全文获取类型
收费全文 | 51262篇 |
免费 | 3390篇 |
国内免费 | 5203篇 |
专业分类
59855篇 |
出版年
2023年 | 782篇 |
2022年 | 1118篇 |
2021年 | 1263篇 |
2020年 | 1212篇 |
2019年 | 1899篇 |
2018年 | 1697篇 |
2017年 | 1415篇 |
2016年 | 1297篇 |
2015年 | 1231篇 |
2014年 | 2463篇 |
2013年 | 3156篇 |
2012年 | 1884篇 |
2011年 | 2385篇 |
2010年 | 1794篇 |
2009年 | 2397篇 |
2008年 | 2501篇 |
2007年 | 2738篇 |
2006年 | 2449篇 |
2005年 | 1981篇 |
2004年 | 1702篇 |
2003年 | 1649篇 |
2002年 | 1431篇 |
2001年 | 1254篇 |
2000年 | 1042篇 |
1999年 | 939篇 |
1998年 | 891篇 |
1997年 | 830篇 |
1996年 | 825篇 |
1995年 | 807篇 |
1994年 | 790篇 |
1993年 | 701篇 |
1992年 | 697篇 |
1991年 | 690篇 |
1990年 | 534篇 |
1989年 | 542篇 |
1988年 | 484篇 |
1987年 | 475篇 |
1986年 | 450篇 |
1985年 | 701篇 |
1984年 | 945篇 |
1983年 | 744篇 |
1982年 | 800篇 |
1981年 | 641篇 |
1980年 | 649篇 |
1979年 | 579篇 |
1978年 | 445篇 |
1977年 | 415篇 |
1976年 | 380篇 |
1974年 | 250篇 |
1973年 | 265篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
191.
The relative appearance of the parietal muscles in the development of the zooids has been studied in several ctenostomatous and “cheilostomatous” species. A comparison of the different on-togenetical sequences demonstrated that a “cheilostomatous” type of organization of the zooids with a great probability has been achieved in minimum three times independentl and originated from different ctenostomatous sub-grous: the Membranidea from plesiomorgic victorelloids (ancestors of Bulbella with not yet developed peristomial tube), the Inoviceiata (Aetea) from advanced forms of victorelloids with reduced primary parietal muscles (perhaps stcies related to Pottsiella), and Penetrantia from arachnidioid or vesicularioid ancestors (?). Therefore, the classical orders α“Ctenostomata” and “Cheilostomata” represent only “stage groups” but no monohyletic systematical units. Because of the new concept and interpretation I propose a new name for the united group: Cteno-Cheilostomata, supra-ord. nov. 相似文献
192.
Motility and orientation has been studied in the unicellular photosynthetic flagellate, Euglena gracilis, using real time image analysis capable of tracking up to 200 cells simultaneously in the slow rotating centrifuge microscope (NIZEMI) which allows one to observe the cells' swimming behavior during centrifugation accelerations between 1 g and 5 g. At 1 g the cells show a weak negative gravitaxis, which increases significantly at higher accelerations up to about 3 g. Though most cells were capable of swimming even against an acceleration of 4.5 g, the degree of gravitaxis decreased and some of the cells were passively moved downward by the acceleration force; this is true for most cells at 5 g. The velocity of cells swimming against 1 g is about 10% lower than that of cells swimming in other directions. The velocity decreases even more drastically in cells swimming against higher acceleration forces than those at 1 g. The degree of gravitactic orientation drastically decreases after short exposure to artificial UV radiation which indicates that gravitaxis may be due to an active physiological perception rather than a physical effect such as an asymmetry of the center of gravity within the cell.
Offprint requests to: D.-P. Häder 相似文献
193.
T. N. Ananthakrishnan R. Senrayan S. Murugesan R. S. Annadurai 《Journal of biosciences》1991,16(3):111-119
Kairomones from moth scales tend to influence the parasitic potential byTrichogramma chilonis Ishii. Hexatriacontane, pentacosane, heptadecane, docosane and 2, 6, l0-dodecatrienal-3, 7, 11-trimethyl were identified
from the active moth scale extract ofHeliothis armigera Hubner (its natural host) andCorcyra cephalonica Stainton (a laboratory host). The significance of an array of compounds from moth scales with kairomonal activity for manipulating
entomophagous insects in biological control programmes is discussed. 相似文献
194.
