首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1328篇
  免费   129篇
  国内免费   1篇
  2021年   21篇
  2020年   15篇
  2019年   14篇
  2018年   17篇
  2017年   24篇
  2016年   45篇
  2015年   46篇
  2014年   62篇
  2013年   78篇
  2012年   81篇
  2011年   76篇
  2010年   50篇
  2009年   44篇
  2008年   65篇
  2007年   59篇
  2006年   60篇
  2005年   59篇
  2004年   66篇
  2003年   65篇
  2002年   63篇
  2001年   39篇
  2000年   38篇
  1999年   33篇
  1998年   21篇
  1997年   15篇
  1996年   12篇
  1995年   14篇
  1994年   13篇
  1992年   15篇
  1991年   11篇
  1990年   12篇
  1989年   14篇
  1988年   10篇
  1987年   13篇
  1986年   9篇
  1985年   8篇
  1983年   8篇
  1982年   6篇
  1978年   5篇
  1976年   6篇
  1975年   9篇
  1974年   10篇
  1973年   12篇
  1972年   9篇
  1971年   5篇
  1969年   6篇
  1968年   14篇
  1967年   9篇
  1966年   8篇
  1965年   5篇
排序方式: 共有1458条查询结果,搜索用时 62 毫秒
11.
Zusammenfassung Die Ausdrucksbewegungen der Sichelente,Anas (Eunetta) falcata Georgi, werden beschrieben, soweit möglich vorläufig analysiert und mit denen nächstverwandter Arten, vor allem denen vonAnas (Nettium) crecca crecca L.,Anas (Chaulelasmus) strepera L. undAnas (Mareca) penelope L. verglichen. Dieser Vergleich ergibt, genau wie der morphologischer Merkmale des Gefieders und der Knochentrommel, eine systematische Stellung vonfalcata genau zwischen den drei genannten Arten, näher den beiden erstgenannten als der dritten. Die Verteilung der Merkmal-Gemeinsamkeiten einerseits mitcrecca, andererseits mitstrepera, läßt den Schluß zu, daß die drei Arten divergent aus einer gemeinsamen Ahnenform entstanden seien. Keine gemeinsamen Merkmale, die bei anderen Arten fehlen, verbinden die drei genannten Formen zu einer Gruppe.Beim Gesellschaftsspiel der Sichelerpel sind, im Gegensatz zu dem aller anderen bisher daraufhin untersuchten Schwimmentenarten, alle beteiligten Bewegungsweisen, einschließlich des einleitenden Schüttelns, zur Ente hin orientiert, die hier, wie beicrecca undstrepera, am Spiel der Erpel sehr regen Anteil nimmt. Dagegen fehlen solche Bewegungen, die durch Ritualisierung aus Angriffsverhalten entstanden sind, beim Gesellschaftsspiel völlig, spielen aber eine große Rolle, analog dem Triumphgeschrei der Gänse und Tadorninen, beim Zusammenhalt der bereits verpaarten Tiere.  相似文献   
12.
A duct system was arranged through which flowed air of controlled ion concentration. In series was a chamber with a flexible dielectric floor which made possible electrical detection of activity.Exposure to various ion concentrations of either polarity produced pronounced effects.In addition to gross motor activity six other behavioural parameters were recorded. The lowest ion concentration were most effective. In addition to presenting a controlled ambient ion concertration the ion current drawn by the animal was measured. This varied markedly from rat to rat.
Zusammenfassung Ein Kanalsystem zum Durchfluss von Luft bekannter Ionenkonzentration wurde eingerichtet. Angeschlossen war eine Kammer mit einem beweglichen dielektrischen Boden, der die Registrierung der Aktivität von Ratten ermöglichte. Neben der motorischen Aktivität wurden 6 andere Verhaltensformen der Tiere registriert.Die niedrigste Ionenkonzentration war am wirksamsten zur Anregung der Aktivität. Neben der Ionenkonzentration in der Kammer wurde der von den Tieren eingeatmete Ionenstrom gemessen; er variierte stark von Tier zu Tier.

Resume On a construit un canal dans lequel il est possible de faire circuler de l'air dont la concentration en ions est contrôlable. On y a adjoint une cellule dont le sol est constitué par un diélectrique flexible permettant de déceler électriquement l'activité de petits animaux.En exposant des rats à des concentrations ioniques variables et de polarité différente, on peut constater des effets prononcés sur le comportement des animaux. En effet, en plus de l'activité motrice, on a enregistré six autres formes de ce comportement. Ce sont les plus faibles concentrations qui produisent le plus d'effets. A part la concentration en ions à l'intérieur de la cellule, on a également mesuré la quantité d'ions absorbés par les bêtes par voie respiratoire. Cette quantité varie fortement d'un individu à l'autre.


