全文获取类型
收费全文 | 994753篇 |
免费 | 108332篇 |
国内免费 | 1684篇 |
专业分类
1104769篇 |
出版年
2018年 | 9419篇 |
2017年 | 8784篇 |
2016年 | 12633篇 |
2015年 | 17577篇 |
2014年 | 20312篇 |
2013年 | 28871篇 |
2012年 | 32141篇 |
2011年 | 32541篇 |
2010年 | 22115篇 |
2009年 | 20169篇 |
2008年 | 28763篇 |
2007年 | 29373篇 |
2006年 | 27761篇 |
2005年 | 26722篇 |
2004年 | 26573篇 |
2003年 | 25494篇 |
2002年 | 24634篇 |
2001年 | 44432篇 |
2000年 | 44675篇 |
1999年 | 35592篇 |
1998年 | 12840篇 |
1997年 | 13409篇 |
1996年 | 12650篇 |
1995年 | 11705篇 |
1994年 | 11422篇 |
1993年 | 11240篇 |
1992年 | 29293篇 |
1991年 | 28567篇 |
1990年 | 27779篇 |
1989年 | 27173篇 |
1988年 | 25171篇 |
1987年 | 23811篇 |
1986年 | 22030篇 |
1985年 | 22003篇 |
1984年 | 18347篇 |
1983年 | 15415篇 |
1982年 | 11791篇 |
1981年 | 10660篇 |
1980年 | 9963篇 |
1979年 | 16867篇 |
1978年 | 13149篇 |
1977年 | 12071篇 |
1976年 | 10989篇 |
1975年 | 12268篇 |
1974年 | 13212篇 |
1973年 | 13056篇 |
1972年 | 11708篇 |
1971年 | 10706篇 |
1970年 | 9309篇 |
1969年 | 9021篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
The gastroenteropancreatic (GEP) endocrine system of bowfin (Amia calva) was described using light and electron microscopy and immunological methods. The islet organ (endocrine pancreas) consists of diffusely scattered, mostly small islets and isolated patches of cells among and within the exocrine acini. The islets are composed of abundant, centrally located B cells immunoreactive to bovine and lamprey insulin antisera and D cells showing a widespread distribution and specificity to somatostatin antibodies. A and F cells are present at the very periphery of the islets and are immunoreactive with antisera against glucagon (and glucagon-like peptide) and several peptides of the pancreatic polypeptide (PP)-family, respectively. The peptides of the two families usually collocates within the same peripheral islet cells and are the most common immunoreactive peptides present in the extra-islet tissue. Immunocytochemistry and fine structural observations characterised the granule morphology for B and D cells and identified two cell types with granules immunoreactive to glucagon antisera. These two putative A cells had similar granules, which were distinct from either B or D cells, but one of the cells had rod-shaped cytoplasmic inclusions within cisternae of what appeared to be rough endoplasmic reticulum. The inclusions were not immunoreactive to either insulin or glucagon antisera. Only small numbers of cells in the stomach and intestine immunoreacted to antisera against somatostatin, glucagon, and PP-family peptides. The paucity of these cells was reflected in the low concentrations of these peptides in intestinal extracts. The GEP system of bowfin is not unlike that of other actinopterygian fishes, but there are some marked differences that may reflect the antiquity of this system and/or may be a consequence of the ontogeny of this system in this species. 相似文献
112.
113.
114.
115.
116.
