全文获取类型
收费全文 | 4514篇 |
免费 | 231篇 |
国内免费 | 342篇 |
出版年
2023年 | 30篇 |
2022年 | 81篇 |
2021年 | 84篇 |
2020年 | 104篇 |
2019年 | 149篇 |
2018年 | 123篇 |
2017年 | 146篇 |
2016年 | 191篇 |
2015年 | 161篇 |
2014年 | 205篇 |
2013年 | 305篇 |
2012年 | 147篇 |
2011年 | 269篇 |
2010年 | 160篇 |
2009年 | 314篇 |
2008年 | 331篇 |
2007年 | 268篇 |
2006年 | 260篇 |
2005年 | 215篇 |
2004年 | 186篇 |
2003年 | 184篇 |
2002年 | 129篇 |
2001年 | 84篇 |
2000年 | 79篇 |
1999年 | 101篇 |
1998年 | 75篇 |
1997年 | 72篇 |
1996年 | 78篇 |
1995年 | 58篇 |
1994年 | 39篇 |
1993年 | 52篇 |
1992年 | 34篇 |
1991年 | 32篇 |
1990年 | 36篇 |
1989年 | 20篇 |
1988年 | 25篇 |
1987年 | 22篇 |
1986年 | 10篇 |
1985年 | 22篇 |
1984年 | 20篇 |
1983年 | 17篇 |
1982年 | 29篇 |
1981年 | 23篇 |
1980年 | 27篇 |
1979年 | 20篇 |
1978年 | 14篇 |
1977年 | 10篇 |
1976年 | 14篇 |
1975年 | 5篇 |
1973年 | 17篇 |
排序方式: 共有5087条查询结果,搜索用时 15 毫秒
121.
亚磁空间生物学效应研究的实验系统 总被引:1,自引:0,他引:1
直径2 m的补偿式亚磁空间中,内置非铁磁性智能化多功能实验箱系统,箱内温控范围20~40℃,精度0.1℃,过温报警,湿度可控范围40%~80%,通风和光照任意。箱内中央空间(长×宽×高=66.0 cm ×40.0 cm ×28.3 cm)中,80%、15%和5%的位点剩余磁场分别平均为地磁场的0.5%~0.6%、1.3%~1.5%和2.7%~4.2%。箱内中央空间的高度为43.0 cm时,55%、35%和10%的位点剩余磁场分别平均为地磁场的1.9%、2.3%和3.3%。可用于多种生物学效应的观察和研究。 相似文献
122.
通过对典型农业生态工程结构的研究,分析了生态农业系统的运行机制.结果表明,结构多样性与主导性关系的协调问题,是目前制约农业生态系统良性循环的根本问题.山东省西单村农业生态系统之所以前期能够保持持续稳定发展,后期出现减缓趋势,主要原因也是由于其系统的结构多样性比较高,结构多样性指数最高达0.6556,结构链内部各组分之间的协调性超过90%;而系统实际主导性则较低,结构主导性指数最高为0.2574,系统的最高协调性指数只有0.1327.因此,要保证系统的可持续发展,必须保证系统结构的协词性. 相似文献
123.
Gustavo Baeza Larios John Sivinski Tim Holler Martin Aluja 《Biocontrol Science and Technology》2002,12(2):205-215
Suppression of Mediterranean fruit fly (medfly), Ceratitis capitata (Wiedemann), populations may be achieved through the mass-rearing and augmentative aerial release of opiine braconid parasitoids. Typically, aerial release techniques require up to one hour of chilling of adult parasitoids at temperatures as low as 3.5°C prior to their dissemination. Such chilling potentially could affect the subsequent performance of the insects. Among three species of the genus Diachasmimorpha longicaudata (Ashmead), tryoni (Cameron), and krausii (Fullaway) there was little or no affect of chilling in the laboratory on female longevity, production of daughters, or offspring sex ratio. This is consistent with previous experiments that found chilling to have no discernable effect on the short-term mortality of D. tryoni or on its ability to take flight immediately after aerial release. While there was little effect of chilling on longevity and fecundity in a species from another opiine genus, Fopius arisanus (Sonan), exposure to low temperatures did result in a significantly more male-biased offspring sex ratio. 相似文献
124.
