全文获取类型
收费全文 | 2320篇 |
免费 | 146篇 |
国内免费 | 3篇 |
专业分类
2469篇 |
出版年
2023年 | 16篇 |
2022年 | 21篇 |
2021年 | 37篇 |
2020年 | 30篇 |
2019年 | 32篇 |
2018年 | 39篇 |
2017年 | 30篇 |
2016年 | 80篇 |
2015年 | 113篇 |
2014年 | 140篇 |
2013年 | 162篇 |
2012年 | 193篇 |
2011年 | 209篇 |
2010年 | 108篇 |
2009年 | 103篇 |
2008年 | 143篇 |
2007年 | 152篇 |
2006年 | 132篇 |
2005年 | 127篇 |
2004年 | 78篇 |
2003年 | 112篇 |
2002年 | 83篇 |
2001年 | 7篇 |
2000年 | 9篇 |
1999年 | 6篇 |
1998年 | 20篇 |
1997年 | 19篇 |
1996年 | 10篇 |
1995年 | 10篇 |
1994年 | 8篇 |
1993年 | 18篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 12篇 |
1989年 | 12篇 |
1988年 | 12篇 |
1987年 | 12篇 |
1986年 | 12篇 |
1985年 | 10篇 |
1984年 | 9篇 |
1983年 | 12篇 |
1982年 | 12篇 |
1981年 | 6篇 |
1980年 | 10篇 |
1979年 | 17篇 |
1978年 | 12篇 |
1977年 | 6篇 |
1975年 | 6篇 |
1971年 | 5篇 |
1969年 | 6篇 |
排序方式: 共有2469条查询结果,搜索用时 15 毫秒
21.
22.
Summary At the Mount Athos the Capercaillie is spread in the montane high forests at an altitude between 1140 m and 1340 m above sea level. This isolated occurrence at the southern boundary of the species area and 140 km to the south of the breeding place in the Rhodope Mountains is the southernmost occurrence recorded so far. In former times, the Capercaillie presumably was widespread in the mountain forests of Central and Northern Greece. 相似文献
23.
Jochen Steinmann 《Applied microbiology》1981,41(4):1043-1045
For detection of rotavirus, domestic sewage was concentrated by two different methods: (i) adsorption to and elution from positively charged Seitz filters, followed by ultracentrifugation, and (ii) chemical precipitation. The concentrated fluids were tested by an enzyme-linked immunosorbent assay and electron microscopy. In 6 of 24 (25%) samples, rotavirus was detectable after the combined filtration and ultracentrifugation technique with both an enzyme-linked immunosorbent assay and electron microscopy. No positive results were obtained after chemical precipitation. 相似文献
24.
In a carrot (Daucus carota L.) cell line lacking the ability to undergo somatic embryogenasis, and in carrot and anise (Pimpinella anisum L.) cell lines in which embryogenesis could be regulated by presence or absence of 2,4-dichlorophen-oxyacetic acid (2,4-D), in the medium (+2,4-D=no embryogenesis,-2,4-D=embryo differentiation and development), the levels of endogenous gibberellin(s) (GA) were determined by the dwarfrice bioassay, and the metabolism of [3H]GA1 was followed. Embryos harvested after 14 d of subculture in-2,4-D had low levels (0.2–0.3 g g-1 dry weight) of polar GA (e.g. GA1-like), but much (3–22 times) higher levels of less-polar GA (GA4/7-like); GA1, GA4 and GA7 are native to these cultures. Conversely, the undifferentiated cells in a non-embryogenic strain, and proembryos of an embryogenic strain (+2,4-D) showed very high levels of polar GA (2.9–4.4 g g-1), and somewhat reduced levels of less-polar GA. Cultures of anise undergoing somatic embryo development (-2,4-D) metabolized [3H]GA1 very quickly, whereas proembryo cultures of anise (+2,4-D) metabolized [3H]GA1 slowly. The major metabolites of [3H]GA1 in anise were tentatively identified as GA8-glucoside (24%), GA8 (15%), GA1-glucoside (8%) and the 1(10)GA1-counterpart (2%). Thus, high levels of a GA1-like substance and a reduced ability to metabolize GA1 are correlated with the absence of embryo development, while lowered levels of GA1-like substance and a rapid metabolism of GA1 into GA8 and GA-conjugates are correlated with continued embryo development. Exogenous application of GA3 is known to reduce somatic embryogenesis in carrot cultures; GA4 was found to have the same effect in anise cultures. Thus, a role (albeit negative) in somatic embryogenesis for a polar, biologically active GA is implied.Abbreviations 2,4-D
2,4-dichlorophenoxyacetic acid
- GA
gibberellin(s) or gibberellin-like substances
- GC-RC
gas chromatography-radiochromatogram counting
- HPLC
high-presare liquid chromatography
- Rt
retention time
- TLC
thinlaver chromatography 相似文献
25.
