全文获取类型
收费全文 | 502篇 |
免费 | 35篇 |
专业分类
537篇 |
出版年
2013年 | 6篇 |
2012年 | 5篇 |
2011年 | 9篇 |
2010年 | 16篇 |
2009年 | 8篇 |
2008年 | 9篇 |
2007年 | 10篇 |
2006年 | 9篇 |
2005年 | 5篇 |
2004年 | 5篇 |
2002年 | 5篇 |
1999年 | 7篇 |
1998年 | 9篇 |
1997年 | 8篇 |
1996年 | 5篇 |
1995年 | 10篇 |
1994年 | 10篇 |
1993年 | 9篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 9篇 |
1989年 | 13篇 |
1988年 | 7篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1983年 | 9篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 13篇 |
1978年 | 13篇 |
1977年 | 8篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1973年 | 6篇 |
1972年 | 8篇 |
1971年 | 7篇 |
1959年 | 12篇 |
1958年 | 19篇 |
1957年 | 22篇 |
1956年 | 24篇 |
1955年 | 31篇 |
1954年 | 13篇 |
1953年 | 18篇 |
1952年 | 14篇 |
1951年 | 11篇 |
1950年 | 9篇 |
1949年 | 10篇 |
1948年 | 8篇 |
排序方式: 共有537条查询结果,搜索用时 0 毫秒
1.
DENNIS P. GORDON 《Zoologica scripta》1993,22(2):203-221
Umbonuloid frontal shields arc described in the type species of Lepraliella, Drepanophora, Frurionella, Tessaradoma, Hincksipora, Stephanopora , and Pseudolepralia. Consequences to classification include the following: the family group names Lepralielloidea and Lepralicllidae have subjective priority over Umbonuloidea and Celleporariidae, respectively; Cylindroporeh is excluded from the Tessaradomidae and included in the Gigantoporidae; Tessarudoma bifax Cheetham is included in Srnithsonius (Bifaxariidae); Hincksiporidae is confirmed as a family of Lepralielloidea; Stephanoporu , with newly discovered dimorphic orifices, comprises two species which are the basis for a new exechonellid subfamily Stephanoporinae; and a new umbonulomorph superfamily, Pseudolepralioidea, is established for Pseudolepraliu (Pseudolepraliidae). Kladapheles gen.n., is established for an erect branching species of Lepraliellidae from New Zealand. 相似文献
2.
YORAM YOM-TOV SHLOMITH YOM-TOV GORDON JARRELL 《Biological journal of the Linnean Society. Linnean Society of London》2008,93(4):701-707
Using museum specimens, we studied recent changes in skull size of the American marten Martes americana , in continental Alaska. In Alaska, global warming has resulted in milder winters that may contribute to an improved food supply in the wild. In the present study, we tested the hypothesis that body size of the marten had increased during the second half of the 20th century, in response to global warming. We found that skull size, and by implication body size, increased significantly during the second half of the 20th century, possibly due to an improved food supply and/or lower metabolic demands in winter. Improved food availability in winter may result from the improved nutritional conditions for prey, and/or from increased access to prey resulting from a longer snow-free season. Longitude had a significant positive effect on skull size and a significant negative effect on teeth size. In Alaska, the climate is milder along the western coast and becomes harsher inland. Hence, the milder climate was associated with larger body size providing further support for our prediction that body size of the American marten was influenced by food availability and reduced energy expenditure. The negative relationship between longitude and teeth size may indicate a trend towards a larger prey in inland marten populations, but we have no data to support or refute this hypothesis. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 701–707. 相似文献
3.
The effects of repeated annual application of methiocarb-based slug pellets, broadcast on the soil surface and drilled into the seed bed, on carabid beetle activity were investigated over a four year period on a winter-sown cereal field using pitfall traps in barriered plots. Following applications in late autumn all winter-active carabid populations were severely depressed; total carabid activity falling to less than 5% and 10–15% following broadcast and drilled applications, respectively, compared with untreated plots. Spring and summer-active species, not active at the time of application, were largely unaffected by applications and were responsible for a gradual recovery of total activity from early spring onwards. Activity of all affected winter species remained demonstrably depressed on treated areas for the remainder of their seasonal incidence. However, all except one species, Bembidion obtusum, recovered to normal activity levels in the following season prior to reapplication. Recovery patterns are discussed in terms of the known biology of the species involved. Evidence that a minority of summer-active species were also affected by treatments, sometimes positively and sometimes negatively, were attributed to indirect effects possibly involving prey availability and foraging behaviour. The long-term ecological and short-term agronomic implications of methiocarb effects on carabid populations in winter-sown cereals are discussed. 相似文献
4.
