全文获取类型
收费全文 | 11810篇 |
免费 | 1300篇 |
国内免费 | 306篇 |
出版年
2024年 | 10篇 |
2023年 | 242篇 |
2022年 | 240篇 |
2021年 | 394篇 |
2020年 | 479篇 |
2019年 | 616篇 |
2018年 | 562篇 |
2017年 | 482篇 |
2016年 | 457篇 |
2015年 | 521篇 |
2014年 | 634篇 |
2013年 | 953篇 |
2012年 | 454篇 |
2011年 | 535篇 |
2010年 | 436篇 |
2009年 | 599篇 |
2008年 | 621篇 |
2007年 | 539篇 |
2006年 | 560篇 |
2005年 | 480篇 |
2004年 | 434篇 |
2003年 | 401篇 |
2002年 | 325篇 |
2001年 | 222篇 |
2000年 | 220篇 |
1999年 | 228篇 |
1998年 | 186篇 |
1997年 | 192篇 |
1996年 | 171篇 |
1995年 | 143篇 |
1994年 | 131篇 |
1993年 | 106篇 |
1992年 | 116篇 |
1991年 | 101篇 |
1990年 | 78篇 |
1989年 | 82篇 |
1988年 | 71篇 |
1987年 | 62篇 |
1986年 | 53篇 |
1985年 | 61篇 |
1984年 | 54篇 |
1983年 | 27篇 |
1982年 | 38篇 |
1981年 | 26篇 |
1980年 | 19篇 |
1979年 | 23篇 |
1978年 | 5篇 |
1977年 | 11篇 |
1976年 | 6篇 |
1973年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
961.
962.
To prevent predation on their eggs, prey often avoid patches occupied by predators. As a result, they need to delay oviposition
until they reach predator-free patches. Because many species allocate energy to egg production in a continuous fashion, it
is not clear what kind of mechanism prey use to delay oviposition. We used females of the phytoseiid mite Neoseiulus cucumeris to study these mechanisms. Females were placed in patches with pollen, a food source they use for egg production, and they
were exposed to another phytoseiid mite, Iphiseius degenerans, which is an intraguild predator of N. cucumeris juveniles. We found that the oviposition of N. cucumeris females on patches with the predator was lower than on patches without the predator. Cues left by the intraguild predator
were not sufficient to elicit such behaviour. Females of N. cucumeris reduced oviposition when exposed to the predator by retaining the egg inside their body, resulting in a lower developmental
rate once these eggs were laid. Hence, females are capable of retaining eggs, but the development of these eggs continues
inside the mother’s body. In this way, females gain some time to search for less risky oviposition sites. 相似文献
963.
Effects of physical connection and genetic identity of neighbouring ramets on root-placement patterns in two clonal species 总被引:5,自引:0,他引:5
Root-placement patterns were examined in the clonal species Glechoma hederacea and Fragaria vesca when grown with different types of neighbours. Three different patterns were predicted as consequences of different types of interactions between roots: the avoidance pattern if root growth decreases in the presence of neighbouring roots; the intrusive pattern if root growth increases towards neighbouring roots; and the unresponsive pattern if root growth is unaffected by neighbouring roots. Experiments were conducted in which physical connection between ramets, and the genetic identity of neighbouring ramets, were manipulated. The patterns of distribution of entire root systems and elongation rates of individual roots were measured. Root systems and individual roots of G. hederacea avoided contact with roots of neighbouring ramets, irrespective of connection to the neighbour and its genetic or specific identity. In contrast, F. vesca roots grew equally towards and away from intraspecific ramet neighbours and their elongation was stimulated by contact with roots of G. hederacea ramets. These results demonstrate that root-placement patterns of plants grown with different types of neighbours vary between species, and suggest that factors additional to resource depletion could be involved in their development. 相似文献
964.
Deveau A Palin B Delaruelle C Peter M Kohler A Pierrat JC Sarniguet A Garbaye J Martin F Frey-Klett P 《The New phytologist》2007,175(4):743-755
965.
966.
Plant–herbivore interactions have strong ecological and evolutionary consequences, but have been traditionally overlooked
in marine higher plants. Despite recent advances in seagrass ecology that highlight the importance of herbivory, the mechanisms
that regulate the feeding behaviour of seagrass consumers remain largely unknown. Herbivores have been shown to reduce the
sexual reproductive success of seagrasses through direct consumption of inflorescences and seeds, but we know little about
intraspecific variation in susceptibility to grazing of different seagrass tissues. We contrasted the relative palatability
of reproductive and vegetative tissues of the temperate seagrass Posidonia oceanica in the field, and we assessed the feeding preferences among these tissues of the main consumers of the plant, the fish Sarpa salpa and the urchin Paracentrotus lividus. Moreover, we identified the plant traits that explained the observed feeding behaviour. We provide strong evidence for herbivore
selectivity among seagrass tissues. In the field, 70–90% of inflorescences were damaged by herbivores compared to 3–60% of
leaves of similar age. In feeding assays, the urchin P. lividus showed over a twofold preference for reproductive tissue at various stages of development. By contrast, we detected no feeding
activity on either leaves or inflorescences from the fish S. salpa, which is known to migrate to deeper waters soon after flowering starts and during the period of fruit maturation. Despite
being the preferred food of urchins, inflorescences were chemically defended, had higher levels of phenolics and lower nutrient
and calorific content than leaves. We experimentally demonstrated that leaf structural defences are the primary factor in
determining urchin feeding preferences. Removal of plant structure results in a drastic shift in urchin selectivity towards
the most nutritious and less chemically defended leaf tissue, indicating that multiple mechanisms of defence to herbivory
may coexist in seagrasses. 相似文献
967.
