全文获取类型
收费全文 | 12537篇 |
免费 | 1246篇 |
国内免费 | 8篇 |
出版年
2022年 | 97篇 |
2021年 | 166篇 |
2020年 | 99篇 |
2019年 | 128篇 |
2018年 | 164篇 |
2017年 | 147篇 |
2016年 | 255篇 |
2015年 | 483篇 |
2014年 | 509篇 |
2013年 | 646篇 |
2012年 | 790篇 |
2011年 | 814篇 |
2010年 | 556篇 |
2009年 | 464篇 |
2008年 | 655篇 |
2007年 | 704篇 |
2006年 | 685篇 |
2005年 | 668篇 |
2004年 | 646篇 |
2003年 | 626篇 |
2002年 | 680篇 |
2001年 | 130篇 |
2000年 | 102篇 |
1999年 | 151篇 |
1998年 | 185篇 |
1997年 | 143篇 |
1996年 | 130篇 |
1995年 | 124篇 |
1994年 | 118篇 |
1993年 | 121篇 |
1992年 | 99篇 |
1991年 | 101篇 |
1990年 | 116篇 |
1989年 | 105篇 |
1988年 | 99篇 |
1987年 | 105篇 |
1986年 | 90篇 |
1985年 | 95篇 |
1984年 | 112篇 |
1983年 | 119篇 |
1982年 | 135篇 |
1981年 | 117篇 |
1980年 | 95篇 |
1979年 | 65篇 |
1978年 | 86篇 |
1977年 | 75篇 |
1976年 | 65篇 |
1975年 | 59篇 |
1974年 | 66篇 |
1973年 | 54篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
We analyzed data from Section 404 permits issued in California from January 1971 through November 1987 that involved impacts to wetlands and required compensatory mitigation (wetland creation, restoration, or preservation). The purpose of this study was to determine patterns and trends in permitting activity and to document cumulative effects of associated management decisions on the California wetland resource. The 324 permits examined documented that 387 compensatory wetlands (1255.9 ha) were required as mitigation for impacts to 368 wetlands (1176.3 ha). The utility of the data on wetland area was limited, however, since 38.0% of the impacted wetlands and 41.6% of the compensatory wetlands lacked acreage data. The wetland type most frequently impacted (37.8% of impacted wetlands) and used in compensation (38.2% of compensatory wetlands) was palustrine forested wetlands. Estuarine intertidal emergent wetlands had the most area impacted (52.3%) and compensated (62.5%). The majority of the wetlands were small (less than or equal to 4.0 ha in size). Wildlife habitat was the most frequently listed function of impacted wetlands (90.7% of the permits) and objective of compensatory wetlands (83.3%). Endangered species were listed as affected in 20.4% of impacted and 21.0% of compensatory projects. The number of permits requiring compensatory mitigation and the number of impacted and compensatory wetlands increased from 1971 to 1986.Documentation of the details of Section 404 permit decisions was inadequate for the permits we examined. Area information and specific locations of impacted and compensatory wetlands were lacking or of poor quality. Follow-up information was also inadequate. For example, project completion dates were specified in the permit for only 2.2% of compensatory wetlands. Furthermore, less than one-third (31.5%) of the permits required the compensatory wetland to be monitored by at least one site visit. We recommend improved documentation, regular reporting, and increased monitoring for better evaluation of the Section 404 permitting system. 相似文献
82.
83.
Mark A. Wolters Mary E. Bodnar Mary D. Strem Scott E. Bingham 《Applied microbiology and biotechnology》1991,35(1):56-59
Summary In order to deregulate arginine biosynthesis in Synechococcus sp. PCC7942, d-arginine-resistant cell lines were selected following ethyl methanesulfonate mutagenesis of wild-type (WT) cells. Three of these arginine-producing mutant (APM) cell lines, APM1, APM31 and APM40, were putative regulatory mutants based upon secretion of l-arginine into their growth medium. HPLC of lyophilized post-harvest supernatants of APM 31 and 40 resolved two predominant amino acids, arginine and citrulline. In-vitro activity of N-acetylglutamate kinase (NAGK), the proposed regulatory enzyme of the arginine pathway, was about 100-fold less sensitive to l-arginine inhibition in extracts from APM 31 and 40 than the enzyme in WT extracts. The enzyme from APM 1 was 20-fold less sensitive to l-arginine inhibition than WT. The most likely site of mutation in each of the APM cell lines is in the gene for NAGK, rendering the enzymes insensitive to l-arginine feedback control. These strains can be utilized for the phototrophic production of arginine.
Offprint requests to: S. E. Bingham 相似文献
84.
Quantitative receptor autoradiography was used to examine the binding of [125I]-human CGRP in the dorsal horn of the L4 spinal segment of rats with a chronic constriction injury (CCI) of the sciatic nerve or unilateral dorsal rhizotomies of spinal segments L1–L6. At the times selected for study, we found no change in the amount of CGRP binding in any areas examined following CCI. In contrast, our results showed a temporally related increase in the amount of CGRP binding in areas within laminae I–II and in lateral lamina V of the dorsal horn ipsilateral to the rhizotomies. These results indicate that CGRP binding sites are regulated, most likely, by changes in the release of CGRP. Further, our results suggest that the release of CGRP from primary afferent neurons is unchanged in animals with a CCI. 相似文献
85.
86.
