全文获取类型
收费全文 | 29352篇 |
免费 | 2867篇 |
国内免费 | 27篇 |
出版年
2022年 | 271篇 |
2021年 | 451篇 |
2020年 | 262篇 |
2019年 | 364篇 |
2018年 | 406篇 |
2017年 | 399篇 |
2016年 | 579篇 |
2015年 | 947篇 |
2014年 | 983篇 |
2013年 | 1340篇 |
2012年 | 1540篇 |
2011年 | 1520篇 |
2010年 | 1119篇 |
2009年 | 879篇 |
2008年 | 1146篇 |
2007年 | 1186篇 |
2006年 | 1096篇 |
2005年 | 1090篇 |
2004年 | 1132篇 |
2003年 | 1048篇 |
2002年 | 1024篇 |
2001年 | 837篇 |
2000年 | 867篇 |
1999年 | 773篇 |
1998年 | 381篇 |
1997年 | 376篇 |
1996年 | 376篇 |
1995年 | 351篇 |
1994年 | 283篇 |
1993年 | 312篇 |
1992年 | 657篇 |
1991年 | 542篇 |
1990年 | 580篇 |
1989年 | 580篇 |
1988年 | 467篇 |
1987年 | 463篇 |
1986年 | 414篇 |
1985年 | 423篇 |
1984年 | 426篇 |
1983年 | 333篇 |
1982年 | 296篇 |
1981年 | 289篇 |
1980年 | 251篇 |
1979年 | 339篇 |
1978年 | 277篇 |
1977年 | 293篇 |
1976年 | 219篇 |
1975年 | 238篇 |
1974年 | 244篇 |
1973年 | 215篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
181.
M. W. A. Verstegen A. Siegerink W. Van der Hel R. Geers C. Brandsma 《Journal of thermal biology》1987,12(4):257-261
The aim of the experiments was to investigate the effect of air velocity on the temperature preferred by growing pigs 12–14 weeks old. Pigs displayed a temperature preference by means of operant supplemental heating. They pushed a button connected to heating lamps. Six experiments of three weeks each and with two treatments with a group of 8 pigs each were made. Animals were housed in groups and weighed 14–20 kg at the start of the experiments. Air velocity was 0.08, 0.25 and 0.40 m/s. At each air velocity four replicates were made. Mean temperatures preferred were 17.9°C at 0.08 m/s, 20.5°C at 0.25 m/s and 21.7°C at 0.40 m/s. Within a day temperature preference fluctuated with 5.7 K at 0.08 m/s, 4.3 K at 0.25 m/s and 4.2 K at 0.4 m/s. Temperatures preferred were highest during day time. 相似文献
182.
C J Van Noorden W M Frederiks D C Aronson F Marx K Bosch G N Jonges I M Vogels J James 《Virchows Archiv. B, Cell pathology including molecular pathology》1987,52(6):501-511
Extrahepatic cholestasis induced by ligation and transsection of the common bile duct caused a change in the parenchyma/stroma relationship in rat liver. Two weeks after ligation, the periportal zones of the parenchyma were progressively invaded by expanding bile ductules with surrounding connective tissue diverging from the portal areas. Parenchymal disarray developed and small clumps of hepatocytes or isolated hepatocytes were scattered within the expanded portal areas. These cells showed normal activity of lactate, succinate and glutamate dehydrogenase and may, therefore, be considered to be functionally active. After cholestasis the remainder of the liver parenchyma showed adaptational changes with respect to glucose homeostasis, as demonstrated by histochemical means. Glycogen stores disappeared completely whereas glycogen phosphorylase activity increased about ten fold. The increased glycogen phosphorylase activity and glycogen depletion indicate a greater glycogenolytic capacity in liver parenchyma after bile duct ligation to maintain as far as possible a normal plasma glucose concentration. The parenchymal distribution pattern of glucose-6-phosphatase activity did not change significantly after bile duct ligation. The isolated hepatocytes within the expanded portal tracts showed a high activity of this enzyme whereas the pericentral parenchyma was only moderately active. The distribution patterns of glucose-6-phosphate dehydrogenase and lactate dehydrogenase activity in the liver parenchyma were also largely unchanged after bile duct ligation, but the histochemical reaction for glucose-6-phosphate dehydrogenase activity demonstrated infiltration of the remainder of the parenchyma by non-parenchymal cells, possibly Küpffer cells and leucocytes as part of an inflammatory reaction. Under normal conditions the mitochondrial enzymes succinate and glutamate dehydrogenase show an opposite heterogenous distribution pattern in liver parenchyma. Following cholestasis both enzymes became uniformly distributed. The underlying regulatory mechanism for these different changes in distribution patterns of enzyme activities is not yet understood. 相似文献
183.