Abstract: The effect of timing of application on the incompatibility of three fungicides (metalaxyl, mancozeb, copper oxide) and isolate MK2001 of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin was assessed in vitro . Isolate MK 2001 at 1 × 107 conidia/ml concentration was highly pathogenous to adults of Lygus lineolaris (Hemiptera) (84% mortality at day 4 post-treatment). The fungicides at the manufacturer recommended rate (1X) were strongly fungistatic and inhibited the fungal isolate radial growth on Sabouraud dextrose agar (SDA). Mancozeb, metalaxyl and copper oxide exhibited insecticidal activity of 24, 40 and 48% respectively, on L. lineolaris adults. It is clear that application of the fungal isolate MK2001 2–4 days before applying fungicides metalaxyl, mancozeb or copper oxide synergized the insecticidal effect of the isolate. On the contrary, application of metalaxyl, mancozeb, copper oxide 2–4 days before applying isolate MK2001 antagonized the insecticidal effect. The simultaneous use of each fungicide (metalaxyl, mancozeb or copper oxide) and the isolate gave lesser insect mortality. This study revealed that although some fungicides are incompatible for use alongside fungal isolates, the proper evaluation of their time of use could be beneficial in biological control or IPM programmes. Furthermore, the application of B. bassiana isolate MK2001 followed by fungicide application could synergize insecticidal activity. 相似文献
117.
118.
In temperate climates, Pustula tragopogonis is rarely found on cultivated sunflower. In Europe, it was so far of little economic impact on other Asteraceae, except for some regions in the Mediterranean. In 2003, P. tragopogonis was found for the first time in sunflower fields in southern Germany. The pathogen has a widespread occurrence there, especially in the region around Stuttgart, BW. Fatty acid profiling, ultrastructural investigation and ITS sequencing revealed a high similarity to an 2002 isolate from southern Africa and an 2005 isolate from Australia, but revealed significant differences to P. tragopogonis s.l. on Cirsium arvense, a common weed, growing on or in the vicinity of sunflower fields in Germany. P. tragopogonis from this host can therefore be excluded from being the source of the reported infection. 相似文献
119.
Large numbers of European ash have died in Poland in all age classes during the last ten years. The characteristic symptom occurring on shoots of planted and self‐sown seedlings was bark necroses starting from the shoot apex, necrotic buds, or leaf and twig scars. The results showed that in the bud tissue of cold acclimated European ash extracellular and intracellular ice formation occurred at approximately ?9 and ?32°C, respectively. In deacclimated plants in spring water supercooling is limited by the heterogenous ice nucleation temperature and consequently the cold tolerance is ?9 to ?4°C for bud tissues and ?13 to ?9°C for shoots. Isolations of fungi were performed from dead buds and from necroses occurring on the main stem. Alternaria alternata, Fusarium lateritium and Phomopsis scobina were among the fungi occurring in both these organs at frequencies of more than 7%. Cylindrocarpon heteronemum, Diplodia mutila and Tubercularia vulgaris from necroses were only isolated in frequencies; 3.3, 1.2 and 5.4%, respectively. It seems likely that freezing injury is the inciting factor, which combined with fungal colonization manifests itself as fatal damage to European ash buds and shoots. 相似文献
120.
Beavers Castor spp. are generalist herbivores, feeding on the bark, shoots and leaves of woody plants, terrestrial herbs and forbs, ferns and aquatic vegetation. As central-place foragers, beavers move out from water to select and cut trees and vegetation, and then transport it back to their refuge. These terrestrial forays are energetically costly; therefore, beavers should concentrate their foraging activity near the central place and increase the degree of selectivity for specific sizes or species of food with increasing distance from the water. The aim of this study was to test the predictions of the central place and the optimal foraging theories for the food selection of the Eurasian beaver Castor fiber , and show the foraging preferences of the focal species in the boreal conifer forest zone of Europe. Foraging intensity by beavers and the abundance of woody species were surveyed in transects positioned randomly at seven beaver territories. In compliance with the central-place foraging theory, the foraging intensity declined with increasing distance from the river. Beavers fed preferentially on willows ( Salix ), rowan ( Sorbus ) and birches ( Betula ), although alders ( Alnus ) dominated their diet. Size selectivity showed similar patterns to previous North American studies, which were also carried out in habitats with predominantly small saplings. The probability of selection of small saplings dropped as distance increased, which is consistent with the predictions of optimal foraging models that larger prey items are more likely to be favoured with increasing provisioning distance. 相似文献