全文获取类型
收费全文 | 93509篇 |
免费 | 288篇 |
国内免费 | 812篇 |
出版年
2023年 | 7篇 |
2022年 | 12篇 |
2021年 | 17篇 |
2020年 | 12篇 |
2019年 | 13篇 |
2018年 | 11848篇 |
2017年 | 10682篇 |
2016年 | 7459篇 |
2015年 | 611篇 |
2014年 | 298篇 |
2013年 | 321篇 |
2012年 | 4240篇 |
2011年 | 12827篇 |
2010年 | 12007篇 |
2009年 | 8239篇 |
2008年 | 9802篇 |
2007年 | 11363篇 |
2006年 | 267篇 |
2005年 | 509篇 |
2004年 | 967篇 |
2003年 | 1024篇 |
2002年 | 787篇 |
2001年 | 263篇 |
2000年 | 164篇 |
1999年 | 26篇 |
1998年 | 9篇 |
1997年 | 24篇 |
1996年 | 15篇 |
1994年 | 8篇 |
1993年 | 30篇 |
1992年 | 25篇 |
1991年 | 38篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1988年 | 18篇 |
1987年 | 17篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 8篇 |
1983年 | 17篇 |
1981年 | 3篇 |
1975年 | 5篇 |
1972年 | 246篇 |
1971年 | 276篇 |
1970年 | 5篇 |
1965年 | 13篇 |
1962年 | 24篇 |
1956年 | 5篇 |
1944年 | 12篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
242.
Johan U. Grobbelaar 《Journal of applied phycology》1989,1(4):333-340
It has been suggested that turbulence with the resultant light/dark cycle and light gradient through which phytoplankton move,
enhances their productivity. The stationary bottle incubation technique for estimating rates of primary productivity has mainly
been criticized because of bottle effects, the elimination of natural turbulence and the presence of photo-inhibition. In
a series of experiments where productivity was measured over static profiles and compared to the productivity in a mixed system,
no definite conclusion could be reached regarding the effect of varying light/dark cycles of medium frequency (seconds to
minutes). It appeared as though the ratio of the euphotic depth to mixing depth (Z
eu/Z
m) influenced productivity more than the duration of the light/dark cycle. The static bottle incubation method gave higher
integral productivities than the mixed samples at low ratio's ofZ
eu/Z
m. It is suggested that mixing has two separate, but synergistic effects i.e. it not only moves the phytoplankton cells through
a light/dark cycle, but also decreases the boundary layer, which increases the rate of exchange through the cell wall of nutrients
and metabolites. In doing so more nutrients are available and light could be utilized more efficiently and therefore, productivity
is increased. 相似文献
243.
Across the world there is a prevailing view that freshwater algae are cosmopolitan. The notion has seldom been tested and
is unlikely to be true in genetic terms. Nonetheless, some morphospecies of several groups of algae do have a worldwide distribution.
Others have restricted distributions and may be regarded as endemic to a region. However there is always the possibility that
they will be discovered in far away places. Australia has a rather large element of endemicity in its algal flora. From the
early days of Australian phycology many new genera and species of freshwater algae have been described. Some are of such distinctive
appearance or novelty as to be regarded as ‘flagship’ taxa. There is little doubt about their endemicity and their existence
increases the probability of less-distinguished species also being endemic. The degree of endemicity is probably masked by
the ‘force-fitting’ of European names to Australian species.
Some Australian endemics are robust and are widely distributed in a variety of types of water body. Others, the frail endemics,
the ones of greatest novelty and phylogenetic significance, have a very restricted range with their strongholds in dystrophic
coastal lagoons where tracts or remnant patches of native vegetation survive. Their survival and the conservation of their
biodiversity depends on recognition of the significance of coastal lagoons and swamps. 相似文献
244.