The -carboxyl groups of Glu-43() and Glu-22() of hemoglobin-S (HbS), two intermolecular contact residues of deoxy protein, are activated by carbodiimide atp H 6.0. The selectivity of the modification by the two nucleophiles, glycine ethyl ester (GEE) and glucosamine, is distinct. Influence ofN-hydroxysulfosuccinimide, a reagent that rescues carbodiimide-activated carboxyl (O-acyl isourea) as sulfo-NHS ester, on the overall selectivity and efficiency of the coupling of Glu-22() and Glu-43() with nucleophiles has been investigated. Sulfo-NHS increases the extent of coupling of nucleophiles to HbS. The rescuing efficiency of sulfo-NHS(increase in modification) with GEE and galactosamine as nucleophiles is 2.0 and 2.8, respectively. In the presence of sulfo-NHS, the extent of modification of a carboxyl group is a direct reflection of the extent to which it is activated (i.e., the protonation state of the carboxyl group). The modification reaction exhibits very high selectivity for Glu-43() with GEE and galactosamine (GA) in the presence of sulfo-NHS. From the studies of the kinetics of amidation of oxy-HbS at its Glu-43() (i.e., chemical reactivity) as a function of thepH in the region of 5.5–7.5, the apparentpKa of its -carboxyl group has been calculated to be 6.35. Deoxygenation of HbS, nearly doubles the chemical reactivity of Glu-43() of HbS atpH 7.0. It is suggested that the increased hydrophobicity of the microenvironment of Glu-43(), which occurs on deoxygenation of the protein, is reflected as the increased chemical reactivity of the -carboxyl group and could be one of the crucial preludes to the polymerization process. 相似文献
195.
Summary The opioidergic, sympathetic and neuropeptide Y-positive innervation of the sphincter of Oddi (common bile duct sphincter and pancreatic duct sphincter), as well as other segments of the extrahepatic biliary tree was studied in the monkey by use of immunohistochemistry. Methionine-enkephalin-positive nerves were seen to innervate the smooth muscle of all portions of the sphincter of Oddi and also local ganglion cells. No methionine-enkephalin-positive nerves could be detected in the common bile duct, pancreatic duct or gallbladder. Tyrosine hydroxylase-positive nerves occurred between smooth muscle bundles and also ran to local ganglion cells as well as along the common bile duct. Neuropeptide Y-positive nerves were observed within smooth muscle of the sphincter of Oddi (all portions), common bile duct, pancreatic duct and gallbladder. No evidence of any differential innervation of the pancreatic duct and common bile duct sphincters could be detected with these markers. 相似文献
196.
In this paper, the attainability of ESS of the evolutionary game among n players under the frequency-independent selection is studied by means of a mathematical model describing the dynamical development and a concept of stability (strongly determined stability). It is assumed that natural selection and small mutations cause the phenotype to change gradually in the direction of fitness increasing. It is shown that (1) the ESS solution is not always evolutionarily attainable in the evolutionary dynamics, (2) in the game where the interaction between two species is completely competitive, the Nash solution is always attainable, and (3) one of two species may attain the state of minimum fitness as a result of evolution. The attainability of ESS is also examined in two game models on the sex ratio of wasps and aphids in light of our criterion of the attainability of ESS. 相似文献
197.
The implant of a femoral prosthesis is a critical process because of the relatively high temperature values reached at the bone/cement interface during the cementation of the infibulum. In fact, the cement is actually a polymer that polymerizes in situ generating heat. Moreover, the conversion of monomer into polymer is never 100%; this is dangerous because of the toxicity of the monomer. In this paper, we present a 3-D axisymmetric mathematical model capable of taking into account both the geometry of the implant and the chemical/physical properties of the cement. This model, together with its numerical simulation, thus represents a useful tool to set up the optimal conditions for the new materials developed in this orthopaedic field. The real complex geometry is assumed to be a bone/cement/metallic system having cylindrical symmetry, thus allowing the model to be reduced to two space variables. The cementation process is described by the Fourier heat equation coupled with a suitable polymerization kinetics. The numerical approximation is accomplished by semi-implicit finite differences in time and finite elements in space with numerical quadrature. The full discrete scheme amounts to solve linear positive definite symmetric systems preceded by an elementwise algebraic computation. We present various numerical simulations which confirm some critical aspects of this orthopaedic fixing technique such as thermal bone necrosis and the presence of unreacted residual monomer.This work was partially supported by MURST (Fondi per la Ricerca Scientifica 40%) and CNR (IAN, Contract 880032601, and Progetto Finalizzato Sistemi Informatici e Calcolo Parallelo, Sottoprogetto Calcolo Scientifico per Grandi Sistemi) of Italy 相似文献
198.
JAMES B. COUPLAND 《Ecological Entomology》1991,16(1):11-15
Abstract.
- 1 The oviposition behaviour of Simulium reptans L. is described from two sites on the River Spey, Scotland. Female aggregations were observed immediately downstream of oviposition sites and were composed mainly of gravid flies (range 60–80%0) together with smaller numbers that were either freshly bloodfed (range 2–17%) or infected with mermithid nematodes (range 0–30%).
- 2 The time from landing on the oviposition sites to the onset of oviposition was recorded. The time in the presence of greater than 1-day-old eggs did not significantly differ from sites with no eggs present. However, the presence of freshly laid or I-day-old eggs significantly shortened the time to onset of oviposition. The cues that elicit oviposition are unknown but it is speculated that they may involve a pheromone.
199.
W. C. Holmes 《Plant Systematics and Evolution》1991,175(1-2):87-92
For the first time dioecy inMikania and in the tribeEupatorieae is described and discussed. The condition is known only in members of theMikania swartziana
Griseb. complex, a group of eight species, all endemic to the Greater Antillean Islands of Cuba, Hispaniola, and Jamaica. 相似文献
200.