全文获取类型
收费全文 | 9542篇 |
免费 | 933篇 |
国内免费 | 1篇 |
专业分类
10476篇 |
出版年
2023年 | 58篇 |
2022年 | 144篇 |
2021年 | 275篇 |
2020年 | 154篇 |
2019年 | 203篇 |
2018年 | 236篇 |
2017年 | 177篇 |
2016年 | 303篇 |
2015年 | 473篇 |
2014年 | 566篇 |
2013年 | 552篇 |
2012年 | 765篇 |
2011年 | 754篇 |
2010年 | 449篇 |
2009年 | 353篇 |
2008年 | 523篇 |
2007年 | 472篇 |
2006年 | 446篇 |
2005年 | 344篇 |
2004年 | 336篇 |
2003年 | 308篇 |
2002年 | 260篇 |
2001年 | 158篇 |
2000年 | 138篇 |
1999年 | 120篇 |
1998年 | 77篇 |
1997年 | 56篇 |
1996年 | 44篇 |
1995年 | 45篇 |
1994年 | 61篇 |
1993年 | 60篇 |
1992年 | 97篇 |
1991年 | 67篇 |
1990年 | 68篇 |
1989年 | 89篇 |
1988年 | 58篇 |
1987年 | 95篇 |
1986年 | 86篇 |
1985年 | 81篇 |
1984年 | 65篇 |
1983年 | 67篇 |
1982年 | 54篇 |
1981年 | 45篇 |
1980年 | 55篇 |
1979年 | 70篇 |
1976年 | 39篇 |
1975年 | 45篇 |
1974年 | 47篇 |
1973年 | 38篇 |
1972年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
93.
94.
The distribution pattern of silver-NORs was studied in cells of six-day blastocysts and kidney fibroblasts of the rabbit using
the Ag-AS technique. At metaphase and interphase there was a binomial distribution of the number of stained sites in both
populations but blastocysts had a greater percentage of cells with larger numbers of stained sites. Up to 7 of the 8 chromosomes
known to bear NORs were stained in cells from blastocysts while a maximum of 6 were stained in fibroblasts. A significant
difference was found between the mean numbers of chromosomal NORs per cell in metaphases from blastocysts and fibroblasts,
where they were 4.2 and 3.3 respectively. Similarly, the mean number of NORs in interphase was significantly greater in cells
from blastocysts. The distribution of staining on chromosome pair 13 was related to cell type. Significantly more cells in
blastocysts than fibroblasts showed staining in this chromosome pair. 相似文献
95.
T. E. Devine C. D. Foy A. L. Fleming C. H. Hanson T. A. Campbell J. E. McMurtrey III J. W. Schwartz 《Plant and Soil》1976,44(1):73-79
Summary Aluminum toxicity limits root growth in acid subsoils that are difficult to lime. An alternative to subsoil liming is the development of plants having greater tolerance to Al. Alfalfa (Medicago sativa L.) is considered an Al-susceptible species. Preliminary studies indicated that alfalfa cultivars differ in Al tolerance, but the extreme plant-to-plant variation within cultivars prevented the establishment of clearcut cultivar differences.Tolerant and susceptible plants were selected from each of six cultivars (DuPuits, Atlantic, Team, Buffalo, Grimm, and Sirsa 9) grown on an Al-toxic Bladen soil at pH 4.1 to 4.3. The tolerant selections were repotted and interpollinated to form one population of polycross seed. Susceptible selections were treated similarly to form a second population. These two populations, tolerant and susceptible, were subjected to an additional cycle of recurrent phenotypic selection for tolerance and susceptibility, respectively, to Al-toxic Bladen soil at pH 4.6.Plants from the population selected for tolerance to the acid Bladen soil were significantly higher in both root and top vigor on Al-toxic Tatum soil than plants from the population selected for susceptibility. The results indicated that Al tolerance is a heritable trait in these alfalfa populations and that recurrent selection can be used effectively to develop strains having differential tolerance to Al-toxic soils. The observation that only 2% of the plants from the tolerant population were in the most tolerant class suggests a good opportunity for more progress in selecting toward Al tolerance. 相似文献
96.