《Saudi Journal of Biological Sciences》2022,29(5):3326-3337
COVID-19 (coronavirus disease-2019) is a contagious illness that has been declared a global epidemic by the World Health Organization (WHO). The coronavirus causes diseases ranging in severity from the common cold to severe respiratory diseases and death. Coronavirus primarily affects blood pressure by attaching to the angiotensin converting enzyme 2 (ACE 2) receptor. This virus has an impact on multiple organ systems, including the central nervous system, immune system, cardiovascular system, peripheral nervous system, gastrointestinal tract, endocrine system, urinary system, skin, and pregnancy. For the prevention of COVID-19, various vaccines such as viral-like particle vaccines, entire inactivated virus vaccines, viral vector vaccines, live attenuated virus vaccines, subunit vaccines, RNA vaccines, and DNA vaccines are now available. Some of the COVID-19 vaccines are reported to cause a variety of adverse effects that range from mild to severe in nature. SARS-CoV-2 replication is controlled by the RNA-Dependent RNA-Polymerase enzyme (RdRp). The availability of FDA-approved anti-RdRp drugs (Ribavirin, Remdesivir, Sofosbuvir, Galidesivir, and Tenofovir) as potent drugs against SARS-CoV-2 that tightly bind to its RdRp may aid in the treatment of patients and reduce the risk of the mysterious new form of COVID-19 viral infection. RdRp inhibitors, such as remdesivir (an anti-Ebola virus experimental drug) and favipiravir (an anti-influenza drug), inhibit RdRp and thus slow the progression of COVID-19 and associated clinical symptoms, as well as significantly shorten recovery time. Molnupiravir, an orally active RdRp inhibitor and noval broad spectrum antiviral agent, is an isopropyl pro-drug of EIDD-1931 for emergency use. Galidesivir's in vitro and in vivo activities are limited to RNA of human public health concern. Top seeds for antiviral treatments with high potential to combat the SARS-CoV-2 strain include guanosine derivatives (IDX-184), setrobuvir, and YAK. The goal of this review is to compile scattered information on available COVID-19 vaccines and other treatments for protecting the human body from their harmful effects and to provide options for making better choices in a timely manner. 相似文献
125.
Nitrogen removal from secondary effluent by using integrated constructed wetland system 总被引:4,自引:0,他引:4
The treatment capacity of an integrated constructed wetland system (CWS) that was designed to reduce nitrogen (N) from secondary effluent was explored. The integrated CWS consisted of vertical-flow constructed wetland, floating bed and sand filter. The vertical-flow wetland was filled with gravel, steel slag and peat from the bottom to the top. Vetiver zizanioides was selected to grow in the vertical-flow constructed wetland and Coix lacrymajobi L. was grown in the floating bed. The results showed that the integrated CWS displayed superior removal efficiency for nitrate nitrogen (NO3−-N), ammonia nitrogen (NH4+-N), nitrite nitrogen (NO2−-N), and total nitrogen (TN). The average NO3−-N, NO2−-N, NH4+-N and TN removal efficiencies of the integrated CWS were 98.83%, 95.60%, 98.05% and 92.41%, respectively, during the whole experimental operation. The integrated CWS may have a good potential for removing N from secondary effluent. 相似文献
126.
Biochar derived from agricultural biomass waste is increasingly recognized as a multifunctional material for agricultural and environmental applications. Three novel magnetic biochars (MOP250, MOP400, MOP700) were prepared by chemical co-precipitation of Fe3+/Fe2+ on orange peel powder and subsequently pyrolyzing under different temperatures (250, 400 and 700 °C), which resulted in iron oxide magnetite formation and biochar preparation in one-step. The MOP400 was comprised of nano-size magnetite particles and amorphous biochar, and thus exhibited hybrid sorption capability to efficiently remove organic pollutants and phosphate from water. For organic pollutants, MOP400 demonstrated the highest sorption capability, and even much larger than the companion non-magnetic biochar (OP400). For phosphate, magnetic biochars, especially MOP250, demonstrated much higher sorption capability than the companion non-magnetic biochars. No significantly competitive effect between organic pollutant and phosphate was observed. These suggest that the magnetic biochar is a potential sorbent to remove organic contaminants and phosphate simultaneously from wastewater. 相似文献
127.
Six natural media were examined for growth and sporulation of six isolates of the nematophagous fungus Hirsutella rhossiliensis , using solid and/or liquid culture. Twenty carbohydrates, 19 nitrogen (N) compounds, and nine vitamins were also tested for their effects on growth, sporulation, and spore germination of a further three isolates (ATCC46487, OWVT-1 and JA16-1). Variations in nutritional requirements existed among the fungal isolates. In general, V-8 juice agar (VA), cornmeal agar and potato dextrose agar were good media for growth, and malt extract agar, VA and yeast dextrose agar were good for sporulation of all six isolates. Glycogen was the best and sucrose, inulin, D- ( + ) - trehalose and soluble starch were also good carbon (C) sources for growth and spore germination of the three isolates ATCC46487, OWVT-1 and JA16-1 in both liquid and solid culture. None of the isolates utilized D- ( + )xylose as a C source. L- sorbose, D- ribose, citric acid and D- fructose were poor for growth of all isolates. The best C source for sporulation was D- ( + )-trehalose for ATCC46487, D- sorbitol for OWVT-1 and D- ( + )-cellobiose for JA16-1. Casein was the best N source for growth of ATCC46487 and OWVT-1, while peptone was best for JA16-1. L- asparagine, L- proline, and peptone were also good for growth of all three isolates. L - cystine was not utilized by H. rhossiliensis and DL- methionine inhibited growth of all isolates. Spore germination of all isolates was well supported by most N compounds examined but was inhibited by L- cystine. No significant difference in sporulation of ATCC46487 was observed among the N sources. DL- threonine was the best N source for spore production by OWVT-1 and L- phenylalanine was best for JA16-1. Vitamins generally enhanced fungal growth and sporulation, with thiamine having the greatest influence. Excluding some vitamins individually from the medium containing all other test vitamins sometimes increased growth and/or sporulation of certain isolates. 相似文献
128.
