全文获取类型
收费全文 | 1501127篇 |
免费 | 135950篇 |
国内免费 | 1623篇 |
出版年
2021年 | 18014篇 |
2019年 | 16204篇 |
2018年 | 18836篇 |
2017年 | 17636篇 |
2016年 | 28862篇 |
2015年 | 42627篇 |
2014年 | 50658篇 |
2013年 | 76783篇 |
2012年 | 41066篇 |
2011年 | 31412篇 |
2010年 | 45757篇 |
2009年 | 46478篇 |
2008年 | 29373篇 |
2007年 | 27886篇 |
2006年 | 31816篇 |
2005年 | 33109篇 |
2004年 | 31987篇 |
2003年 | 29391篇 |
2002年 | 27476篇 |
2001年 | 41188篇 |
2000年 | 38940篇 |
1999年 | 36983篇 |
1998年 | 26239篇 |
1997年 | 26123篇 |
1996年 | 25423篇 |
1995年 | 23649篇 |
1994年 | 23353篇 |
1993年 | 22493篇 |
1992年 | 31886篇 |
1991年 | 30418篇 |
1990年 | 29285篇 |
1989年 | 29818篇 |
1988年 | 27668篇 |
1987年 | 26393篇 |
1986年 | 24797篇 |
1985年 | 26708篇 |
1984年 | 25565篇 |
1983年 | 22549篇 |
1982年 | 21840篇 |
1981年 | 20939篇 |
1980年 | 19700篇 |
1979年 | 21762篇 |
1978年 | 19809篇 |
1977年 | 18945篇 |
1976年 | 18163篇 |
1975年 | 18135篇 |
1974年 | 18952篇 |
1973年 | 19285篇 |
1972年 | 16612篇 |
1971年 | 15270篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Winnie W.Y. Lau Marlene M. Martinez 《Journal of experimental marine biology and ecology》2003,295(1):1-21
Hydrodynamic forces can affect survival as well as limit the movement of motile benthic animals. An animal's danger of dislodgement depends on the hydrodynamic forces it experiences in its microhabitat relative to the force required to dislodge it (tenacity) from the substratum. We measured water flow and substratum characteristics in two different habitats of the shore crab Pachygrapsus crassipes: a wave-swept rocky shore and an intertidal mudflat. The maximum water velocities and accelerations in the microhabitats of the crabs at the wave-swept site were three times and two times greater, respectively, than at the mudflat site. In the laboratory, we measured the tenacity of crabs of various sizes on different substrata, and also measured their drag, lift and added-mass coefficients. Using these data, we calculated the flow conditions under which crabs would be overturned or sheared off the substratum in their two habitats. The net horizontal force (drag plus acceleration reaction) required to dislodge a crab on a rugose rock substratum was an order of magnitude greater than on smooth rock and two orders of magnitude greater than on mud. Our calculations indicate that, under non-storm conditions, crabs will not be dislodged from the substratum in either the mudflat or the wave-swept habitat when grasping the substratum with maximum tenacity. Moving crabs have lower tenacity and our calculations predict that hydrodynamic forces will restrict the mobility of large crabs more than that of small ones on smooth, but not on rugose rock. 相似文献
962.
The Vascular System of Maize Stems Revisited: Implications for Water Transport and Xylem Safety 总被引:3,自引:0,他引:3
The plexus of vascular bundles in the nodes of grasses is notoriouslycomplex, where long axial bundles pass through a network oftransverse bundles. The xylem pathways for water in maize stemshave been investigated anatomically and with dye and particulatetracers, revealing some of the details of this complexity. Onlyapprox. 3% of axial vessels pass through nodes without beinginterrupted by end walls. Axial bundles at nodes differ fromthose in internodes in having the metaxylem and protoxylem vesselsconnected by small tracheary elements. So it is only at nodesthat exchange of sap occurs between the large vessels withina bundle. End walls, acting as filters for particles and gasbubbles, always separate axial vessels from vessels in transversebundles. The high redundancy of bundle connections in the nodalplexus is interpreted as providing alternative water pathwaysto bypass embolisms and damaged or diseased sections of thexylem. The pores in the filters at the base of nodes and betweenaxial and transverse vessels within nodes are <20 nm in diameter.Where axial vessels connect to transverse vessels, a varietyof unusual shapes of vessel elements mediate two- and three-wayconnections within the plexus.Copyright 2000 Annals of BotanyCompany Zea mays, cryoSEM, maize, node, pits, pit membranes, vessel ends, vessels, xylem embolism, xylem pathogens 相似文献
963.