Careful investigation of the influence of palmitic and lauric acid on the interaction of progesterone and testosterone with several batches of untreated and defatted bovine and human serum albumins have revealed that, by contrast with published data for studies with progesterone as well as nonsteroid ligands, there is a surprising stimulation rather than inhibition of binding, albeit with a reduction of the apparent number of binding sites in almost all instances. Furthermore, fatty acid tends to minimize or eliminate the well-known differences in affinity between bovine and human albumin for interactions with these two steroids. The values for binding affinity in the interaction of testosterone with these batches of human serum albumin are significantly higher than those previously published by us and other authors and the value for progesterone-bovine albumin interaction is not in accordance with the "polarity rule". Studies of these same interactions by ultraviolet difference spectroscopy give further evidence of the augmentation in binding but, in the case of defatted bovine albumin only, the aromatic difference troughs are indicative of tyrosine perturbation whereas refatted bovine albumin, defatted and refatted human albumin manifest tryptophan perturbation. Quantitative correlation of perturbation with level of bound steroid suggests that fatty acid alters the ratio (possibly hydrogen-bonded to non hydrogen-bonded) of two forms of bound steroid. There is also further evidence that the binding sites for testosterone and progesterone are not identical. 相似文献
97.
Extensive data on 102 patients who presented with rheumatoid disease within a year of onset were gathered by a prospective study to assess the prognostic value of early features. Outcome was evaluated at a mean 4-5 years from onset on the basis of functional grade, extent of joint disease, early morning stiffness, and grip strength. Twenty-six patients improved, 14 pursued a mild steady course, and 62 had a persistently severe or deteriorating condition. The features recorded at the first visit were correlated with outcome. Those indicating a poor prognosis were: older age at onset, being underweight, poor grip strength, many affected joints, involvement of wrist or metatarsophalangeal joints, poor functional status, fulfilment of many of the American Rheumatism Association criteria for rheumatoid disease, raised erythrocyte sedimentation rate, seropositivity on sheep cell agglutination or latex tests, low haemoglobin level, raised blood urea level, and early erosions on x-ray films. 相似文献
98.
There is increasing evidence that type II alveolar cells are capable of synthesizing surface active material like that obtained from the airways. However a number of problems remain to be solved before it can be stated conclusively that type II cells synthesize the surface active material of the terminal airspace. Among these problems is that of secretion. A number of previous studies have given evidence of the release of lamellar bodies by merocrine secretion. In this study morphologic evidence is presented which supports the view that secretion of lamellar bodies is accomplished by exocytosis. At the apical surface of type II cells, sites can be found where the limiting membrane of the lamellar body is clearly fused with the type II cell plasma membrane and an open channel exists between the contents of the lamellar body and the alveolar space. At these sites the lamellar contents extrude into the airspace with consequent loss of the highly compact organization of intracellular lamellar bodies. The intactness and continuity of the membranes can be traced for the full extent of the exocytosis site. Freeze-etch replicas of the membranes of type II cells show depressions which may represent the sites of discharged lamellae. In addition, tongue-like folds are seen which could be explained as the extensions of cytoplasm which surround the releasing lamellar body and which may flap over the exocytosis pit after discharge. Micrographs of the alveolar space show disorganized lamellar whorls which appear to be unravelling to produce tubular myelin. In view of the unusually large size and lipid composition of lamellar bodies, a mechanism involving hydration of mucopolysaccharide contents as an aid to expulsion of lamellar contents is suggested. 相似文献
99.
100.
William G. Ambrose Jr. Paul E. Renaud William L. Locke V Finlo R. Cottier J?rgen Berge Michael L. Carroll Benjamin Levin Stuart Ryan 《Polar Biology》2012,35(3):345-354
Growth patterns of two common circumpolar bivalves, the Greenland cockle (Serripes groenlandicus), and the hairy cockle (Clinocardium ciliatum) have been used in previous studies to reconstruct environmental conditions in the arctic. To date, there has been no direct
determination that growth lines in either species are deposited periodically, and there has been no examination of factors
affecting growth. We placed calcein-marked individuals of both species on oceanographic moorings in two fjords (Rijpfjord
and Kongsfjord) in the Svalbard archipelago for one and two (Kongsfjord only) years. Growth patterns were compared with concurrent
in situ temperature and fluorescence data in order to assess environmental controls on growth. Dark growth lines are evident
on the outer shell surface and internally in shell cross section in both S. groenlandicus and C. ciliatum, and both species deposited only one line per year, unequivocally confirming that internal lines are deposited annually. Growth
line deposition in both species began in late summer to early fall, before the seasonal decline in temperature. There was
no difference in growth of S. groenlandicus between the two fjords despite differences in water temperature (3°C), fluorescence (nearly threefold) and the onset and
duration of the winter season. C. ciliatum, however, grew approximately 2.8 times faster in the warmer, more food-rich Kongsfjord than in Rijpfjord. Subannual lines
were counted in two individuals of each species from each fjord, but deposition of these lines was not clearly related to
number of growing days estimated by temperature and fluorescence. 相似文献