全文获取类型
收费全文 | 22768篇 |
免费 | 2342篇 |
国内免费 | 6篇 |
专业分类
25116篇 |
出版年
2022年 | 234篇 |
2021年 | 448篇 |
2020年 | 242篇 |
2019年 | 305篇 |
2018年 | 415篇 |
2017年 | 350篇 |
2016年 | 573篇 |
2015年 | 1023篇 |
2014年 | 1058篇 |
2013年 | 1254篇 |
2012年 | 1589篇 |
2011年 | 1553篇 |
2010年 | 973篇 |
2009年 | 835篇 |
2008年 | 1225篇 |
2007年 | 1245篇 |
2006年 | 1169篇 |
2005年 | 1098篇 |
2004年 | 1020篇 |
2003年 | 978篇 |
2002年 | 875篇 |
2001年 | 280篇 |
2000年 | 242篇 |
1999年 | 276篇 |
1998年 | 322篇 |
1997年 | 235篇 |
1996年 | 218篇 |
1995年 | 200篇 |
1994年 | 208篇 |
1993年 | 212篇 |
1992年 | 253篇 |
1991年 | 199篇 |
1990年 | 186篇 |
1989年 | 181篇 |
1988年 | 181篇 |
1987年 | 177篇 |
1986年 | 147篇 |
1985年 | 190篇 |
1984年 | 208篇 |
1983年 | 180篇 |
1982年 | 191篇 |
1981年 | 175篇 |
1980年 | 166篇 |
1979年 | 135篇 |
1978年 | 108篇 |
1977年 | 110篇 |
1976年 | 119篇 |
1975年 | 103篇 |
1974年 | 129篇 |
1973年 | 104篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Homozygous osteogenesis imperfecta unlinked to collagen I genes 总被引:4,自引:1,他引:3
Katherine Aitchison Donald Ogilvie Mary Honeyman Elizabeth Thompson Bryan Sykes 《Human genetics》1988,78(3):233-236
Summary In a consanguineous pedigree in which a severe type of osteogenesis imperfecta was segregating as an autosomal recessive trait, analysis of genetic markers for both collagen I structural loci COL1A1 and COL1A2 showed that the phenotype was unlinked to either locus. 相似文献
102.
Marijo G. Kent Robert N. Shoffner Allan Hunter Keith O. Elliston Wesley Schroder Elizabeth Tolley Stephen S. Wachtel 《Human genetics》1988,79(4):321-328
Summary An inherited genetic disorder causes XY embryos of the horse to develop as mares. On the basis of our study of 38 such mares, we have identified four grades or classes of XY sex reversal according to this scheme: class I, nearly normal female, of which some are fertile; class II, female with gonadal dysgenesis, normal mullerian development; calss III, intersex mare with gonadal dysgenesis, abnormal mullerian development, enlarged clitoris; class IV, virilized intersex characterized by high levels of testosterone. In general, class I and calss II mares were typed H-Y antigen-negative whereas class III and class IV mares were typed H-Y antigen-positive. 相似文献
103.
Internal waves, primary production and the compensation depth of marine phytoplankton 总被引:2,自引:0,他引:2
Internal waves increase the average light intensity experiencedby phytoplankton and augment the compensation depth below whichno net photosynthesis occurs. These effects may be quite largein eutrophic waters with moderate or high light attenuationcoefficients. Data on internal waves and light attenuation canbe used to correct standard estimates of (new) primary productionin the lower euphotic zone based on uptake rates of carbon ornitrogen isotopes. 相似文献
104.
Elizabeth B. Andrews Peter M. Taylor 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1988,158(2):247-262
Summary The heart, main blood vessels, and associated structures ofLittorina littorea were examined by scanning and transmission electron microscopy. The auricle is subdivided into two compartments, one receiving blood from the gill and opening to the nephridial gland vein, the other connecting with the latter anteriorly and the ventricle posteriorly.Video recordings were made of the beating heart in vivo and revealed that the auricle expelled blood not only to the ventricle, but also the nephridial gland vein at systole and provided further evidence of tidal flow of blood in the vein. There is clear indication that the constant volume mechanism of auricular re-filling is not strictly true inLittorina.Blood pressure in the heart and major vessels was measured using a servo-nulling micropressure system. The rate of formation of urine (derived by filtration of blood through the auricular wall) was measured using [51Cr] EDTA as a blood marker.Basal blood pressure was slightly above ambient (0.7 cm H2O). Peak systolic pressure in the ventricle (3.8 cm H2O) was synchronised with a subambient trough in pericardial pressure (–1.0 cm H2O); these pressure pulses were out of phase with that of the auricle (2.3 cm H2O) at systole. The observations are consistent in broad terms with a constant volume mechanism, but this does not take into account urine formation or filling of the nephridial gland vein.A filtration pressure of 1.5 cm H2O has been demonstrated across the auricular wall throughout the cardiac cycle. Colloidal back pressure appears to be negligible. The mean rate of urine formation is 0.26 l g–1 min–1. 相似文献
105.