Jochen Martens 《Zoomorphology》1979,92(1):77-93
Zusammenfassung Alle Arten der Sironidae tragen als sekundäres männliches Geschlechtsmerkmal ein epidermales Drüsenorgan im letzten Tarsalglied des 4. Laufbeines. Das Sekret tritt dort dorsal, exponiert auf einer kanülenartigen Apophyse, dem Adenostyl, nach außen.BeiSiro duricorius (Joseph, 1868) besteht das komplexe Drüsenorgan aus zahlreichen funktionellen Einheiten, deren Sekret einem gemeinsamen Kanal zugeführt wird; er öffnet sich auf dem Adenostyl. Jede funktionelle Drüseneinheit umfaßt 5 Zellen: 3 Drüsenzellen (DZ, Dreiergruppe), 1 Hüllzelle (HZ), 1 Kanalzelle (KZ). Die Sekrete werden in ein gemeinsames Reservoir abgegeben.HZ verknüpft manschettenartig die DZ mit der KZ. Eine Randeinfaltung am Apex der DZ gewährleistet innige Verfalzung von diesen mit der HZ.DZ und HZ sind mit gut entwickeltem Mikrovillisaum ausgestattet; beide Zelltypen sezernieren.Der ableitende Kanal der GZ besteht aus 2 Abschnitten: ein distaler schwammig-fibrillärer und ein proximaler massiver; beide Teile werden von nur einer Zelle (KZ) begleitet. Der basale Teil mündet in den Hauptkanal, der zum Adenostyl zieht.Neben diesem komplexen Drüsenorgan liegt ein ähnlich gebautes, aber kleineres ventral im Tarsus; sein Sammelkanal zieht zum Kanal der Hauptdrüse.Isolierte funktionelle Einheiten sind in der Tarsenepidermis weit verstreut; sie leiten ihr Sekret unmittelbar auf die Tarsusoberfläche.Die DZ produzieren nach elektronenoptischer Evidenz ein Protein. Das Sekret der HZ läßt sich bisher nicht zuordnen; es ist von dem der DZ verschieden.
Abkürzungen A Adenostyl - aSe austretendes Sekret - C Kutikulin - Cu Kutikula - dK distaler Kanal - DK Kern der Drüsenzelle - DZ Drüsenzelle - ER rauhes endoplasmatischesm, Retikulum - G Golgi-Apparat - HK Kern der Hüllzelle - HZ Hüllzelle - K Kralle des Tarsus - Ke Kern - KK Kern der Kanalzelle - KZ Kanalzelle - M Mikrovilli - Mi Mitochondrium - pK proximaler Kanal - R Randeinfaltung der DZ - Re extrazelluläres Reservoir - Ri Ribosomen - Se Sekret - SK zentraler Sammelkanal - T Mikrotubuli - Va Vakuole - Va1-Va6 Vakuolen mit Sekret unterschiedlichen Reifegrades 相似文献
Ultrastructure of the tarsal gland ofSiro duricorius (Joseph)
Summary All species of Sironidae possess as a secondary male sex character an epidermal gland organ in the last tarsal joint of the fourth leg. The glandular secretion emanates from a conical apophysis, the adenostyl, which is located dorsally on the joint.InSiro duricorius (Joseph, 1868) the complex organ consists of numerous functional units, the secretion of which flows into the common channel opening on the adenostyl.Every gland unit contains 5 cells: 3 gland cells (DZ), 1 enveloping cell (HZ), 1 duct cell (KZ). The enveloping cell connects the gland cells with the duct cell like a sleeve. A marginal fold of the gland cell apex provides tight connection of gland cells and enveloping cell.Gland cells and enveloping cell are equipped with microvilli, and both cell types secrete in a common reservoir.The duct of the duct cell is divided into two parts: 1) a distal fibrillaceous-spongy one and 2) a proximal massive one. Both are accompanied only by a single duct cell.Besides this complex gland organ a similar one is located in the ventral part of the tarsus. Its collecting channel opens into the channel of the main gland.Isolated functional gland units are scattered over the epidermis of the tarsus; their secretion flows immediately on the tarsal surface.The gland cells proper produce a protein, the fact is indicated by the electronmicrographs. The secretion of the enveloping cell is a different one, but at this time cannot be attached to a chemical group.
