental data of T. J. Colatsky (
J.Physiol., Lond.
305, 215-234 (1980)) and A. M. Brown, K. S. Lee and T. Powell (
J.Physiol., Lond.
, Lond. 318, 479-500 (1981)). The equations correctly reproduce the range and magnitude of the sodium ‘window’ current. The second inward current is based in part on the data of H. Reuter and H. Scholz (
J. Physiol., Lond
. 264, 17-47 (1977)) and K. S. Lee and R. W. Tsien (
Nature, Lond
. 297,498-501 (1982)) so far as the ion selectivity is concerned. However, the activation and inactivation gating kinetics have been greatly speeded up to reproduce the very much faster currents recorded in recent work. A major consequence of this change is that Ca current inactivation mostly occurs very early in the action potential plateau. The sodium-potassium exchange pump equations are based on data reported by D. C. Gadsby (
Proc. natn. Acad. Sci. U. S. A.
77, 4035-4039 (1980)) and by D. A. Eisner and W. J. Lederer (
J. Physiol., Lond
. 303, 441-474 (1980)). The sodium-calcium' exchange current is based on L. J. Mullins’ equations (
J. gen.. Physiol.
70, 681-695 (1977)). Intracellular calcium sequestration is represented by simple equations for uptake into a reticulum store which then reprimes a release store. The repriming equations use the data of W. R. Gibbons & H. A. Fozzard (
J. gen. Physiol
. 65, 367-384 (1975
b
)). Following Fabiato & Fabiato’s work (
J. Physiol., Lond.
249, 469-495 (I975)), Ca release is assumed to be triggered by intracellular free calcium. The equations reproduce the essential features of intracellular free calcium transients as measured with aequorin. The explanatory range of the model entirely includes and greatly extends that of the M.N.T. equations. Despite the major changes made, the overall time-course of the conductance changes to potassium ions strongly resembles that of the M.N.T. model. There are however important differences in the time courses of Na and Ca conductance changes. The Na conductance now includes a component due to the hyperpolarizing-activated current,
i
r
, which slowly increases during the pacemaker depolarization. The Ca conductance changes are very much faster than in the M.N.T. model so that in action potentials longer than about 50 ms the primary contribution of the fast gated calcium channel to the plateau is due to a steady-state ‘window’ current or non-inactivated component. Slower calcium or Ca-activated currents, such as the Na-Ca exchange current, or Ca-gated currents, or a much slower Ca channel must then play the dynamic role previously attributed to the kinetics of a single type of calcium channel. This feature of the model in turn means that the repolarization process should be related to the inotropic state, as indicated by experimental work. The model successfully reproduces intracellular sodium concentration changes produced by variations in [Na]0, or Na-K pump block. The sodium dependence of the overshoot potential is well reproduced despite the fact that steady state intracellular Na is proportional to extracellular Na, as in the experimental results of D. Ellis
J. Physiol., Lond
. 274, 211-240 (1977)). The model reproduces the responses to current pulses applied during the plateau and pacemaker phases. In particular, a substantial net decrease in conductance is predicted during the pacemaker depolarization despite the fact that the controlling process is an increase in conductance for the hyperpolarizing-activated current. The immediate effects of changing extracellular [K] are reproduced, including: (i) the shortening of action potential duration and suppression of pacemaker activity at high [K ]; (ii) the increased automaticity at moderately low [K ]; and (iii) the depolarization to the plateau range with premature depolarizations and low voltage oscillations at very low [K]. The ionic currents attributed to changes in Na-K pump activity are well reproduced. It is shown that the apparent
K
m
for K activation of the pump depends strongly on the size of the restricted extracellular space. With a 30% space (as in canine Purkinje fibres) the apparent
K
m
is close to the assumed real value of 1 mM . When the extracellular space is reduced to below 5% , the apparent
K
m
increases by up to an order of magnitude. A substantial part of the pump is then not available for inhibition by low [K]
b
. These results can explain the apparent discrepancies in the literature concerning the
K
m
for pump activation.
Tập 307 Số 1133 - Trang 353-398 - 1985
Late quaternary history of vegetation and climate of the Rajasthan desert, India
The results from stratigraphy, radiocarbon dating and pollen analysis of three salt-lake deposits at Sambhar, Lunkaransar and Didwana in western Raj'asthan, and one freshwater lake deposit at Pushkar in the Aravalli Hills, are described in conjunction with pollen analysis of some archaeological soil samples from the Indus Valley site at Kalibangan in northern Rajasthan. The salt-lake deposits studied are stratigraphically divisible into (a) pre-lacustrine, (b) lacustrine and (c) post-lacustrine sections. The pre-lacustrine section is characterized by a thick bed of aeolian sand underlying lacustrine sediments, while the lacustrine and post-lacustrine sections are broadly circumscribed by laminated clay and nonlaminated silt respectively. The pollen record from the four lake profiles studied is divided into local pollen zones. Four regional pollen assemblage zones are delineated for the area west of the Aravalli Range in Rajasthan. The environmental history deduced from the pollen record is divisible into phases I-V, of which phases II-V follow the regional pollen assemblage zones. Phase I is stratigraphically determined, and is representative of severe arid environments under which the sand dunes, now stabilized, are suggested to have been active. The plant microfossils first appear in phase II with the deposition of lacustrine sediments dated to around 10000 b . p . The vegetation comprises an openland steppe which is rich in grasses, Artemisia and sedges and poor in halophytes.
Artemisa, Typha angustata, Mimosa rubicaulis
and
Oldenlandia
, which now grow under areas of comparatively higher average annual rainfall (above 50 cm), appear to have flourished in the semi-arid belt, while the first two plant taxa had encroached even as far as the arid belt, both suggesting that a general westward shift of the rainfall belts had taken place. Vegetation destruction through burning by man is suggested to have started together with the first occurrence of Cerealia-type pollen at about 7500 b . c . and continued thereafter throughout phases III and IV. The increase in swamp vegetation and the intensification of vegetation cover inland together with the maxima of all mesophytic elements in phase IV ( ca. 3000 b .c . to ca. 1000 b .c .) indicate an increase in the rainfall, apart from a short relatively drier time about 1800-1500 b . c . at Sambhar which correlates with the decline of the Indus culture in northwest India. Phase IV is immediately followed by aridity for which there is stratigraphic evidence that the salt lakes started drying. At Pushkar, there is evidence that the vegetation showed a marked change in the Aravallis. The onset of this aridity is suggested to have been widespread. The climate did not ameliorate until about phase V (? early centuries a .d . to present) at which time the Rangmahal culture perhaps flourished in Rajasthan, the remains of which imply good water supply. In conclusion it is suggested that the Rajasthan desert is primarily natural, its history punctuated by at least one more vegetated, humid period during the Holocene, the climatic control of which as indicated by the vegetation history is consistent with climatic events elsewhere in the world.
Tập 267 Số 889 - Trang 467-501 - 1974