全文获取类型
收费全文 | 1408篇 |
免费 | 127篇 |
国内免费 | 1篇 |
学科分类
生物科学 | 1536篇 |
出版年
2022年 | 22篇 |
2021年 | 36篇 |
2020年 | 27篇 |
2019年 | 16篇 |
2018年 | 23篇 |
2017年 | 14篇 |
2016年 | 34篇 |
2015年 | 65篇 |
2014年 | 74篇 |
2013年 | 92篇 |
2012年 | 102篇 |
2011年 | 99篇 |
2010年 | 78篇 |
2009年 | 47篇 |
2008年 | 65篇 |
2007年 | 73篇 |
2006年 | 76篇 |
2005年 | 67篇 |
2004年 | 74篇 |
2003年 | 60篇 |
2002年 | 64篇 |
2001年 | 16篇 |
2000年 | 15篇 |
1999年 | 16篇 |
1998年 | 18篇 |
1997年 | 12篇 |
1996年 | 7篇 |
1995年 | 13篇 |
1994年 | 7篇 |
1993年 | 10篇 |
1992年 | 14篇 |
1991年 | 13篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 9篇 |
1987年 | 17篇 |
1986年 | 12篇 |
1985年 | 6篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 9篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 10篇 |
1974年 | 5篇 |
1973年 | 7篇 |
1971年 | 6篇 |
1969年 | 5篇 |
排序方式: 共有1536条查询结果,搜索用时 0 毫秒
1.
Contrary to our expectations, soil salinity and moisture explained little of the spatial variation in plant establishment in the upper intertidal marsh of three southern California wetlands, but did explain the timing of germination. Seedlings of 27 species were identified in 1996 and 1997. The seedlings were abundant (maximum densities of 2143/m2 in 1996 and 1819/m2 in 1997) and predominantly annual species. CCAs quantified the spatial variation in seedling density that could be explained by three groups of predictor variables: (1) perennial plant cover, elevation and soil texture (16% of variation), (2) wetland identity (14% of variation) and (3) surface soil salinity and moisture (2% of variation). Increasing the spatial scale of analysis changed the variables that best predicted patterns of species densities. Timing of germination depended on surface soil salinity and, to a lesser extent, soil moisture. Germination occurred after salinity had dropped below a threshold or, in some cases, after moisture had increased above a critical level. Between 32% and 92% of the seedlings were exotic and most of these occurred at lower soil salinity than native species. However, Parapholis incurva and Mesembryanthemum nodiflorum were found in the same environments as the native species. In 1997, the year of a strong El Niño/Southern Oscillation event with high rainfall and sea levels, the elevation distribution of species narrowed and densities of P. incurva and other exotic species decreased but densities of native and rare species did not change. The ‘regeneration niche’ of wetland plant communities includes the effects of multiple abiotic and biotic factors on both the spatial and temporal variations in plant establishment. 相似文献
2.
3.
V. I. Joy Royes 《Grana》2013,52(2):151-157
A knowledge of the pollen and fungal spores which comprise the air spora is useful as a preliminary approach to the problem of respiratory allergy. Therefore, this study of the qualitative and quantitative aspects of the air spora was done. Fungal spores were found to be numerically dominant, comprising 97.73% whilst pollen comprised 0.40% of the total material observed. A small number of types made up the majority of the fungal air spora, namely, Cladosporium, the Sporobolomycetaceae group, Diatrype, Glomerella, hyaline and coloured basidiospores, and septate fusiform spores. Seasonal periodicity studies on twenty-five fungal types showed that a high number of spores were trapped for sixteen during wet months, four during cooler months, and that five showed no seasonal trends. Mean diurnal periodicity studies for the year on the same twenty-five spore types showed that all had a maximum number of spores trapped at some time during the day. Investigation of the effect of rainfall on the numbers of spores released showed that the amount and duration of rainfall, the time of day rain occurs, and the length of the dry period preceding rain were of varying importance to particular spore types. 相似文献
4.
Lee D. Major Thomas S. Partridge Joy Gardner Stephen J. Kent Robert de Rose Andreas Suhrbier Wayne A. Schroder 《PloS one》2013,8(2)
SerpinB2, also known as plasminogen activator inhibitor type 2, is a major product of activated monocytes/macrophages and is often strongly induced during infection and inflammation; however, its physiological function remains somewhat elusive. Herein we show that SerpinB2 is induced in peripheral blood mononuclear cells following infection of pigtail macaques with CCR5-utilizing (macrophage-tropic) SIVmac239, but not the rapidly pathogenic CXCR4-utilizing (T cell-tropic) SHIVmn229. To investigate the role of SerpinB2 in lentiviral infections, SerpinB2−/− mice were infected with EcoHIV, a chimeric HIV in which HIV gp120 has been replaced with gp80 from ecotropic murine leukemia virus. EcoHIV infected SerpinB2−/− mice produced significantly lower anti-gag IgG1 antibody titres than infected SerpinB2+/+ mice, and showed slightly delayed clearance of EcoHIV. Analyses of published microarray studies showed significantly higher levels of SerpinB2 mRNA in monocytes from HIV-1 infected patients when compared with uninfected controls, as well as a significant negative correlation between SerpinB2 and T-bet mRNA levels in peripheral blood mononuclear cells. These data illustrate that SerpinB2 can be induced by lentiviral infection in vivo and support the emerging notion that a physiological role of SerpinB2 is modulation of Th1/Th2 responses. 相似文献
5.
