全文获取类型
收费全文 | 13136篇 |
免费 | 1165篇 |
国内免费 | 5篇 |
出版年
2023年 | 42篇 |
2022年 | 93篇 |
2021年 | 272篇 |
2020年 | 149篇 |
2019年 | 176篇 |
2018年 | 243篇 |
2017年 | 197篇 |
2016年 | 337篇 |
2015年 | 619篇 |
2014年 | 612篇 |
2013年 | 772篇 |
2012年 | 1033篇 |
2011年 | 1090篇 |
2010年 | 659篇 |
2009年 | 565篇 |
2008年 | 914篇 |
2007年 | 906篇 |
2006年 | 808篇 |
2005年 | 765篇 |
2004年 | 796篇 |
2003年 | 672篇 |
2002年 | 676篇 |
2001年 | 132篇 |
2000年 | 103篇 |
1999年 | 124篇 |
1998年 | 172篇 |
1997年 | 120篇 |
1996年 | 113篇 |
1995年 | 101篇 |
1994年 | 86篇 |
1993年 | 77篇 |
1992年 | 70篇 |
1991年 | 73篇 |
1990年 | 50篇 |
1989年 | 54篇 |
1988年 | 50篇 |
1987年 | 48篇 |
1986年 | 63篇 |
1985年 | 49篇 |
1984年 | 50篇 |
1983年 | 49篇 |
1982年 | 44篇 |
1981年 | 44篇 |
1980年 | 39篇 |
1979年 | 30篇 |
1978年 | 26篇 |
1977年 | 21篇 |
1976年 | 25篇 |
1974年 | 22篇 |
1973年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
Primary roots of maize (Zea mays L.) and pea (Pisum sativum L.) exhibit strong positive gravitropism. In both species, gravistimulation induces polar movement of calcium across the root tip from the upper side to the lower side. Roots of onion (Allium cepa L.) are not responsive to gravity and gravistimulation induces little or no polar movement of calcium across the root tip. Treatment of maize or pea roots with inhibitors of auxin transport (morphactin, naphthylphthalamic acid, 2,3,5-triiodobenzoic acid) prevents both gravitropism and gravity-induced polar movement of calcium across the root tip. The results indicate that calcium movement and auxin movement are closely linked in roots and that gravity-induced redistribution of calcium across the root cap may play an important role in the development of gravitropic curvature.Abbreviations 9-HFCA
9-hydroxyfluorenecarboxylic acid
- NPA
naphthylphthalamic acid
- TIBA
2,3,5-triiodobenzoic acid
- IAA
indole-3-acetic acid 相似文献
172.
Schizophyllan (SPG)-treated macrophages and anti-tumor activities against syngeneic and allogeneic tumor cells 总被引:1,自引:0,他引:1
We tested anti-tumor activities of macrophages treated with a neutral polysaccharide, schizophyllan (SPG), against syngeneic and allogeneic tumor cell lines. SPG was a macrophage stimulant which was not mitogenic to lymphocytes. That made a sharp contrast with the data that Corynebacterium parvum, BCG, and muramyl dipeptide (MDF) were macrophage stimulants which had lymphocyte-activating properties. Treatment of SPG-treated PEC with Thy12 monoclonal antibody and guinea pig complement did not affect the capabilities of tumor-cell-growth suppression by the treated PEC. Thus, the effector cells were peritoneal adherent cells (macrophages morphologically) and effector-to-target contact seemed to be necessary for effective tumor-cell-growth inhibition, although contradictory data exist for this. Murine peritoneal adherent cells harvested 4 days after a single IP injection of SPG at a dose of 100 mg/kg body weight of mouse showed the most prominent cytostatic and cytotoxic activities against syngeneic and allogeneic tumor cells. The distribution of anti-tumor activity in macrophages of various sizes followed the same pattern as macrophages treated with C. Parvum, i.e., larger macrophages showed more remarkable anti-tumor activity. Crude nonadherent peritoneal cells incubated with SPG at a concentration of 10 micrograms/ml, 100 micrograms/ml, or 1 mg/ml did not secrete lymphokine that rendered macrophages cytotoxic, while ConA-treated nonadherent cells did so. Furthermore, spleen cells treated with SPG in vivo did not secrete macrophage-activating lymphokine in the presence of SPG. On the other hand, addition of 1 mg/ml of SPG-treated peritoneal adherent cells and bone-marrow-derived macrophages in vitro rendered them cytotoxic to a moderate degree. This implies that SPG may activate macrophages directly, allowing them to become cytotoxic in the peritoneal cavity. Lastly, SPG could induce production of II-1-like factor to a moderate degree. SPG, whose molecular structure is well elucidated, will provide us with a strong tool to analyze the mechanism of macrophage activation both in vitro and in vivo. 相似文献
173.
