全文获取类型
收费全文 | 6602篇 |
免费 | 545篇 |
国内免费 | 495篇 |
出版年
2023年 | 144篇 |
2022年 | 109篇 |
2021年 | 175篇 |
2020年 | 200篇 |
2019年 | 242篇 |
2018年 | 181篇 |
2017年 | 209篇 |
2016年 | 205篇 |
2015年 | 201篇 |
2014年 | 221篇 |
2013年 | 360篇 |
2012年 | 213篇 |
2011年 | 243篇 |
2010年 | 209篇 |
2009年 | 309篇 |
2008年 | 363篇 |
2007年 | 360篇 |
2006年 | 394篇 |
2005年 | 324篇 |
2004年 | 329篇 |
2003年 | 302篇 |
2002年 | 242篇 |
2001年 | 234篇 |
2000年 | 211篇 |
1999年 | 181篇 |
1998年 | 146篇 |
1997年 | 159篇 |
1996年 | 143篇 |
1995年 | 101篇 |
1994年 | 136篇 |
1993年 | 107篇 |
1992年 | 108篇 |
1991年 | 93篇 |
1990年 | 72篇 |
1989年 | 59篇 |
1988年 | 50篇 |
1987年 | 46篇 |
1986年 | 42篇 |
1985年 | 38篇 |
1984年 | 35篇 |
1983年 | 29篇 |
1982年 | 20篇 |
1981年 | 19篇 |
1980年 | 10篇 |
1979年 | 20篇 |
1978年 | 17篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1972年 | 3篇 |
排序方式: 共有7642条查询结果,搜索用时 15 毫秒
61.
The correlations between intertree distances and stem volumina of trees in an artificial pine wood with trees in rows are studied. Several stochastic models are suggested, and methods of the theory of random measures are used. A method for estimating the produce of woods of the type considered is suggested. 相似文献
62.
Moshe Tal 《In vitro cellular & developmental biology. Plant》1994,30(4):175-180
Summary In recent years attempts have been made to supplement traditional breeding for the production of salt-tolerant plants with
variability existing in cell culture. The potential causes suggested as an explanation for the limited success of the in vitro
approach include: a) lack, or loss during selection, of regeneration capability; b) the development of epigenetically adapted
cells; c) lack of correlation between the mechanisms of tolerance operating in cultured cells and mechanisms that operate
in cells in the intact plant; and d) multigenicity of salt tolerance. The recent successful production of healthy, fertile,
and genetically stable salt-tolerant regenerants from cells obtained from highly morphogenic explants which are selected early
in culture (using one-step or short-term strategies) for salt tolerance, together with the demonstration that salt-sensitive
plants can become tolerant by mutations in one or few genes, suggest that some of the potential limitations can be overcome
and that some of them may not exist at all. 相似文献
63.
The statistical analyses of populations of first-generation transgenic plants are commonly based on mean and variance and
generally require a test of normality. Since in many cases the assumptions of normality are not met, analyses can result in
erroneous conclusions. Transformation of data to normality, the use of other distributions, or distribution-free statistical
tests should then be used to obtain valid conclusions from these populations. 相似文献
64.
65.
Alex D. Twyford 《American journal of botany》2017,104(10):1433-1436
Parasitic plants demonstrate a diversity of growth strategies, life histories, and developmental and physiological characteristics. Most research to date has focused on a narrow range of parasitic taxa, particularly in the Orobanchaceae, while the other independent origins of parasitism have largely gone unstudied. One type of parasite that has received relatively little attention are the endophytic parasites, which have a fascinating growth strategy where the parasite is embedded within the host tissue, with the flower the only externally visibly plant part. Endophytic growth makes it challenging to understand basic aspects of species biology, such as the size of a given parasite, the number of parasites per host, and the genetic diversity of populations. Recent studies by Barkman et al. (2017) and Pelser et al. (2017) have used microsatellite genotyping to investigate the population biology of endoparasitic Rafflesiaceae species in Asia. They show the potential for extensive parasite spread within a host vine and the strong partitioning of genetic diversity by host. These species are also shown to have an outcrossing mating system. However, these studies suggest different reproductive strategies, one supporting monoecy and one suggesting dioecy. Overall, these studies partly “lift the lid” on the cryptic biology of Rafflesia and the Rafflesiaceae and open the door for future comparative studies between endophytic and free-living parasitic plants. 相似文献
66.