Abkürzungen A Adenostyl - aSe austretendes Sekret - C Kutikulin - Cu Kutikula - dK distaler Kanal - DK Kern der Drüsenzelle - DZ Drüsenzelle - ER rauhes endoplasmatischesm, Retikulum - G Golgi-Apparat - HK Kern der Hüllzelle - HZ Hüllzelle - K Kralle des Tarsus - Ke Kern - KK Kern der Kanalzelle - KZ Kanalzelle - M Mikrovilli - Mi Mitochondrium - pK proximaler Kanal - R Randeinfaltung der DZ - Re extrazelluläres Reservoir - Ri Ribosomen - Se Sekret - SK zentraler Sammelkanal - T Mikrotubuli - Va Vakuole - Va1-Va6 Vakuolen mit Sekret unterschiedlichen Reifegrades 相似文献
26.
Ken-ichi Asahi Jochen Born Heinz Tiedemann Hildegard Tiedemann 《Development genes and evolution》1979,187(3):231-244
Summary A highly purified vegetalizing factor induces endoderm preferentially in amphibian gastrula ectoderm. After combination of this factor with less pure fractions, a high percentage of trunks and tails with notochord and somites are induced. The induction of these mesodermal tissues depends on secondary factors which may act on plasma membrane receptors of the target cells. The secondary factors are probably proteins as they are inactivated by trypsin or cellulose-bound proteinase K. They are not inactivated by thioglycolic acid.The implication of these findings for tissue determination and differentiation in normal development in relation to the anlageplan for endoderm and mesodermal tissues is discussed. 相似文献
27.
Jochen Martens 《Journal of Ornithology》1975,116(4):369-433
Zusammenfassung Die fünf Formen der UntergattungParus (Periparus) des südlichen Zentral-Asien(rufonuchalis, rubidiventris, beavani, melanolophus, aemodius) werden morphologisch, biologisch-ökologisch und vor allem akustisch differenziert. Die Spezifität der Gesänge gilt als wesentliches Art-Kriterium; sie wird mit Attrappen-Versuchen getestet.1.Rufonuchalis, rubidiventris undbeavani (Fichtenmeisen-Gruppe) bilden im Nepal-Himalaya Arealgrenzen aus.Rufonuchalis undrubidiventris leben dort sympatrisch,rubidiventris undbeavani allopatrisch ohne Introgressionszone.
Rubidiventris undbeavani haben übereinstimmende ökologische Ansprüche;rufonuchalis als größere und trockenadaptierte Form weicht merklich ab. Die Arealgrenzen vonrufonuchalis undrubidiventris sind im westlichen Nepal-Himalaya klimatisch bedingt (Monsun!), nicht durch Konkurrenz mit anderenPeriparus-Formen.Reviergesänge der farblich fast ununterscheidbarenrufonuchalis undbeavani divergieren sehr, die vonrubidiventris undbeavani sind gleich.
Rufonuchalis verfügt über 2 Gesangstypen: Trillergesang dient der Reviermarkierung, Pfiffgesang der Revierverteidigung. Beide Gesangsformen entsprechen nicht demParus-Gesangsschema, wohl aber der Klappergesang vonrubidiventris undbeavani.