Summary Wound repair and proliferation were examined in the injured newt atrium with light- and electron-microscopic techniques including autoradiography. Hearts were injured by removing a piece approximately 0.5 mm2 of the atrial wall. The five-day wound was an endothelial and mesothelial-lined blood clot bordered by a 150-m necrotic zone. Repair progressed from the periphery inward with areas of macrophage activity replaced by fibroblasts and connective tissue. The wound at 25 days consisted of a scar with few myocytes. There was no difference in the proliferative behavior between the right and left atria. Proliferative cells were localized to a 500-m reactive zone surrounding the wound. The maximum mesothelial cell thymidine-labeling index of 20.5% and mitotic index of 1.4% were seen 5 days after injury. The peak connective tissue cell thymidine-labeling index of 10.2% and mitotic index of 0.4% were seen 10 days after wounding. The peak thymidine-labeling index of 9.8% for myocardial cells was recorded 10 days after injury with a mitotic index of 0.2%. Proliferation returned to control levels by 25 days post-injury. Electron microscopy demonstrated that myocytes engaged in DNA synthesis were indistinguishable from control myocytes. Z-band material was not observed in mitotic myocytes, but myofilaments and junctions were present. 相似文献
174.
At concentrations inhibitory to the elongation of corn (Zea mays L.) roots, the auxins, indole-3-acetic acid (IAA) and α-naphthaleneacetic acid (α-NAA), cause an increase in the pH of the
bathing medium; this increase occurs with an average latent period shorter than the latent period for the inhibitory effect
of these auxins on elongation. Indole-2-carboxylic acid, an inactive structural analogue of IAA, and β-naphthaleneacetic acid,
an inactive analogue of α-NAA, affect neither growth nor the pH of the medium. Since acid pH is known to promote and basic
pH to inhibit root elongation, the data are consistent with the hypothesis that hormone-induced modification of cell-wall
pH plays a role in the control of elongation of roots, as has been proposed for elongation of stems and coleoptiles. 相似文献
175.
Summary Conventional and Cl-selective liquid ion-exchanger intracellular microelectrodes were employed to study the effects of extracellular ionic substitutions on intracellular Cl activity (aCl
i
) inNecturus gallbladder epithelium. As shown previously (Reuss, L., Weinman, S.A., 1979;J. Membrane Biol.
49:345), when the tissue was exposed to NaCl-Ringer on both sidesaCl
i
was about 30mm, i.e., much higher than the activity predicted from equilibrium distribution (aCleq) across either membrane (5–9mm). Removal of Cl from the apical side caused a reversible decrease ofaCl
i
towards the equilibrium value across the basolateral membrane. A new steady-stateaCl
i
was reached in about 10 min. Removal of Na from the mucosal medium or from both media also caused reversible decreases ofaCl
i
when Li, choline, tetramethylammonium or N-methyl-d-glucamine (NMDG) were employed to replace Na. During bilateral Na substitutions with choline the cells depolarized significantly. However, no change of cell potential was observed when NMDG was employed as Na substitute. Na replacements with choline or NMDG on the serosal side only did not changeaCl
i
. When K substituted for mucosal Na, the cells depolarized andaCl
i
rose significantly. Combinations of K for Na and Cl for SO4 substitutions showed that net Cl entry during cell depolarization can take place across either membrane. The increase ofaCl
i
in depolarized cells exposed to K2SO4-Ringer on the mucosal side indicates that the basolateral membrane Cl permeability, (P
Cl) increased. These results support the hypothesis that NaCl entry at the apical membrane occurs by an electroneutral mechanism, driven by the Na electrochemical gradient. In addition, we suggest that Cl entry during cell depolarization is downhill and involves an increase of basolateral membraneP
Cl. 相似文献
176.
Timothy P. Spira 《American journal of botany》1980,67(3):278-284
Diversity in flower size and degree of exsertion of anthers and stigma from the corolla in the California species of Trichostema appear to be correlated with breeding system and pollinator type (bee vs. bird). Autogamous (self-pollinating) species unlike xenogamous (cross-pollinating) species lack spatially separate anthers and stigma and nototribic pollination. The outcrossing species have significantly larger flowers, significantly larger nectar volumes and significantly higher pollen-ovule ratios than do selfing species. Because autogamous species are less reliant on flower visitors to facilitate pollination, there may be relaxed selection for large nectar producing flowers. Pollen-ovule ratios are correlated with breeding system and reflect the efficiency of pollen transport. Data on floral parameters suggest xenogamous species expend more energy per floral unit and are less efficient seed producers than closely related autogamous species. 相似文献
177.