Human elongation factor 1α: a polymorphic and conserved multigene family with multiple chromosomal localizations 总被引:1,自引:0,他引:1
184.
Detection of ionizing radiations with the SOS Chromotest, a bacterial short-term test for genotoxic agents 总被引:1,自引:0,他引:1
The effects of 3 types of ionizing radiation, gamma-rays, neutrons and accelerated alpha-particles, were examined using the SOS Chromotest, a bacterial colorimetric assay for genotoxic agents based on the measurement of the SOS response in Escherichia coli. The SOS Chromotest appeared to be a sensitive and simple assay to detect quantitatively these radiations as well as their biological effects. The range of adsorbed doses for which induction was observed was similar for the 3 types of radiation, the minimum inducing doses being in the order of 2.5-5 Gy. We discuss the possible use of these observations to study the molecular action of radiations and to compare their genotoxic effects with those of chemicals. 相似文献
185.
Using a modified Salmonella typhimurium TA98 Ames-test system, more than 150 red, white and rose wines were analyzed for direct-acting and microsomal enzyme-enhanced mutagenic activity. The following conclusions were reached from analysis of this wine mutagenicity data base. White and rose wines, as well as grape juices, exhibited little or no detectable direct-acting or microsomal enzyme-enhanced mutagenic activity. However, red wine samples contained highly variable amounts of mutagens, ranging from undetectable to levels 30-fold above the sensitivity limit of the assay system. The variations in red wine mutagenicity were unrelated to grape variety, vintage, aging methods or production region. Hence, individual winery production practices must represent the most significant contribution to the variations observed. 相似文献
186.
Gynodioecy, the coexistence of hermaphrodites and male steriles, is frequent in populations of Plantago lanceolata L. A condition for the maintenance of gynodioecy in an obligatory outbreeding species like this is an increase in female fitness in male steriles compared with hermaphrodites. One of the possible underlying mechanisms, a lower cyanide-resistant respiration in male steriles, which could lead to a higher metabolic efficiency, was investigated. For the experiments adult plants were used, because the effects which compensate for male sterility have been found in characters like seed production and longevity. No general correlation between sex phenotype and cyanide-resistant respiration capacity, or with any other respiration component, was found. Only in a single cross a strong correlation between cyanide-resistant respiration activity and sex phenotype was established, male steriles possessing the higher activity. The conclusion from these experiments is that there is no pleiotropic relationship between respiration levels and sex phenotype. The strongly significant correlation mentioned is ascribed to chromosomal linkage. 相似文献
187.
Maintenance of low cl concentrations in mesophyll cells of leaf blades of barley seedlings exposed to salt stress
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The concentrations of vacuolar Na+ and Cl− in the epidermal and mesophyll cells of the leaf blade and sheath of Hordeum vulgare seedlings (cv California Mariout and Clipper) were measured by means of quantitative electron probe x-ray microanalysis. A preferential accumulation of Cl− in vacuoles of epidermal cells in both blade and sheath and a low level in mesophyll cells of the blade were evident in plants grown in full strength Johnson solution. The concentration of Cl− in the mesophyll cells of the blade remained at a low level after exposure to 50 or 100 millimolar NaCl for 1 day or to 50 millimolar for 4 days, while at the same time the concentration of Cl− in the epidermis and mesophyll of the sheath showed a dramatic increase. Clipper generally contained more Cl− in the mesophyll cells of the blade than California Mariout. A greater accumulation of Na+ in the mesophyll of the sheath relative to that of the blade was only apparent after treatment with 100 millimolar NaCl for 1 day or 50 millimolar for 4 days. These results confirm the suggestion that sheath tissue is capable of accumulating excess Cl− (and to a lesser extent Na+) and suggest that the site of regulation of Cl− concentration in the barley leaf is located in the mesophyll cells of the blade. 相似文献
188.