Beavani reagiert im Attrappen-Test nur auf denrufonuchalis-Pfiffgesang und besitzt vergleichbare Lautäußerungen.Rufonuchalis-Trillergesang findet beibeavani ebenfalls Entsprechungen, jedoch nicht beirubidiventris.Spezialisierte Trillergesang-Elemente vonrufonuchalis undbeavani werden als homolog betrachtet. Bei diesen allopatrischen Formen konnten sie erhalten bleiben, der mitrufonuchalis sympatrischerubidiventris hat sie reduziert.Die gleichfarbigenrufonuchalis undbeavani haben das Himalaya-System nie kontinuierlich besiedelt; ihre Areale standen ursprünglich über die Waldzonen Zentralasiens miteinander in Verbindung.Akustische Analyse und Verbreitungsmuster weisenrufonuchalis undrubidiventris (mit den Subspeziesrubidiventris undbeavani) als eigene Spezies aus; sie werden zu einer Superspezies verklammert.2. Die Areale vonP. ater aemodius undP. melanolophus stehen im zentralen Himalaya in sekundärem Kontakt; sie bilden dort eine Hybrid-(Introgressions-)Zone von 70 km Breite.An keiner Stelle desaemodius- bzw.melanolophus-Areals kommen die beiden Ausgangsformen bzw. Hybriden und Ausgangsformen nebeneinander vor. Merkliche Zuwanderung in das Hybridgebiet aus östlichen oder westlichen Arealteilen findet nicht statt.Es existieren Hybriden, die nicht intermediär zwischen den Ausgangsformen stehen, sondern von ihnen in Farbmuster und Färbung markant abweichen (rostbäuchige Hybriden).Vom Gebiet reineraemodius-Phäne zu dem reinermelanolophus-Phäne verläuft eine Merkmalsprogression:aemodius-Beige der Unterseite zu Rostrot;aemodius-Beige der Flanken zumelanolophus-Grau. Rostrot der Unterseite schlägt fast ohne Übergang zumelanolophus-Grau um (bisher nur für eine Population belegt).Alle Populationen des Hybridgürtels sind voll fortpflanzungsfähig (Reviergesang, Revierabgrenzung, -Brutfleck, -Gonadenentwicklung). Die Ausgangsformen haben (noch) keine verschiedenen adaptiven Niveaus erreicht; die Hybriden sind bei der Paarbildung (offensichtlich) nicht unterlegen.Isolationsmechanismen zwischen Ausgangsformen und Hybriden sind (heute) nicht (mehr) entwickelt; füraemodius undmelanolophus werden gering entwickelte akustische (und optische) postuliert (Hinweise durch Hybridisation in Gefangenschaft).Rostfarbigkeit wird als altes Merkmal derPeriparus-Gruppe angesehen, sein Auftreten bei den Hybriden als Atavismus.Reviergesänge aus derater/melanolophus-Gruppe sind einander sehr ähnlich. Der Gesang mitteleuropäischera. ater und der vonmelanolophus zeichnet sich durch ansteigende Elemente aus, diea. aemodius fehlen. Auf solche Gesangstypen reagiertaemodius nur mangelhaft.Die Gesänge in den Hybridpopulationen gleichen jenen vonaemodius; das wird auf Lernen desaemodius-Repertoires der Hybriden während des ersten Kontaktes vonmelanolophus undaemodius zurückgeführt.Süddeutschea. ater reagieren nur zur Zeit höchster Revierverteidigungs-Bereitschaft auf einen bestimmtenaemodius-Strophentyp. Auf diesen Strophentyp reagierten auch syntop lebendeP. major; er enthält Merkmale der Gesänge beider Arten.Die genetische Divergenz vonaemodius undmelanolophus — kenntlich an den rostbäuchigen Hybriden — legt nahe, den Artstatus beider bedingt aufrechtzuerhalten: SuperspeziesP. ater mit den SemispeziesP. ater undP. melanolophus.