酶法制备海洋活性肽及其功能活性研究进展 总被引:1,自引:0,他引:1
海洋生物活性肽(Marine biological active peptide)是从海洋生物中提取的具有优化机体代谢环境、有益于机体健康的一类多肽。酶法制备海洋生物活性肽是目前最常用的制备方法,是通过适当的蛋白酶水解海洋生物蛋白来制备生物活性肽的一种方法。海洋生物活性肽在降血压、抗氧化、抗凝血及抗菌等方面效果显著,对治疗和预防疾病具有巨大潜力。介绍海洋生物活性肽在酶解制备及其生物学功能方面国内外研究进展,为进一步开展海洋活性多肽研究提供参考。 相似文献
129.
Increased isolation of two Biosphere Reserves and surrounding protected areas (WAP ecological complex, West Africa) 总被引:1,自引:0,他引:1
Nicola Clerici Antonio Bodini Hugh Eva Jean-Marie Grgoire Dominique Dulieu Carlo Paolini 《Journal for Nature Conservation》2007,15(1):26-40
Protected areas such as nature reserves have been found to be effective in preventing habitat destruction and protecting ecosystems within their borders. Recent studies however found extensive loss of tropical forest habitat around protected areas, vastly contributing to increase the levels of ecological isolation. Using high-resolution satellite data we investigated the isolation trend occurring in the W-Arly-Pendjari (WAP) ecological complex in West Africa. A land-cover change analysis was performed for the period 1984–2002: savanna vegetation extension and loss were derived within the complex and in a 30 km peripheral buffer. Sample regions in the buffer were also analysed using selected spatial indicators to quantify temporal trends in habitat fragmentation. Implications for change in relative capacity to conserve biodiversity were discussed through the calculation of the species richness capacity (SRC). More than 14.5% of savanna habitat was lost in the WAP peripheral areas, while 0.3% was converted inside the complex. The degree of fragmentation of remnant savanna habitat has also drastically increased. Despite the effectiveness of the park conservation programme, we found through the SRC approach that the WAP complex is decreasing its potential capacity to conserve species richness. This process is mainly due to the rapid and extended agricultural expansion taking place around the complex. A better understanding of the ecological dynamics occurring in the peripheral regions of reserves and the consideration of development needs are key variables to achieve conservation goals in protected areas. 相似文献
130.
Carlos García-Robledo & Carol C. Horvitz 《Entomologia Experimentalis et Applicata》2009,131(2):115-120
Leaf volatile chemicals are known to reduce herbivory rates by repelling or intoxicating insect herbivores and by attracting the predators and parasitoids of herbivores. However, leaf volatiles may also be used by insect herbivores as cues to locate their host plants. Leaf volatiles are suggested to be important host search cues for herbivores in structurally complex and diverse habitats, such as tropical rain forests. A group of insect herbivores, the rolled-leaf beetles (Coleoptera: Chrysomelidae: Hispinae), have maintained a highly specialized interaction with Neotropical gingers (Zingiberales) for ca. 60 million years. In this study, we explored chemical attraction to host plants under controlled laboratory conditions, using four sympatric rolled-leaf beetle species, Cephaloleia dorsalis Baly, Cephaloleia erichsonii Baly, Cephaloleia fenestrata Weise, and Cephaloleia placida Baly. For each beetle species, we investigated (i) whether it was repelled or attracted by leaf scents produced by four host and four non-host plant species, including Neotropical gingers in the families Marantaceae, Costaceae, and Zingiberaceae; and (ii) its ability to use scents to detect its host plant. We found that rolled-leaf beetles can detect and are attracted by leaf volatiles from both host and non-host gingers. Additionally, when beetles were simultaneously exposed to leaf volatiles from host and non-host plants, three rolled-leaf beetle species were significantly more attracted by volatiles from their host plants than from non-hosts. Only one of the beetle species was not able to discriminate between host and non-host scents. 相似文献