This work was supported by Public Health Service Research Grant #AP00305-02 from the Division of Air Pollution, Bureau of State Services and also by a grant from the Atmospheric Science Research Center of the State University of New York.  相似文献   
13.
Summary A hypervariable DNA marker is closely linked to one of the most severe forms of night blindness, X-linked retinitis pigmentosa (RP). Affected individuals with X-linked RP, obligate carriers, and ophthalmologically identifiable carriers of the disease were included in a linkage study. The diagnosis was established in five sibships by funduscopic and electrophysiological investigations. When the X-linked probe M27 was used, 2 recombinants out of 29 informative meioses were detected (=0.07 at a maximum lod of 4.75). The hypervariable probe detected two different alleles in 38 of 39 females tested. M27 is therefore a potentially very useful probe for carrier detection and prenatal diagnosis, as well as for addressing the question of heterogeneity of X-linked RP.  相似文献   
14.
 Accessory function allows antigen-presenting cells to produce sufficient secondary signals for optimum T cell proliferation and interleukin-2 (IL-2) production. Alveolar macrophages are inferior accessory cells compared to monocytes (PBM). We report here that the accessory index (AI) of alveolar macrophages and PBM of patients with lung metastases of solid tumors treated with inhalations of human natural IL-2 (hnIL-2) increased following its administration (P<0.005). The accessory index was significantly elevated from baseline values after 2 weeks of inhalation of 300 000 IU hnIL-2/day (8.2±10.2 compared to 1.1±1; P<0.001). The inhalation of 150 000 IU also induced increases in the index (AI = 2.3±1.9), however, without reaching statistical significance. In addition at 300 000 IU IL-2/day a significant increase in the accessory index was observed for PBM (4±2.5; P<0.05). The indices of PBM and alveolar macrophages prior to inhalation showed a significant negative correlation with the age of the patients (r s =  – 0.5; r s =  – 0.8, respectively; P<0.03 for all comparisons). Our data demonstrate that the inhalational application of hnIL-2 enhances the accessory function of alveolar macrophages and, to lesser extent, the accessory index of PBM, indicating the occurrence of pharmacological immunostimulation. Received: 16 August 1995 / Accepted: 4 January 1996  相似文献   
15.
In a captive group of long-tailed macaques, tool-using behavior by a single competent individual had a significant effect on the synchronous manipulative behavior of naïve animals. Group members engaged in manipulations on the same object class more frequently during times when the model was working than when it was not. The form of their behavior, however, in no way resembled the technique used by the model. All three animals that later became successful tool users were among the few subjects that exhibited a significant increase in manipulations on the same object class while the model was working. Possible causal relationships between this stimulus enhancement and the transmission of the new behavior to other group members are analyzed and discussed.  相似文献   
16.
The phosphorylation and dephosphorylation of proteins on tyrosyl residues are key regulatory mechanisms in T-cell signal transduction and are controlled by the opposing activities of protein tyrosine kinases and phosphotyrosyl phosphatases (PTPs). In T cells, several nontransmembrane protein tyrosine kinases are associated with receptors; for example, Lck is bound to the coreceptors CD4 and CD8 and becomes activated upon their stimulation. In comparison, little is known about the role of nontransmembrane PTPs in early T-cell signaling. SH-PTP1 (PTP1C, HCP, SHP) is a nontransmembrane PTP expressed primarily in hematopoietic cells, including T cells. We have found that SH-PTP1 is basally phosphorylated on serine in resting T cells. Upon stimulation of CD4 or CD8 either in a T-cell hybridoma cell line or in primary thymocytes, SH-PTP1 becomes tyrosyl phosphorylated. Moreover, SH-PTP1 is constitutively phosphorylated on tyrosine in the Lck-overexpressing lymphoma cell line LSTRA. SH-PTP1 is also a good substrate for recombinant Lck in vitro. Comparisons of the tryptic phosphopeptide maps of wild-type SH-PTP1 and deletion and point mutations establish that the two sites (Y-536 and Y-564) which are directly phosphorylated by Lck in vitro are also phosphorylated in vivo in LSTRA cells. One of these sites (Y-564) is phosphorylated in T cells in response to Lck activation. We conclude that SH-PTP1 undergoes Lck-dependent tyrosyl phosphorylation in T cells and likely plays a role in early T-cell signaling.  相似文献   
17.