Mit einem Jahresstipendium des Deutschen Akademischen Austauschdienstes und einer Sachbeihilfe der Deutschen Forschungsgemeinschaft. — Ergebnisse der Nepal-Reisen 1969/70, 1973 und 1974, Nr. 36. — Nr. 35: Senckenbergiana biol., 56 (4/6): 247–256, 1975. 相似文献
Acoustic differentiation of phylogenetic relations in theParus (Periparus) group from investigations in the Nepal Himalayas
Summary In the Nepal Himalayas 5 taxa of the subgenusParus (Periparus) exist — the greatest concentration of this subgenus in the whole Palearctic region. The taxa belong to 2 groups: 1.rufonuchalis, rubidiventris, beavani; 2. aemodius, melanolophus (fig. 1). All forms settle in the same altitude belt from about 2800 to 4200 m — three of them even in the same locality.Rufonuchalis is adapted to dry habitats mainly of West Central Asia and is thus restricted in Nepal to the Western parts (fig. 3, 4).The up to now unclarified status of all forms is determined as follows: Distinct species areP. rufonuchalis andP. rubidiventris (with highly different subspeciesP. r. rubidiventris andP. r. beavani) and they are regarded as a superspecies.P. rufonuchalis andP. r. rubidiventris live syntopically in West Nepal and display different territorial songs.Both differ remarkably in body size and bill size. Certain acoustic similarities indicating close relationship could be shown betweenrufonuchalis and the allopatric (fig. 3, 4)rubidiventris beavani; r. beavani reacts to the playback of whistle song (Pfiffgesang, fig. 10) ofP. rufonuchalis (see also fig. 14).The areas ofP. r. rubidiventris andP. r. beavani are separated apparently only by the Bhote Kosi river in East Nepal (fig. 4). Introgression is not (yet) known; the songs are identical (fig. 8). P. melanolophus andP. ater aemodius hybridize in West Nepal in a zone of secondary contact. Their areas are linked by a zone of introgression (fig. 17, 18). In that zone cinnamomeous bellied hybrids locally occur differing thus strikingly in colour from both parental species (fig. 1). Sympatric occurrence ofmelanolophus andater aemodius is not known and is not expected either.Territorial songs ofa. aemodius (fig. 21) andmelanolophus (fig. 25) are very similar and both species obviously do not differ in their ecology. Despite the small introgression belt, similar songs and apparently very similar or even identical adaptive peaks,P. ater andP. melanolophus are still treated as distinct species but reduced to semispecies level. In doing so the genetic divergence betweenater aemodius andmelanolophus indicated by the cinnamomeous bellied hybrids is emphasized.
Mit einem Jahresstipendium des Deutschen Akademischen Austauschdienstes und einer Sachbeihilfe der Deutschen Forschungsgemeinschaft. — Ergebnisse der Nepal-Reisen 1969/70, 1973 und 1974, Nr. 36. — Nr. 35: Senckenbergiana biol., 56 (4/6): 247–256, 1975. 相似文献
28.
How does landscape context contribute to effects of habitat fragmentation on diversity and population density of butterflies? 总被引:20,自引:0,他引:20
Aim Studies on habitat fragmentation of insect communities mostly ignore the impact of the surrounding landscape matrix and treat all species equally. In our study, on habitat fragmentation and the importance of landscape context, we expected that habitat specialists are more affected by area and isolation, and habitat generalists more by landscape context. Location and methods The study was conducted in the vicinity of the city of Göttingen in Germany in the year 2000. We analysed butterfly communities by transect counts on thirty‐two calcareous grasslands differing in size (0.03–5.14 ha), isolation index (2100–86,000/edge‐to‐edge distance 55–1894 m), and landscape diversity (Shannon–Wiener: 0.09–1.56), which is correlated to percentage grassland in the landscape. Results A total of 15,185 butterfly specimens belonging to fifty‐four species are recorded. In multiple regression analysis, the number of habitat specialist (n = 20) and habitat generalist (n = 34) butterfly species increased with habitat area, but z‐values (slopes) of the species–area relationships for specialists (z = 0.399) were significantly steeper compared with generalists (z = 0.096). Generalists, but not specialists, showed a marginally significant increase with landscape diversity. Effects of landscape diversity were scale‐dependent and significant only at the smallest scale (landscape context within a 250 m radius around the habitat). Habitat isolation was not related to specialist and generalist species numbers. In multiple regression analysis the density of specialists increased significantly with habitat area, whereas generalist density increased only marginally. Habitat isolation and landscape diversity did not show any effects. Main conclusions Habitat area was the most important predictor of butterfly community structure and influenced habitat specialists more than habitat generalists. In contrast to our expectations, habitat isolation had no effect as most butterflies could cope with the degree of isolation in our study region. Landscape diversity appeared to be important for generalist butterflies only. 相似文献
29.