Michael I. Lerman Farida Latif Gladys M. Glenn Lambert N. Daniel Hiltrud Brauch Shigeto Hosoe Krista Hampsch John Delisio Mary Lou Orcutt O. Wesley McBride Karl-Heinz Grzeschik Takashi Takahashi John Minna Patrick Anglard W. Marston Linehan Berton Zbar 《Human genetics》1991,86(6):567-577
Summary A collection of 2,000 lambda phage-carrying human single-copy inserts (> 700 bp) were isolated from two chromosome-3 flow-sorted libraries. The single-copy DNA fragments were first sorted into 3p and 3q locations and about 700 3p fragments were regionally mapped using a deletion mapping panel comprised of two humanhamster and two-human-mouse cell hybrids, each containing a chromosome 3 with different deletions in the short arm. The hybrids were extensively mapped with a set of standard 3p markers physically localized or ordered by linkage. The deletion mapping panel divided the short arm into five distinct subregions (A-E). The 3p fragments were distributed on 3p regions as follows: region A, 26%; B, 31%; C, 4%; D, 4% and E, 35%. We screened 300 single-copy DNA fragments from the distal part of 3p (regions A and B) with ten restriction endonucleases for their ability to detect restriction fragment length polymorphisms (RFLPs). Of these fragments 110 (36%) were found to detect useful RFLPs: 35% detected polymorphisms with frequency of heterozygosity of 40% or higher, and 25% with frequency of 30% or higher. All polymorphisms originated from single loci and most of them were of the base pair substitution type. These RFLP markers make it possible to construct a fine linkage map that will span the distal part of chromosome 3p and encompasses the von Hippel-Lindau disease locus. The large number of single-copy fragments (2,000) spaced every 100–150 kb on chromosome 3 will make a significant contribution to mapping and sequencing the entire chromosome 3. The 300 conserved chromosome 3 probes will increase the existing knowledge of man-mouse homologies. 相似文献
87.
Synopsis The ability of embryos of the viviparous scorpaenidSebastes melanops to take up nutrients from an exogenous substrate was demonstrated by incubating embryos at various stages of development
(18–30 days after fertilization) in14C-labeled glycine for 24 h. Uptake was highest for embryos at the latest stages (28–30 days) and increased at a linear rate
during the incubation period. Nutrient uptake was not time dependent in embryos at the early stages (18–22 days). Nutrient
utilization byS. melanops embryos was measured by the oxidation of14C-labeled glycine to14CO2. The amount of respired14CO2 by the oldest embryos increased significantly at a linear rate over the 24 h incubation period. There was no evidence of
nutrient utilization by the youngest embryos. The developmental changes we observed in the uptake and utilization of exogenous
glycine are supported by our previous findings that the oldest embryos have fully developed mouths and guts, and require additional
nutrition from intraovarian sources at this stage of development. 相似文献
88.
The Mu Transposable Elements of Maize: Evidence for Transposition and Copy Number Regulation during Development 总被引:14,自引:8,他引:6
下载免费PDF全文
![点击此处可从《Genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The Mu transposon of maize exists in a highly mutagenic strain called Robertson's Mutator. Plants of this strain contain 10-50 copies of the Mu element, whereas most maize strains and other plants have none. When Mutator plants are crossed to plants of the inbred line 1S2P, which does not have copies of Mu, the progeny plants have approximately the same number of Mu sequences as did their Mutator parent. Approximately one-half of these copies have segregated from their parent and one-half have arisen by transposition and are integrated into new positions in the genome. This maintenance of copy number can be accounted for by an extremely high rate of transposition of the Mu elements (10-15 transpositions per gamete per generation). When Mutator plants are self-pollinated, the progeny double their Mu copy number in the first generation, but maintain a constant number of Mu sequences with subsequent self-pollinations. Transposition of Mu and the events that lead to copy number maintenance occur very late in the development of the germ cells but before fertilization. A larger version of the Mu element transposes but is not necessary for transposition of the Mu sequences. The progeny of crosses with a Mutator plant occasionally lack Mutator activity; these strains retain copies of the Mu element, but these elements no longer transpose. 相似文献
89.
Limnologic consequences of the decline in hemlock 4800 years ago in three Southern Ontario lakes 总被引:2,自引:2,他引:0
Mary M. Boucherle John P. Smol Tracie C. Oliver S. R. Brown R. McNeely 《Hydrobiologia》1986,143(1):217-225
Four thousand eight hundred years ago hemlock (Tsuga canadensis) populations were decimated throughout eastern North America. We have studied the effects of this loss from the terrestrial community on three Southern Ontario lakes: Little Round Lake, Sunfish Lake, and McKay Lake. This study includes the use of cladocerans, diatoms, chrysophytes, and bacterial pigments to assess the limnologic changes that occurred in these lakes. Each lake experienced a change in trophic status that coincided with the loss of hemlock from its catchment, but the change in the aquatic biota was different in each lake. The lakes' size may have been the most influential factor governing the response to this terrestrial disturbance. 相似文献
90.
Comparison of the roles of Ostracods and Cladocerans in regulating community structure and metabolism in freshwater microcosms 总被引:1,自引:1,他引:0
Laboratory microcosms were used to compare the effects of the littoral ostracod Cypridopsis vidua and the planktonic cladoceran Daphnia magna on community structure and metabolism. Filter-feeding by cladocerans, both in the presence and absence of ostracods, greatly
reduced the abundance of planktonic algae when D. magna reached peak density around day 50; rotifers and euglenids were then limited to flocculent matter on the container bottom.
Both net production and community respiration rates decreased as community composition changed. Microcosms containing ostracods
as the only microcrustacean showed little reduction in total algal numbers but the otherwise dominant alga, Scenedesmus spp., was replaced by Ankistrodesmus spp. when peak ostracod density was reached around day 100. Rotifers were completely eliminated but euglenids were able to
coexist with ostracods. Ostracods impacted community metabolism less than cladocerans, but depressed respiration slightly
more than net production. 相似文献