The host suitability of five of the most common weed species occurring in maize (Zea mays L.) fields in South Africa to Pratylenchus zeae was tested. Based on the number of nematodes per root unit, mealie crotalaria (Crotalaria sphaerocarpa) was a good host; goose grass (Eleusine indica), common pigweed (Amaranthus hybridus), and thorn apple (Datura stramonium) were moderate hosts; and khaki weed (Tagetes minuta) was a poor host. Only the root residues of khaki weed suppressed the P. zeae infestation of subsequently grown maize. When goose grass, khaki weed, and mealie crotalaria were grown in association with maize in soil infested with P. zeae, goose grass and khaki weed severely suppressed maize root development; this resulted in a low number of nematodes per maize root system and a high number of nematodes per maize root unit. Mealie crotalaria did not restrict maize root growth and did not affect nematode densities per maize root system or maize root unit. Special attention should be given to the control of mealie crotalaria, which is a good host for P. zeae, and goose grass, which, in addition to its ability to compete with maize, is also a suitable host for P. zeae. 相似文献
106.
Several prototype macrophage (MO) populations were compared for differences in ectoenzyme phenotype and polyamine content. Resident peritoneal MO and Corynebacterium parvum (CP)-activated peritoneal MO expressed unique ectoenzyme phenotypes, while bone marrow derived MO (BMDMO), obtained from stem cells after 7 days in culture with colony stimulating factor, and thioglycollate (TG)-elicited peritoneal MO exhibited a similar ectoenzyme phenotype. All of the MO populations, however, differed in polyamine accumulation patterns. These results suggest that ectoenzyme phenotypes do not serve as completely selective markers of MO differentiation. Moreover, BMDMO do not resemble steady state tissue peritoneal MO but appear to resemble inflammatory MO in several respects. Therefore activated BMDMO do not appear to provide an accurate model system for their continued use in studies to characterize the development of resident tissue MO. 相似文献
107.
Christine L. Truitt Elizabeth A. Weaver William G. Haldenwang 《Molecular & general genetics : MGG》1988,212(1):166-171
Summary The E-37 gene ctc was inactivated by a site-specific insertion into the Bacillus subtilis chromosome. The resulting mutation inhibited sporulation by 95% at elevated temperatures (48° C). If the ctc
- mutation is placed in a strain that carries a mutation in the closely linked but distinct spoVC gene, ctc now affects both growth and sporulation at elevated temperatures. Growth of the ctc
- spoVC285 strain was transiently inhibited when exponentially growing cultures were shifted from 37° C to 48° C. A similar, but less pronounced growth lag, was also seen in a B. subtilis strain carrying only the spoVC-285 mutation. This finding suggests that both the ctc and spoVC products function in vegetatively growing B. subtilis. 相似文献
108.
Summary The European starling Sturnus vulgaris preferentially incorporates fresh sprigs of particular plant species for use as nesting material. Chemicals found in these plants may act to reduce pathogen and ectoparasite populations normally found in nest environments. The present experiments were performed to test this Nest Protection Hypothesis. In the fild, we experimentally determined that wild carrot Daucus carota, a plant species preferred as nest material, effectively reduced the number of hematophagous mites found within nests relative to control nests without green vegetation. Chicks from nests containing wild carrot had higher levels of blood hemoglobin than chicks from control nests. However, there were no differences in weight or feather development. In the laboratory, we found that wild carrot and fleabane, Erigeron philadelphicus, (also preferred by starlings as nest material) substantially reduced the emergence of feeding instars of mites, while garlic mustard, Alliaria officinalis, (commonly available but not preferred) had little effect on the emergence of mites. We infer that preferred plant material may act to inhibit feeding or otherwise delay reproduction of mites, thereby reducing risk of anemia to developing nestlings. 相似文献
109.
Inhibitor-induced enzyme activation in organic solvents 总被引:8,自引:0,他引:8
The enzymatic activity of the protease subtilisin in anhydrous organic solvents can be dramatically increased by pretreating the enzyme before it is placed in the nonaqueous medium. For instance, lyophilization of subtilisin from aqueous solution containing competitive inhibitors (followed by their removal) created an enzyme which was up to 100 times more active than the enzyme lyophilized in the absence of such ligands. This phenomenon of ligand-induced "enzyme memory" also extends to the stability, affinity, and substrate specificity of subtilisin in organic solvents. 相似文献
110.
The joint probability distribution of the number of distinct (not identical by descent) genes from each founder of the Equus przewalskii population that survive in the five horses of the Calgary Zoological Gardens breeding group has been calculated. The dependence structure of this distribution is investigated, and informative marginal distributions are given, among them the distributions of the genetic contributions of each founder to the Calgary horses and the distribution of wild-type genes in these horses. The dependence pattern is found to be complex; there is no substitute for exact calculation of the full joint probability distribution of numbers of surviving genes. Probabilities of gene survival give a more complete summary of the genetic structure of a set of individuals than is provided by more routine measures such as heterozygosity or founder contributions. The feasibility of computing these probabilities for small groups of current individuals descended from few founders via long and complex pedigrees, provides a new approach to assessing such groups, and could be used also in selecting animals to form the founder stock of propagules for future reintroduction programs. 相似文献