Ludmilla Figueiredo Jochen Krauss Ingolf Steffan‐Dewenter Juliano Sarmento Cabral 《Ecography》2019,42(12):1973-1990
Extinction debt refers to delayed species extinctions expected as a consequence of ecosystem perturbation. Quantifying such extinctions and investigating long‐term consequences of perturbations has proven challenging, because perturbations are not isolated and occur across various spatial and temporal scales, from local habitat losses to global warming. Additionally, the relative importance of eco‐evolutionary processes varies across scales, because levels of ecological organization, i.e. individuals, (meta)populations and (meta)communities, respond hierarchically to perturbations. To summarize our current knowledge of the scales and mechanisms influencing extinction debts, we reviewed recent empirical, theoretical and methodological studies addressing either the spatio–temporal scales of extinction debts or the eco‐evolutionary mechanisms delaying extinctions. Extinction debts were detected across a range of ecosystems and taxonomic groups, with estimates ranging from 9 to 90% of current species richness. The duration over which debts have been sustained varies from 5 to 570 yr, and projections of the total period required to settle a debt can extend to 1000 yr. Reported causes of delayed extinctions are 1) life‐history traits that prolong individual survival, and 2) population and metapopulation dynamics that maintain populations under deteriorated conditions. Other potential factors that may extend survival time such as microevolutionary dynamics, or delayed extinctions of interaction partners, have rarely been analyzed. Therefore, we propose a roadmap for future research with three key avenues: 1) the microevolutionary dynamics of extinction processes, 2) the disjunctive loss of interacting species and 3) the impact of multiple regimes of perturbation on the payment of debts. For their ability to integrate processes occurring at different levels of ecological organization, we highlight mechanistic simulation models as tools to address these knowledge gaps and to deepen our understanding of extinction dynamics. 相似文献
30.
Llorenç Milà i Canals Christian Bauer Jochen Depestele Alain Dubreuil Ruth Freiermuth Knuchel Gérard Gaillard Ottar Michelsen Ruedi Müller-Wenk Bernt Rydgren 《The International Journal of Life Cycle Assessment》2007,12(1):5-15
Background, Aim and Scope
Land use by agriculture, forestry, mining, house-building or industry leads to substantial impacts, particularly on biodiversity
and on soil quality as a supplier of life support functions. Unfortunately there is no widely accepted assessment method so
far for land use impacts. This paper presents an attempt, within the UNEP-SETAC Life Cycle Initiative, to provide a framework
for the Life Cycle Impact Assessment (LCIA) of land use.
Materials and Methods:
This framework builds from previous documents, particularly the SETAC book on LCIA (Lindeijer et al. 2002), developing essential
issues such as the reference for occupation impacts; the impact pathways to be included in the analysis; the units of measure
in the impact mechanism (land use interventions to impacts); the ways to deal with impacts in the future; and bio-geographical
differentiation.
Results:
The paper describes the selected impact pathways, linking the land use elementary flows (occupation; transformation) and parameters
(intensity) registered in the inventory (LCI) to the midpoint impact indicators and to the relevant damage categories (natural
environment and natural resources). An impact occurs when the land properties are modified (transformation) and also when
the current man-made properties are maintained (occupation).
Discussion:
The size of impact is the difference between the effect on land quality from the studied case of land use and a suitable reference
land use on the same area (dynamic reference situation). The impact depends not only on the type of land use (including coverage
and intensity) but is also heavily influenced by the bio-geographical conditions of the area. The time lag between the land
use intervention and the impact may be large; thus land use impacts should be calculated over a reasonable time period after
the actual land use finishes, at least until a new steady state in land quality is reached.
Conclusions:
Guidance is provided on the definition of the dynamic reference situation and on methods and time frame to assess the impacts
occurring after the actual land use. Including the occupation impacts acknowledges that humans are not the sole users of land.
Recommendations and Perspectives:
The main damages affected by land use that should be considered by any method to assess land use impacts in LCIA are: biodiversity
(existence value); biotic production potential (including soil fertility and use value of biodiversity); ecological soil quality
(including life support functions of soil other than biotic production potential). Bio-geographical differentiation is required
for land use impacts, because the same intervention may have different consequences depending on the sensitivity and inherent
land quality of the environment where it occurs. For the moment, an indication of how such task could be done and likely bio-geographical
parameters to be considered are suggested. The recommendation of indicators for the suggested impact categories is a matter
of future research. 相似文献