A quantitative endonuclease assay, which relies on the introduction of single and double strand breaks into supercoiled plasmid DNA, was used to study the activity of the extracellular nuclease of Serratia marcescens SM6 in buffer and in groundwater. The parallel enzyme concentration-dependent production of relaxed and linear plasmid molecules suggests that the nuclease produces single and double strand breaks in duplex DNA. Bovine serum albumin stimulated the nuclease activity towards DNA and RNA and increased the stability of the enzyme against thermal inactivation. The DNase activity at 4 °C and 50 °C was almost half of that at the optimum temperature (37 °C). The nuclease was active in groundwater, although the specific activity was lower than in buffer. In a groundwater aquifer microcosm, mineral-adsorbed transforming DNA was substantially less accessible to the nuclease than was dissolved DNA. The data suggest that the extracellular nuclease of Serratia marcescens may contribute to DNA turnover in the environment and that adsorption of DNA to minerals provides protection against the nuclease.Abbreviations GW groundwater GWA groundwater aquifer  相似文献   
18.
Mycoplasma-like organisms (MLOs) were constantly detected by the DAPI technique and by restriction fragment length polymorphism (RFLP) analysis of PCR-amplified DNA in trees of Pyrus pyrifolia cvs Hosui and Kosui grafted on P. communis, seedlings rootstock with symptoms similar to the slow form of pear decline. These symptoms included upward curling of the leaves along the midrib. Leaves were abnormally thick and later turned reddish while major veins became swollen and brown. Trees with symptoms were usually 4–5 years old and were growing in the major pear areas of central Italy. The incidence of affected trees was particularly high in one orchard adjacent to a pear orchard strongly affected with the slow form of pear decline. In this case the distribution pattern of affected Nashi trees suggests that the causal agent was introduced from the adjacent pear orchard by an aerial vector. Although oriental pears are well-known hosts of the pear-decline agent when used as rootstocks of French cultivars, this is the first report of pear decline in P. pyrifolia varieties.  相似文献   
19.
The ability of a fetus to heal without scar formation depends on its gestational age at the time of injury and the size of the wound defect. In general, linear incisions heal without scar until late in gestation whereas excisional wounds heal with scar at an earlier gestational age. The profiles of fetal proteoglycans, collagens, and growth factors are different from those in adult wounds. The less-differentiated state of fetal skin is probably an important characteristic responsible for scarless repair. There is minimal inflammation in fetal wounds. Fetal wounds are characterized by high levels of hyaluronic acid and its stimulator(s) with more rapid, highly organized collagen deposition. The roles of peptide growth factors such as transforming growth factor-beta and basic fibroblast growth factor are less prominent in fetal than in adult wound healing. Platelet-derived growth factor has been detected in scarless fetal skin wounds, but its role is unknown. An understanding of scarless tissue repair has possible clinical application in the modulation of adult fibrotic diseases and abnormal scar-forming conditions.  相似文献   
20.
A modified protocol for DNA amplification by polymerase chain reaction (PCR) coupled with laser densitometric determination of the amount of PCR products, which allowed quantitation of target sequence numbers in soil extracts, was developed. The method was applied to monitor target loss during incubation of purified plasmid DNA in natural nonsterile soils. It revealed soil-specific kinetics of target loss. After 60 days, 0.2, 0.05, and 0.01% of the initially added nahA genes on plasmids were detectable by PCR in a loamy sand soil, a clay soil, and a silty clay soil, respectively. Electroporation of Escherichia coli was used in parallel to quantitate plasmid molecules in soil extracts by their transforming activity. It was found that transformation by electroporation was about 20 times more efficient and much less inhibited by constituents of soil extracts than transformation of Ca(2+)-treated cells (G. Romanowski, M.G. Lorenz, G. Sayler, and W. Wackernagel, Appl. Environ. Microbiol. 58:3012-3019, 1992). By electroporation, greater than 10,000-fold plasmid loss was monitored in nonsterile soils. Transforming activity was found up to 60 days after inoculation of the soils. The studies indicate that PCR and electroporation are sensitive methods for monitoring the persistence of extracellular plasmid DNA in soil. It is proposed that plasmid transformation by electroporation can be used for the monitoring in soil and other environments of genetically engineered organisms with recombinant plasmids. The data suggest that genetic material may persist in soil for weeks and even for months after its release from cells.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号