Joy L. Ware Andrew P. Lieberman Karen S. Webb 《Cancer immunology, immunotherapy : CII》1986,21(1):58-62
Summary The human prostate tumor subline 1-LN-PC-3-1A (1-LN) is reproducibly metastatic in adult athymic nude mice. Cells surviving a brief in vitro exposure to ethyl methanesulfonate (EMS) exhibited a profound decrease in capacity for experimental lung metastasis in nude mice. Thirty days after EMS treatment, 1×106 uncloned EMS-treated 1-LN cells (1-LN-EMS-10) were injected IV into groups of 6 to 8-week-old male athymic nude mice (BALB/cAnBOM). A median of 8.5 colonies/lung was observed among 20 1-LN-EMS-10-injected mice, which was significantly different from the median of 51 colonies/lung produced among 14 1-LN-injected mice (P=0.0002). This altered phenotype remained stable during 150 days of continuous culture. However, the 1-LN-EMS-10 cells were tumorigenic in 10/10 nude mice injected SC. Single lung tumor colonies recovered from 1-LN-EMS-10-injected mice and reinjected IV into nude mice produced medians of 32–63 colonies/lung. The altered metastatic phenotype resulting from treatment of 1-LN with EMS was reversed by exposure to a noncytotoxic dose of 5-azacytidine, but unaffected by a second exposure to EMS. Collectively these data demonstrate that the metastatic phenotype of these human tumor cells in athymic nude mice can be heritably altered by in vitro exposure to EMS and 5-azacytidine. Analysis of the mechanisms underlying these phenotypic changes may provide insight into parts of the complex process of tumor cell evolution. 相似文献
6.
Subcellular Localization of Asparaginase and Asparagine Aminotransferase in Pisum sativum Leaves 总被引:1,自引:1,他引:0 下载免费PDF全文
Protoplasts isolated from young and mature pea leaves (Pisum sativum L.) were broken and their contents fractionated by differential centrifugation or on sucrose-density gradients. Asparaginase was found only in the cytosol of young leaves. Asparagine aminotransferase was found in both young and mature leaves and was localized exclusively in the peroxisome. This corroborates the observation that asparagine transamination is catalyzed by the serine:glyoxylate aminotransferase. 相似文献
7.
The use of a cotton gauze swab and subsequent culture of the swab was found to be a more sensitive method for isolating Salmonella from liquid milk than the revised procedure of North. The swab method was found to be as sensitive as the North procedure for recovering Salmonella when incubated at 37 C but more sensitive when incubated at 43 C. Incubation of the swab cultures at the elevated temperature of 43 C gave good results when Salmonella was present at levels as low as one per liter. Swabs exposed to milk contaminated with 100 Salmonella per liter remained positive even when subsequently washed for 2 hr in noncontaminated milk. Bismuth sulfite agar and Brilliant Green sulfadiazine agar were equally effective for isolating Salmonella from broth cultures; use of both media resulted in maximal isolations. 相似文献
8.
A. W. Asscher M. Sussman W. E. Waters Joy A. S. Evans H. Campbell K. T. Evans J. Edmund Williams 《BMJ (Clinical research ed.)》1969,1(5647):804-806
Short courses of nitrofurantoin and ampicillin produced an immediate cure in 80% of adult non-pregnant bacteriuric women. Of the subjects so treated, 55% remained cured at the end of one year. Over the same follow-up period 36% of untreated bacteriuric women developed a spontaneous remission of bacteriuria. Treatment failed to prevent the development of symptomatic infection, and the reinfections which followed successful treatment were more commonly associated with the development of symptoms than the persistent or relapsing infections in untreated or unsuccessfully treated subjects.It is concluded that a search for bacteriuria in non-pregnant women is unlikely to be of value as a preventive measure, since in many instances it fails to detect urinary tract infection at an early stage and since treatment by methods suitable for large-scale use is ineffective. 相似文献
9.
10.
Synopsis The reproductive biology of the coral reef butterflyfish,Chaetodon multicinctus, was investigated by histological examination of gonads sampled over an 18 month period from a shallow inshore population
on Oahu, Hawaii. Most gonads developed directly from previously undifferentiated tissue. Ovarian development (the structural
formation of lamellae and primary oocytes) was observed in fish ≥44 mm and testicular development (the formation of spermatogenic
crypts) in fish ≥62 mm standard length (SL). In addition, testis formation was identified within the ovarian lamellae of several
differentiated but immature fish. It is hypothesized that prematurational sex change may facilitate monogamy within the highly
competitive social structure of this site attached species. Oocyte development in mature females was marked by distinct phases
of primary growth, the formation of yolk vesicles, and vitellogenesis. Spawning activity was histologically identified by
the maturation and hydration of fully yolked oocytes, and presence of postovulatory follicles. Recently spawned females from
field collections and experimental gonadotropin-treatments exhibited postovulatory follicles that were estimated to persist
at least 24 h after ovulation. Atresia of yolked oocytes was classified into four stages of cell degeneration and resorption.
Monthly analyses of oocyte development and atresia within the sample population show thatC. multicinctus has a protracted annual spawning season with a major peak during the early spring and evidence of spawning activity among
some individuals in the fall. Histological analyses of spawning activity provide more accurate and unambiguous information
than do traditional gonadosomatic assays in this and probably other coral reef fishes. 相似文献