Many herbivores elicit biochemical, physiological, or morphological changes in their host plants that render them more resistant
to co-occurring herbivores. Yet, despite the large number of studies that investigate how induced resistance affects herbivore
preference and performance, very few have simultaneously explored the cascading effects of induction on higher trophic levels
and consequences for prey suppression. In our study system, early-season herbivory by leafhoppers elevated plant resistance
to subsequent attack by chrysomelid beetles sharing the same host plant. Notably, beetles feeding on leafhopper-damaged plants
incurred developmental penalties (e.g., prolonged time in early larval instars) that rendered them more susceptible to predation
by natural enemies. As a result, the combined bottom-up effect of leafhopper-induced resistance and the top-down effect of
enhanced predation resulted in the synergistic suppression of beetle populations. These results emphasize that higher trophic
level dynamics should be considered in conjunction with induced resistance to better understand how plants mediate interspecific
interactions in phytophagous insect communities. 相似文献
968.
Predators and parasitoids may contribute to controlling the population sizes of phytophagous insects, and this has been shown
to benefit plants. Phytophagous insects may also be killed by other herbivores (intraguild predation), usually larger-sized
vertebrates that ingest insects accidentally while feeding on common food sources. We studied the intraguild predation on
acorn weevils by ungulates and assessed the consequences for weevil populations. Infested acorns are prematurely abscised
and the weevil larvae finish their development inside the acorns after being dropped. Our results show that weevil larvae
were killed by ungulates eating the infested acorns on the ground. Ungulates did not discriminate between infested and sound
acorns, and the probability of a larva being incidentally eaten was inversely related to acorn availability. Thus, predation
risk was enhanced by the premature drop of infested acorns when acorn availability on the ground was low. Predation rates
on infested acorns were much higher where ungulates were present, and acorn infestation rates were significantly lower. However,
ungulates did not provide the oaks any net benefit, since the reduction of infestation rates was not enough to compensate
for the large amounts of sound acorns eaten by ungulates. Seed predation is usually studied as a progressive loss of seeds
by pre- and post-dispersal predators, but the interactions between them are usually not considered. We show that intraguild
predation on insects by large ungulates had an effect on the structure of the foraging guild, as the proportion of acorns
predated by insects decreased; however, replicating the same experimental design in different ecological scenarios would increase
the strength of these results. In conclusion, the present study shows the importance of considering the multi-trophic interactions
between seed predators in order to have a complete picture of granivory. 相似文献
969.
Xiao X Yang X Liu T Chen Z Chen L Li H Deng L 《Applied microbiology and biotechnology》2007,75(5):1209-1216
The rapid detection and separation of Staphylococcus aureus and group G Streptococcus was based on the affinity chromatography interactions between Fc fragment of human IgG and protein A/G (located on the cell
wall of S. aureus and group G Streptococcus). In this case, immobilization of antibodies had to take place in a different and complementary way than in the case of conventional
immunosensors. In this study, three different kinds of immunomolecular-magnetic beads (IMB) were prepared for rapid detection
and separation of S. aureus and group G Streptococcus (GGS). The Fc regions of the immobilized antibodies were fully accessible to adsorb protein A or protein G. On the contrary,
conventional immunosensors had to have fully accessible Fab regions to facilitate the antigen–antibody recognition. It was
suggested that the worse method of immobilization of the antibodies for conventional use would yield the better results for
this specific use. In this study, we also perfectly solved the nonspecific adsorptions and interaction problems, which were
the most serious critical problems for all kinds of sensors. It was achieved by blocking the excess surface groups of aldehyde
IMB and the Fab region of the immobilized antibodies with aldehyde-dextran. 相似文献
970.
Based on universal thermodynamic principles (Schwarz in Biophys Chem 86:119–129, 2000) it is shown how measured enthalpy changes can be utilized to determine the relevant binding isotherm as well as the variation
of the molar enthalpy change. This is carried out in a novel way involving multiple titration experiments whose evaluation
requires no beforehand assumptions or models whatever. An appropriate specific model mechanism may be discussed afterwards
and developed in view of the given experimental results. The pertinent procedure is demonstrated using micro-calorimetric
data obtained in the case of the local anesthetic dibucaine as it associates with POPC liposomes. Mutual interactions of the
bound ligand molecules could be described in terms of repulsive enthalpic and entropic activity coefficients. Apparently these
are induced by electrostatic forces and by the finite size of binding sites, respectively. 相似文献