The phosphatases that hydrolyze fructose 2,6-bisphosphate in a crude spinach (Spinacia oleracea L.) leaf extract were separated by chromatography on blue Sepharose, into three fractions, referred to as phosphatases I, II, and III, which were further purified by various means. Phosphatase I hydrolyzed fructose 2,6-bisphosphate, with a Km value of 30 micromolar, to a mixture of fructose 2-phosphate (90%) and fructose 6-phosphate (10%). It acted on a wide range of substrates and had a maximal activity at acidic pH. Phosphatase II specifically recognized the osyl-link of phosphoric derivatives and had more affinity for the β-anomeric form. Its apparent Km for fructose 2,6-bisphosphate was 30 micromolar. It most likely corresponded to the fructose-2,6-bisphosphatase described by F. D. Macdonald, Q. Chou, and B. B. Buchanan ([1987] Plant Physiol 85: 13-16). Phosphatase III copurified with phosphofructokinase 2 and corresponded to the specific, low-Km (24 nanomolar) fructose-2,6-bisphosphatase purified and characterized by Y. Larondelle, E. Mertens, E. Van Schaftingen, and H. G. Hers ([1986] Eur J Biochem 161: 351-357). Three similar types of phosphatases were present in a crude extract of Jerusalem artichoke (Helianthus tuberosus) tuber. The concentration of fructose 2,6-bisphosphate decreased at a maximal rate of 30 picomoles per minute and per gram of fresh tissue in slices of Jerusalem artichoke tuber, upon incubation in 50 millimolar mannose. This rate could be accounted for by the maximal extractable activity of the low-Km fructose-2,6-bisphosphatase. A new enzymic method for the synthesis of β-glucose 1,6-bisphosphate from β-glucose 1-phosphate and ATP is described. 相似文献
189.
Modeling long-term crop response to fertilizer and soil nitrogen 总被引:1,自引:1,他引:0
A simple nitrogen balance model to calculate long-term changes in soil organic nitrogen, nitrogen uptake by the crop and recovery
of applied nitrogen, is presented. It functions with time intervals of one year or one growing season. In the model a labile
and a stable pool of soil organic nitrogen are distinguished. Transfer coefficients for the various inputs of nitrogen are
established that specify the fractions taken up by the crop, lost from the system, and incorporated in soil organic nitrogen.
It is shown how input data, model parameters and initial pool sizes can be derived and how the model can be used for calculating
long-term changes in total soil organic nitrogen and uptake by the crop. For nitrogen applied annually as fertilizer or organic
material the time course of nitrogen uptake and recovery of applied nitrogen is calculated. To test the sensitivity of the
model, calculations have been performed for different environmental conditions with higher or lower risks for losses. The
model has also been applied to establish fertilizer recommendations for a certain target nitrogen uptake by the crop. Finally,
for agricultural systems where periods of cropping alternate with peroids of green fallow the time course of nitrogen uptake
by the crop is calculated. 相似文献
190.
P D Nguyen E A O'Rear A E Johnson R Lu B M Fung 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1989,192(3):261-269
The clot-lysing ability of streptokinase (SK) was examined using membrane-bound thrombi. Encapsulation of SK in large unilamellar phospholipid vesicles (liposomes) resulted in entrapping approximately 30% of its original activity. Measurements of streptokinase activity for liposomal-encapsulated streptokinase (LESK) indicated little loss of activity or leakage in Tris-buffered saline over a 24-hr period at temperatures of 4 and 23 degrees C. However, incubation of free SK and LESK in platelet-poor plasma (PPP) at 37 degrees C resulted in a decrease of SK activity. The retention of SK activity in LESK was considerably higher than that of unentrapped SK. Clot-dissolving time (CDT) was measured by monitoring the pressure drop during slow filtration in plasma through membrane-bound thrombi. The results indicated that both LESK and free SK were able to activate the fibrinolytic system. Without prior incubation in PPP at 37 degrees C, the CDT of a SK and PPP mixture (SK/PPP) was 10.7 +/- 1.9 min (n = 12), while that of a LESK and PPP mixture (LESK/PPP) was 12.4 +/- 1.7 min (n = 12). The CDT-detected clot-lysing abilities of both SK and LESK were diminished by incubation in PPP, but to different extents. After 15- and 30-min incubations, the CDT of SK/PPP increased significantly to 15.5 +/- 1.5 and 24.1 +/- 2.4 min (n = 5, P less than 0.05), respectively. In contrast, the CDT of LESK/PPP increased to 13.3 +/- 0.8 min (n = 5) after 15 min of incubation and to 16.0 +/- 1.1 min (n = 5, P less than 0.05) after a 30-min incubation. These results suggest that entrapment of SK in liposomes preserves the thrombolytic potential of the plasminogen activator by limiting its exposure to the components of the plasma. 相似文献