The effect of algae on the production of musty-smelling compounds by actinomycetes was studied. Streptomyces spp., causing intensive musty odor, were isolated from hypertrophic Lake Kasumigaura and cultured in association with algae from the same lake. Isolate E and I effectively utilized the cyanobacteria, Microcystis aeruginosa and Anabaena spiroides, and the diatom, Synedra acus, as a carbon source and produced a musty-smelling 2-methylisoborneol in the shaken sediment cultures. High populations of algae and actinomycetes, and aerobic condition in the sediment seem responsible for the occurrence of musty odor in Lake Kasumigaura. 相似文献
245.
246.
Flavio Alexandre Carvalho Natália Valadares de Moraes Antônio Eduardo Miller Crotti Eduardo José Crevelin André Gonzaga dos Santos 《化学与生物多样性》2023,20(9):e202300492
Casearia species are found in the America, Africa, Asia, and Australia and present pharmacological activities, besides their traditional uses. Here, we reviewed the chemical composition, content, pharmacological activities, and toxicity of the essential oils (EOs) from Casearia species. The EO physical parameters and leaf botanical characteristics were also described. The bioactivities of the EOs from the leaves and their components include cytotoxicity, anti-inflammatory, antiulcer, antimicrobial, antidiabetic, antioxidant, antifungal, and antiviral activities. The main components associated with these activities are the α-zingiberene, (E)-caryophyllene, germacrene D, bicyclogermacrene, spathulenol, α-humulene, β-acoradiene, and δ-cadinene. Data on the toxicity of these EOs are scarce in the literature. Casearia sylvestris Sw. is the most studied species, presenting more significant pharmacological potential. The chemical variability of EOs components was also investigated for this species. Caseria EOs have relevant pharmacological potential and must be further investigated and exploited. 相似文献
247.
Maxim K. Elias 《Pal?ontologische Zeitschrift》1962,36(1):29-37
The type specimen ofGonioloboceras goniolobum (Meek), rediscovered by Spath in the British Museum, is the foundation for a more accurate comparative study of this and other species ofGonioloboceras.Gonioloboceras described asG. goniolobum byElias in 1938 is differentiated asGonioloboceras schmidti, new species. Suture sets (new term) for several growth stages inG. goniolobum (Meek),G. welleriSmith,G. schmidtiElias, G.eliasiMiller &Owen, andG. asiaticumLibrovitch are assembled and used for differentiation of the species.The Kazakhstan goniatite faunule containingG. asiaticum is considered of very late Pennsylvanian age. 相似文献
248.
R. Schoysman B. Lejeune E. Van Roosendaal L. Segal P. Vanderzwalmen M. Nijs B. Vandamme G. Bertin 《Andrologie》1996,6(4):432-439
The authors report their experience with the use of spermatids in TESE programs where mature spermatozoa could not be isolated from testicular biopsies. The details of the indications for spermatid insemination, the technicity of the procedure and the results are exposed. 相似文献
249.
250.
A. Jagannadha Rao N. Mathialagan S. G. Kotagi N. R. Moudgal 《Journal of biosciences》1984,6(2):97-106
The regulation of secretion of chorionic gonadotropin in primates has been studied using bothin vivo andin vitro models.In vivo studies using the pregnant bonnet monkey revealed that at the doses tested, the administration of progesterone or estradiol 17Β in combination or alone did not result in any appreciable change in the duration or magnitude of serum chorionic gonadotropin levels. However, administration of lutropin-releasing hormone by intravenous route resulted in significant increase in chorionic gonadotropin levels within 30–60 min and the extent of stimulation seemed to depend on the state of pregnancy. Forin vitro studies, explants or cells prepared from first trimester human placenta has been used. The functional integrity of these cells has been established by demonstrating the binding of [125I]-labelled human chorionic gonadotropin antibody to the cells as well as the synthesis of [3H]-labelled human chorionic gonadotropin.In vitro studies using the cells revealed that addition of lutropin-releasing hormone caused a significant increase in chorionic gonadotropin and estradiol 17Β secreted into the medium. Thus bothin vivo andin vitro results suggest that lutropin-releasing hormone could be one of the factors involved in regulation of chorionic gonadotropin secretion in primates. 相似文献