首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1047篇
  免费   106篇
  2023年   7篇
  2022年   5篇
  2021年   17篇
  2020年   8篇
  2019年   21篇
  2018年   18篇
  2017年   17篇
  2016年   24篇
  2015年   55篇
  2014年   48篇
  2013年   60篇
  2012年   70篇
  2011年   59篇
  2010年   47篇
  2009年   38篇
  2008年   43篇
  2007年   55篇
  2006年   51篇
  2005年   47篇
  2004年   54篇
  2003年   63篇
  2002年   49篇
  2001年   29篇
  2000年   31篇
  1999年   28篇
  1998年   11篇
  1997年   12篇
  1996年   13篇
  1995年   12篇
  1994年   8篇
  1993年   14篇
  1992年   14篇
  1991年   19篇
  1990年   14篇
  1989年   6篇
  1988年   7篇
  1987年   9篇
  1986年   6篇
  1985年   9篇
  1984年   6篇
  1982年   6篇
  1979年   3篇
  1977年   3篇
  1976年   3篇
  1972年   3篇
  1971年   3篇
  1967年   5篇
  1966年   4篇
  1965年   2篇
  1943年   2篇
排序方式: 共有1153条查询结果,搜索用时 15 毫秒
1.
2.
An ecosystem service is a benefit derived by humanity that can be traced back to an ecological process. Although ecosystem services related to surface water have been thoroughly described, the relationship between atmospheric water and ecosystem services has been mostly neglected, and perhaps misunderstood. Recent advances in land-atmosphere modeling have revealed the importance of terrestrial ecosystems for moisture recycling. In this paper, we analyze the extent to which vegetation sustains the supply of atmospheric moisture and precipitation for downwind beneficiaries, globally. We simulate land-surface evaporation with a global hydrology model and track changes to moisture recycling using an atmospheric moisture budget model, and we define vegetation-regulated moisture recycling as the difference in moisture recycling between current vegetation and a hypothetical desert world. Our results show that nearly a fifth of annual average precipitation falling on land is from vegetation-regulated moisture recycling, but the global variability is large, with many places receiving nearly half their precipitation from this ecosystem service. The largest potential impacts for changes to this ecosystem service are land-use changes across temperate regions in North America and Russia. Likewise, in semi-arid regions reliant on rainfed agricultural production, land-use change that even modestly reduces evaporation and subsequent precipitation, could significantly affect human well-being. We also present a regional case study in the Mato Grosso region of Brazil, where we identify the specific moisture recycling ecosystem services associated with the vegetation in Mato Grosso. We find that Mato Grosso vegetation regulates some internal precipitation, with a diffuse region of benefit downwind, primarily to the south and east, including the La Plata River basin and the megacities of Sao Paulo and Rio de Janeiro. We synthesize our global and regional results into a generalized framework for describing moisture recycling as an ecosystem service. We conclude that future work ought to disentangle whether and how this vegetation-regulated moisture recycling interacts with other ecosystem services, so that trade-offs can be assessed in a comprehensive and sustainable manner.  相似文献   
3.
4.
5.
Abstract. The pupal stage of ten Coelometopini species occurring in Australia, New Guinea, Southeast Asia and the Pacific region are described and a key for their identification is provided. The species are Chrysopeplus expolitus Broun, Derosphaerus hirtipes Kaszab, Hypaulax crenata (Boisduval), Leprocaulus borneensis Kaszab, Metisopus purpureipennis Bates, Promethis carteri Kaszab, P. nigra (Blessig), P. quadraticollis (Gebien), P. quadricollis Pascoe and P. sulcigera (Boisduval). The gin trap structures of D. hirtipes and P. quadraticollis are described in detail using scanning electron micrographs. A summary of antipredator structures of all known Coelometopini pupae is given. The phylogenetic value of pupal characters is assessed at intra‐ and intergeneric levels within the tribe.  相似文献   
6.
In mature rat Leydig cells, the testosterone output (24 ng/10(6) Leydig cells/4hrs.) is increased 10 fold by LH; the addition of serum from either control or castrated or hypophysectomized rams inhibits (60%) the LH-stimulated testosterone production. Similarly, the incubation of immature rat Leydig cells with sera from hypophysectomized patients leads to a diminution (70 and 30% respectively) of both basal (0.98 ng) and LH stimulated (3.44 ng) testosterone biosynthesis. These data suggest the existence of an LH inhibitor (or inhibitors) in blood from ram and human; in addition, this substance is not only of testicular origin and is not an LH-related molecule.  相似文献   
7.