Heidrun Gerbling 《Plant biology (Stuttgart, Germany)》1993,106(5):380-387
Peroxisomes from mung bean hypocotyl (Vigna radiata L.) degrade 2-oxoisocaproate, the transamination product of leucine, via isobutyryl-CoA and propionyl-CoA to acetyl-CoA. The methyl group at the C-3 position forms a barrier to β-oxidation. This barrier is overcome in the peroxisomes by several enzymatic steps. Senecioate (3-methylcrotonate), 2-hydroxyisovalerate, and 2-oxoisovalerate were detected as free acid intermediates. Senecioate, formed from 3-methylcrotonyl-CoA, is transformed by enzymatic hydrolysis to 2-hydroxyisovalerate. 2-Hydroxyisovalerate is then oxidized to 2-oxoisovalerate in an H2O2-producing reaction, exhibiting 1:1 stoichiometry of the products, by a 2-hydroxyacid oxidase which is different from the peroxisomal marker enzyme glycollate oxidase. 2-oxoisovalerate is activated by an NAD-dependent oxidative decarboxylation to isobutyryl-CoA. Accumulation of 2-oxoisovalerate in the presence of arsenite, an inhibitor of oxidative decarboxylations, is a feature of this latter pathway of degradation of isovaleryl-CoA or senecioate. It is concluded that the barrier caused by the methyl group of 2-oxoisocaproate is surmounted in higher plant peroxisomes in a manner different to that in mammalian mitochondria. 相似文献
67.
James J. Krupa Kevin R. Hopper Samuel B. Gruber Jason M. Schmidt James D. Harwood 《Ecology and evolution》2020,10(11):4762-4772
- Plant–animal interactions are diverse and widespread shaping ecology, evolution, and biodiversity of most ecological communities. Carnivorous plants are unusual in that they can be simultaneously engaged with animals in multiple mutualistic and antagonistic interactions including reversed plant–animal interactions where they are the predator. Competition with animals is a potential antagonistic plant–animal interaction unique to carnivorous plants when they and animal predators consume the same prey.
- The goal of this field study was to test the hypothesis that under natural conditions, sundews and spiders are predators consuming the same prey thus creating an environment where interkingdom competition can occur.
- Over 12 months, we collected data on 15 dates in the only protected Highland Rim Wet Meadow Ecosystem in Kentucky where sundews, sheet‐web spiders, and ground‐running spiders co‐exist. One each sampling day, we attempted to locate fifteen sites with: (a) both sheet‐web spiders and sundews; (b) sundews only; and (c) where neither occurred. Sticky traps were set at each of these sites to determine prey (springtails) activity–density. Ground‐running spiders were collected on sampling days. DNA extraction was performed on all spiders to determine which individuals had eaten springtails and comparing this to the density of sundews where the spiders were captured.
- Sundews and spiders consumed springtails. Springtail activity–densities were lower, the higher the density of sundews. Both sheet‐web and ground‐running spiders were found less often where sundew densities were high. Sheet‐web size was smaller where sundew densities were high.
- The results of this study suggest that asymmetrical exploitative competition occurs between sundews and spiders. Sundews appear to have a greater negative impact on spiders, where spiders probably have little impact on sundews. In this example of interkingdom competition where the asymmetry should be most extreme, amensalism where one competitor experiences no cost of interaction may be occurring.
68.
《Critical reviews in biotechnology》2013,33(4):307-312
Heavy metals are found in their different forms in the environment. The distribution, mobility, and toxicity of metals are strongly related to these different forms. This necessitates the exploration of different methods for the remediation and speciation of heavy metals. Some direct and indirect physico-chemical methods such as filtration, chemical precipitation, ion-exchange, electro deposition, and membrane systems have been used for the last four decades. However, it is only in last few years that reliable biological methods have also been used. The biological methods include the use of microorganisms (fungi, algae, bacteria), plants (live or dead) and biopolymers. The use of these methods for the speciation of heavy metals is reviewed here. 相似文献
69.
Callispa keram
sp. n. infesting coconut palm (Cocos nucifera L.) in Kerala, India is described and illustrated. Livistona chinensis R.Br. and Syagrus romanzoffiana (Cham.) Glassman are reported as additional host plants. 相似文献
70.
D. Lupi M. L. Giudici C. Cenghialta A. Rocco C. Jucker M. Colombo 《Journal of Applied Entomology》2013,137(5):355-364
Italy is the largest rice‐producing country in the European Union. In Italy, only japonica cultivars are listed in the Italian National Register. Almost all of the rice production is concentrated in the Po Valley, where the rice water weevil Lissorhoptrus oryzophilus Kuschel was first detected and settled. This study investigated the performance of this pest in terms of feeding, reproduction and plant injury on 10 rice cultivars chosen among the most widely grown in Italy. No‐choice experiments were conducted to evaluate the plant susceptibility to larval attack and to find out how cultivars can influence the adult leaf area consumption. The results gave evidence of different types of attack depending on the density of the insect (0.6 adults/plant vs. 0.9 adults/plant), the cultivar type and climatic conditions. Different cultivars with the same level of infestation gave different results in terms of productivity. Production was significantly affected by the larval presence in 4 of the 10 cultivars tested. A different population structure reflected a different damage severity. Statistically different values for total adult leaf area consumption were found according to adult female age and to the cultivar. 相似文献