High level of divergence of male-reproductive-tract proteins, between Drosophila melanogaster and its sibling species, D. simulans 总被引:1,自引:0,他引:1
We compared male-reproductive-tract polypeptides of Drosophila melanogaster
and D. simulans by using two-dimensional gel electrophoresis. Approximately
64% of male-reproductive-tract polypeptides were identical between two
randomly chosen isofemale lines from these two species, compared with 83%
identity for third-instar imaginal wing-disc polypeptides. Qualitatively
similar differences were found between reproductive tracts and imaginal
discs when D. sechellia was compared with D. melanogaster and with D.
simulans. When genic polymorphism was taken into account, approximately 10%
of male- reproductive-tract polypeptides were apparently fixed for
different alleles between D. melanogaster and D. simulans; this proportion
is the same as that found for soluble enzymes by one-dimensional gel
electrophoresis. Strikingly, approximately 20% of male-reproductive- tract
polypeptides of either D. melanogaster or D. simulans had no detectable
homologue in the other species. We propose that proteins of the Drosophila
male reproductive tract may have diverged more extensively between species
than have other types of proteins and that much of this divergence may
involve large changes in levels of polypeptide expression.
相似文献
5.
GORDON B. CURRY 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1983,16(2):119-127
Microborings in the primary shell layer of Recent brachiopods are clearly seen to avoid endopunctamicroscopic canals pervading the shell fabric and housing papillose extcnlions of the mantle (the caeca). This avoidance confirms the suggestion that the caecal contents inhibit boring organisms (Owen & Williams 1969; Proc. R. Soc. Loud. B, 172 ), and as such the caecum can be considered as an important instrument in protecting the brachiopod shell. A comparison of the relative fecundity of co-habitating impunctate and cndopunctate New Zealand brachiopods provides indirect evidence that the caecum may indeed also function in a nutrient storage capacity. Brachiopods, microborings, primary shell layer, endopuncta, defence, storage. 相似文献
6.
AVRAM HERSHKO PIERRE MAMONT ROBERT SHIELDS GORDON M. TOMKINS 《Nature: New biology》1971,232(33):206-211
A hypothesis has been developed to relate stringent control in bacteria to a set of interactions involved in the regulation of growth of transformed and untransformed mammalian cells. 相似文献
7.
The quantitatively major nonasaccharide (XG9) derived from xyloglucanby digestion with cellulase exhibits anti-auxin activity inthe pea stem segment straight-growth bioassay; the most effectiveconcentration of XG9 is c. 109 M. Previous work had shownthat XG9 owes its biological activity to the presence of a terminal-L-fucopyranose residue. In order to investigate to what extentthe remainder of the XG9 molecule is essential for activity,several fucose-containing compounds were tested for their abilityto mimic the anti-auxin effect of XG9. A fucose-containing pentasaccharideof xyloglucan (XG5; probable structure FucGalXylGlcGlc) was,at 108 M, about as effective an anti-auxin as 109M XG9; unlike XG9, XG5 did not diminish in effectiveness at107 M. The human milk trisaccharide, 2'-fucosyl-lactose[L-fucopyranosyl--(12)-D-galactopyranosyl-ß-(14)-D-glucose],whose FucGal unit is identical with that of XG9, inhibited auxin-inducedelongation over a wide range of concentrations centred on about108 M. 2'-Fucosyl-lactose at 108 M was about aseffective an anti-auxin as 109 M XG9. Free L-fucose andmethyl--L-fucopyranoside were unable to inhibit auxin-inducedgrowth at any concentration tested (1010 M to 106M) and neither compound interfered with the inhibition causedby 109 M XG9 when co-incubated at concentrations up to104 M. The results confirm the essential r?le of an -linkedterminal fucose residue in the anti-auxin activity of XG9 andshow that the sub-terminal galactose residue may also be required.Possible reasons why high concentrations of XG9 fail to antagonizeauxin-induced growth while high concentrations of XG5 and 2'-fucosyl-lactosecontinue to do so are discussed. Key words: Anti-auxin, oligosaccharin, fucose 相似文献
8.
9.
10.
The Strategy of Carbon Utilization in Uniculm Barley: II. THE EFFECT OF CONTINUOUS LIGHT AND CONTINUOUS DARK TREATMENTS 总被引:1,自引:0,他引:1
After a photoperiod of 8.25 h during which the youngest fullyexpanded leaf of uniculm barley plants was allowed to assimilate14CO2 for 30 min, groups of plants were transfered either tocontinuous light or to continuous dark. Plants were harvestedover a 72 h period to examine the effect of the treatments (comparedwith control plants growing in normal light/dark cycles) onthe transport of 14C from the exposed leaf, the distributionof 14C assimilates to the rest of the plant, and the chemicalfate of assimilated 14C. In continuous light a substantial quantity (22% at 72 h) ofthe 14C assimilated by the leaf remained in that leaf in theform of starch and neutral sugars compared with only 4% in thecontrol fed leaf. Also the total amount of 14C respired fromplants maintained in continuous light was significantly less(c. 18% of the total originally fixed by 24 h) than that respiredfrom control plants (c. 36%). The result was that approximatelyequal amounts of 14C were accumulated in the growing leavesand roots of plants given continuous light or normal light/darkcycles. In continuous dark the fate of 14C was similar to that of controlplants. This is probably because the two treatments shared acommon light/dark environment for the first 22 h, during whichtime almost complete distribution and utilization of 14C occurred. 相似文献