Six female and six male adult rhesus macaques were given sticks and nylon balls as an attempt at simple cage enrichment. A latin square design was used to compare behavior during separate 4-week periods with each object and during a control period with no object. Frequency and duration of 15 different behaviors were recorded. Resting was the most common activity which decreased slightly in duration when the stick or nylon ball was present (P less than 0.02). The mean duration of stick use was longer than that of the nylon ball (P less than 0.01). No other behaviors changed significantly, including the frequency of abnormal behaviors such as self-abuse, stereotypic acts, and bizarre postures. Generally, these objects were used infrequently and led to few changes in the behavior of singly-caged adult rhesus macaques. However, they did appear to stimulate activity for some individuals.  相似文献   
8.
The survival, development and mycorrhizal efficiency of a selected strain of Laccaria bicolor along with naturally occurring ectomycorrhizal fungi in a young plantation of Douglas fir was examined. Symbionts were identified and their respective colonization abilities were determined. Eight species of symbiotic fungi, which may have originated in adjacent coniferous forests, were observed on the root systems. Mycorrhizal diversity differed between inoculated (5 taxa) and control (8 taxa) seedlings. Ectomycorrhizal fungi which occurred naturally in the nursery on control seedlings (Thelephora terrestris and Suillus sp.) did not survive after outplanting. Both inoculated and naturally occurring Laccaria species, as well as Cenococcum geophilum, survived on the old roots and colonized the newly formed roots, limiting the colonization by other naturally occurring fungi. Other fungi, such as Paxillus involutus, Scleroderma citrinum and Hebeloma sp. preferentially colonized the old roots near the seedling's collar. Russulaceae were found mainly in the middle section of the root system. Mycorrhizal colonization by Laccaria species on inoculated seedlings (54%) was significantly greater than on controls (13%) which were consequently dominated by the native fungi. Significant differences (up to 239%) were found in the growth of inoculated seedlings, especially in root and shoot weight, which developed mainly during the second year after outplanting. Seedling growth varied with the species of mycorrhizae and with the degree of root colonization. Competitiveness and effectiveness of the introduced strain on improving growth performances of seedlings are discussed.  相似文献   
9.
The in vivo metabolism of 12-(S)-Hydroxy-eicosatetraenoic acid (12-HETE), the end-lipoxygenase product of arachidonic acid in platelets, has been investigated in the rat. Fifty microcuries of 5,6-[3H]-12-HETE (50 Ci/mmol) were injected to anesthetized rats and the radioactivity was followed in plasma. At the end of the experiment, various organs of the animal were removed and the radioactivity attached to them was determined. The label of the plasma plateaued to approximately one third of the initial radioactivity ten minutes after the injection. Among the various organs tested (brain, heart, intestine, kidney, liver, lungs, spleen, testis/uterus) the kidney was far the most active to accumulate 12-HETE and/or its labeled metabolites, and no radioactivity could be detected in urine during the course of the experiment. The analysis of lipid extracts from the various tissues revealed that 12-HETE was not accumulating in its unesterified form but was likely bound to phospholipids. We conclude that, although the label providing from the initial 12-HETE did not completely disappear from plasma, circulating 12-HETE cannot be considered as a circulating marker of cell activation.  相似文献   
10.
The estrous cycle of 16 mature mongrel female dogs was monitored to evaluate the accuracy of teasing, vaginal cytology and quantitative ELISA progesterone assay to determine ovulation. The dogs were presented to male, and blood samples and vaginal swabs were taken daily during proestrus and estrus. Selected serum samples collected during estrus were assayed for endogenous LH by radioimmunoassay (RIA). Plasma samples collected during proestrus and estrus were assayed for progesterone with a commercially avialable ELISA kit. Ovulation was considered to take place 48 h after the preovulatory LH peak. Vaginal cytology smears were stained with Wright's stain and evaluated for the percentage of superficial squamous cells. Day 1 of diestrus (Day 1) was defined as a drop of 20% or more in the total number of superficial cells. Two standard curves (linear and best fitted curves) commonly used with ELISA were compared together and with the RIA progesterone assay. Ovulation was estimated to occur when progesterone concentration was 4.9 +/- 1.0ng/ml (mean +/- SD, n = 15), with a range of 3.4 to 6.6 ng/ml. Based on vaginal cytology, ovulation took place 6.9 +/- 1.6 d (n = 15) after 80% of the squamous cells were superficial and 6.8 +/- 1.4 d (n = 16) before Day 1. Ovulation took place 2.1 +/- 3.9 d (n=11) after the first day of standing estrus and 8.8 +/- 1.5 d (n = 10) before the last day of receptivity. The two standard curves were found parallel to each other and to the RIA progesterone assay. Based on the results of the present study, ELISA progesterone assay and determination of the first day of estrus by vaginal cytology are reliable methods for predicting ovulation, whereas the last day of receptivity as determined by teasing and Day 1 as determined by vaginal cytology are reliable methods to retrospectively estimate ovulation time.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号