E F Hill 《Journal of wildlife diseases》1988,24(1):51-61
Organophosphorus and carbamate pesticides are potent anticholinesterase substances that have killed large numbers of wild birds of various species. Cause of death is diagnosed by demonstration of depressed brain cholinesterase (ChE) activity in combination with chemical detection of anticholinesterase residue in the affected specimen. ChE depression is determined by comparison of the affected specimen to normal ChE activity for a sample of control specimens of the same species, but timely procurement of controls is not always possible. Therefore, a reference file of normal whole brain ChE activity is provided for 48 species of wild birds from North America representing 11 orders and 23 families for use as emergency substitutes in diagnosis of anticholinesterase poisoning. The ChE values, based on 83 sets of wild control specimens from across the United States, are reproducible provided the described procedures are duplicated. Overall, whole brain ChE activity varied nearly three-fold among the 48 species represented, but it was usually similar for closely related species. However, some species were statistically separable in most families and some species of the same genus differed as much as 50%. 相似文献
964.
Of 41 fungicides tested in the laboratory, copper carbonate, copper sulphate, mercuric chloride, Agrosan GN, quintozene, kasugamycin, carboxin, pyracar-bolid, carbendazim, chloroneb, benomyl, Ohric, RH 893 (2-n-octyl-4-isothiazole-3-one) and Terrazole were most inhibitory to the mycelial growth of Rhizoctonia solani on Czapek's agar plates and had EC50 values of less than 1 μg a.i./ml, while copper oxychloride, Udonkor, zineb, ziram, F 319 (3-hydroxy-5-methyl isoxazole) and anilazine were much less toxic, ziram being least inhibitory with an EC50 of 214 μg a.i./ml. Of 17 fungicides tested in the greenhouse as seed treatments, thiabendazole, carbendazim, benomyl, thiophanate-methyl, dichlozoline and Ohric gave 80–90% control of damping-off of mung bean seedlings. A single soil drench with thiophanate-methyl and two drenches with benomyl gave about 90% disease control, More seedlings with R. solani infection survived when thiophanate-methyl was used as a post-inoculation soil drench than when benomyl or chloroneb were used. 相似文献
965.
966.
967.
Sukhraj S. Dhillon Graeme P. Berlyn Jerome P. Miksche 《American journal of botany》1978,65(2):192-196
Five populations of Pinus rigida growing in contrasting ecological situations ranging from North Carolina (35°53'N latitude) to Quebec (45°06'N latitude) showed no significant variation in amounts of nuclear DNA with respect to germinating seedlings. Nuclear volume of dormant nuclei also showed no significant variation between and within populations, a finding that is consistent with the concept that the basic 2C DNA value of Pinus rigida is uniform under all habitats. This finding is in contrast to numerous reports for other coniferous species. 相似文献
968.
Two acrasid cellular slime molds. Guttulinopsis vulgaris and G. nivea, are compared at the ultrastructural level. The amoebae of the two species are indistinguishable except for the presence of intranuclear fibers in G. vulgaris. Both species share some unusual features, including: plate-like cristae in the mitochondria, production of microbody-like organelles in the perinuclear space, spores with thin bilaminar walls, and stalks containing microfilaments bound in striated bundles. These and other observations are discussed with regard to the development of the sorocarps and the relationship of the genus to other members of the Acrasida. 相似文献
969.
970.
Isoproterenol-stimulated cyclic AMP (cAMP) accumulation in lymphocytes is an index of beta-adrenergic receptor functioning. It is not known if a delay between blood collection and subsequent cell preparation influences cAMP accumulation. We compared cAMP levels in lymphocytes processed from whole blood immediately after collection and following a 3.5 hour delay. There were no significant changes in either basal or stimulated levels of cAMP. This information may alleviate time constraints in certain experimental designs where immediate isolation of lymphocytes from whole blood may not be practical. 相似文献