178.
The visual pigment and visual cycle of the lobster,Homarus 总被引:1,自引:0,他引:1
Merle S. Bruno Stephen N. Barnes Timothy H. Goldsmith 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1977,120(2):123-142
Summary The visual pigment of the American lobster,Homarus americanus, has been studied in individual isolated rhabdoms by microspectrophotometry. Lobster rhodopsin has max at 515 nm and is converted by light to a stable metarhodopsin with max at 490 nm. These figures are in good agreement with corresponding values obtained by Wald and Hubbard (1957) in digitonin extracts. Photoregeneration of rhodopsin to metarhodopsin is also observed. The absorbance spectrum of lobster metarhodopsin is invariant with pH in the range 5.4–9, indicating that even after isomerization of the chromophore fromcis totrans, the binding site of the chromophore remains sequestered from the solvent environment. Total axial density of the lobster rhabdom to unpolarized light is about 0.7.As described for several other Crustacea, aldehyde fixation renders the metarhodopsin susceptible to photobleaching, a process that is faster at alkaline than at neutral or acid pH. Small amounts of a photoproduct with max at 370 nm are occasionally seen. A slower dark bleaching of lobster rhabdoms (1/2–2 h) also occurs, frequently through intermediates with absorption similar to metarhodopsin.The molar extinction coefficient of metarhodopsin is about 1.2 times greater than that of rhodopsin, each measured at their respective max. Isomerization of the chromophore fromcis totrans is accompanied by a change in the orientation of the absorption vector of about 3°. The absorption vector of metarhodopsin is either tilted more steeply into the membrane or is less tightly oriented with respect to the microvillar axes.When living lobsters are kept at room temperature, light adaptation does not result in an accumulation of metarhodopsin. At 4 °C, however, the same adapting lights cause a reduction of rhodopsin and an increase in metarhodopsin. There is thus a temperature-sensitive regeneration mechanism that supplements photoregeneration. Following 1 ms, 0.1 joule xenon flashes that convert about 70% of the rhodopsin to metarhodopsin in vivo, dark regeneration occurs in the living eye with half-times of about 25 and 55 min at 22 °C and 15 °C respectively.This work was supported by USPHS research grant EY 00222 to Yale University. S.N.B. was aided by NIH Postdoctoral Fellowship EY 52378. 相似文献
179.
Stephen N. Barnes Timothy H. Goldsmith 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1977,120(2):143-159
Summary Dark adaptation of living lobsters was measured by recording the ERG at several temperatures in the range 5–20 °C following adapting flashes that convert about 70% of the rhodopsin to metarhodopsin. Recovery of log threshold is rapid, and at 10–20° is nearly complete in 10 min. Only at 5 °C is dark adaptation significantly slowed. Comparison of dark adaptation with data on regeneration of pigment (Bruno et al., 1977) is consistent with the hypothesis that as rhodopsin concentration rises and falls, its only effect on sensitivity is to alter the probability of quantum catch. This interpretation is further bolstered by observations on winter lobsters that have a 70% deficiency of rhodopsin without the concomitant increase in metarhodopsin that accompanies light adaptation. No effect of metarhodopsin on sensitivity was detected. These experiments support the growing body of evidence indicating that the relationship between rhodopsin concentration and log threshold is fundamentally different in the rhabdomeric photoreceptors of invertebrates and the rods and cones of vertebrates.This work was supported by USPHS research grant EY 00222 to Yale University. S.N.B. was aided by NIH Postdoctoral Fellowship EY 52378, by funds made available through the Unidel Foundation, and by a grant from the University of Delaware Research Foundation. 相似文献
180.
Studies on the H-Y antigen in rats 总被引:1,自引:0,他引:1
The H-Y antigen has been studied under a variety of experimental conditions in BN and Lewis rats. The results indicate that 1. graft size is crucially important in determining the fate of male skin isografts on females; 2. H-Y incompatible ear skin grafts survive significantly better than those of trunk origin; 3. prior exposure of females to male lymphoid cells greatly increases their capacity to reject male skin isografts; 4. neonatal castration has no influence on the expression of H-Y; 5. multiparity can induce unresponsiveness to H-Y; and 6. although BN females respond better than do Lewis females to H-Y, the antigen is stronger in Lewis males. These findings are compared with the results of similar experiments conducted with mice.Submitted in memory of Dr. Joy Palm, member of the Wistar Institute, who pioneered the genetic analysis of histocompatibility in rats. 相似文献