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71.
REMARKS ON THE APPENDAGES OF TRILOBITES 总被引:1,自引:0,他引:1
JAN BERGSTRÖM 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1969,2(4):395-414
The morphology and function of the exite (the pre-epipodite of Stormer) of the trilobite appendage is discussed. After a review of past work, the morphology is found to be considerably less uniform than previously thought. By an analysis of the morphological variation and by analogy with conditions in recent arthropods, function is thought to have been concerned with different activities such as filtering, swimming, or gill ventilation. Gills may have been present on the ventral side of the body, lateral to the appendages. 相似文献
72.
Genetic variation within and between vegetative compatibility groups (VCGs) of Amylostereum areolatum (Fr.) Boid. and Amylostereum chailletii (Pers.: Fr.) Boid. isolates was investigated. DNA fingerprints were made using the M13 core sequence as a primer. A total of 53 isolates of A. areolatum and 57 isolates of A. chailletii from Lithuania, Sweden, Denmark and Great Britain were studied. In all cases isolates belonging to the same VCG showed identical DNA banding patterns, suggesting that VCGs correspond to clones. In A. areolatum the vast majority of the isolates (spread by Sirex juvencus L.) were assigned to dispersive clones, that have wide geographical distribution (i.e. the same genotypes were detected in Lithuania, Sweden and Denmark), with low genetic variation between the different clones. By contrast, A. chailletii population structure was consistent with the spread of airborne basidiospores produced by outcrossing. Only a small fraction of A. chailletii isolates studied, could be assigned to dispersal clones with a local distribution, spread by Urocerus gigas L. Overall, M13 fingerprinting detected low genetic differentiation in both species in the samples we studied. 相似文献
73.
CHARACTERISTIC patterns resembling those revealed by the fluorochrome techniques1 appear along human mitotic metaphase chromosomes when methanol-acetic acid-fixed preparations are heated to 69° C and stained with Giemsa solution. This procedure is a greatly simplified version of one developed by F. E. Arrighi and T. C. Hsu (personal communication) which heavily stains the centromere regions and produces distinct patterns on the chromosome arms. We chose the conditions for pretreatment of the preparations on the basis of the finding of Marmur and Doty2 that the transition temperature, that is, the temperature at which the double helical configuration of DNA changes to a disordered coiling, for adenine-thymine nucleotide pairs, is 69° C. For cytosine-guanine pairs they found that the transition temperature is 110° C. Heating to the lower transition temperature should therefore denature regions of DNA rich in adenine-thymine pairs, leaving regions rich in cytosine-guanine pairs intact; such differences might be reflected in the staining properties, although the differential staining that occurs after heat treatment is obviously open to alternative interpretations. 相似文献
74.
FRANS WITTE MONIQUE WELTEN MARTIN HEEMSKERK IRENE VAN DER STAP LISANNE HAM HENDRIKUS RUTJES JAN WANINK 《Biological journal of the Linnean Society. Linnean Society of London》2008,94(1):41-52
During the upsurge of the introduced predatory Nile perch in Lake Victoria in the 1980s, the zooplanktivorous Haplochromis ( Yssichromis ) pyrrhocephalus nearly vanished. The species recovered coincident with the intense fishing of Nile perch in the 1990s, when water clarity and dissolved oxygen levels had decreased dramatically due to increased eutrophication. In response to the hypoxic conditions, total gill surface in resurgent H. pyrrhocephalus increased by 64%. Remarkably, head length, eye length, and head volume decreased in size, whereas cheek depth increased. Reductions in eye size and depth of the rostral part of the musculus sternohyoideus, and reallocation of space between the opercular and suspensorial compartments of the head may have permitted accommodation of larger gills in a smaller head. By contrast, the musculus levator posterior, located dorsal to the gills, increased in depth. This probably reflects an adaptive response to the larger and tougher prey types in the diet of resurgent H. pyrrhocephalus . These striking morphological changes over a time span of only two decades could be the combined result of phenotypic plasticity and genetic change and may have fostered recovery of this species. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 41–52. 相似文献
75.
INA FRANZN BERNARD SLIPPERS RIMVYDAS VASILIAUSKAS JAN STENLID 《Molecular ecology resources》2006,6(3):870-872
Seven polymorphic microsatellite markers were developed for the wood‐decay basidiomycete Phlebia centrifuga. The primers were identified using two techniques, based on intersimple sequence repeats (ISSR) and amplified fragment length polymorphism (AFLP), respectively. The markers were screened on 27 isolates from Europe and North America. Two markers varied only on a worldwide scale, but not within Europe. The other five showed variation on both scales. These markers will now be used to characterize populations of P. centrifuga, which is red‐listed as near‐threatened in its natural habitat due to human disturbance. 相似文献
76.
1. High water column NO3? concentrations, low light availability and anoxic, muddy sediments are hypothesised to be key factors hampering growth of rooted submerged plants in shallow, eutrophic fresh water systems. In this study, the relative roles and interacting effects of these potential stressors on survival, growth, allocation of biomass and foliar nutrient concentrations of Potamogeton alpinus were determined in a mesocosm experiment using contrasting values of each factor (500 versus 0 μmol L?1 NO3?; low irradiance, corresponding to the eutrophic environment, versus ambient irradiance; and muddy versus sandy sediment). 2. Low irradiance, high NO3? and sandy sediment led to reduced growth. In a muddy sediment, plants had lower root : shoot ratios than in a sandy sediment. 3. Growth at high NO3? and on the sandy sediment resulted in lower foliar N and C concentrations than in the contrasting treatments. The C : N ratio was higher at high NO3? and on the sandy sediment. Foliar P was higher on the muddy than on the sandy sediment but was not affected by irradiance or NO3?. The N : P ratio was lowest at high NO3? on the sandy sediment. 4. Total foliar free amino acid concentration was lowest on sand, low irradiance and high NO3?. Total free amino acid concentration and growth were not correlated. 5. Turbidity and ortho‐PO43? concentration of the water layer were lower at high water column NO3? indicating that the growth reduction was not associated with increased algal growth but that physiological mechanisms were involved. 6. We conclude that high water column NO3? concentrations can significantly reduce the growth of ammonium preferring rooted submerged species such as P. alpinus, particularly on sediments with a relatively low nutrient availability. Further experiments are needed to assess potential negative effects on other species and to further elucidate the underlying physiological mechanisms. 相似文献
77.
JACQUES F. G. M. MEIS JAN PETER VERHAVE PAUL H. K. JAP ROBERT E. SINDEN JOSEPH H. E. T. MEUWISSEN 《The Journal of eukaryotic microbiology》1983,30(2):361-366
The invasion of liver parenchymal cells by sporozoites of Plasmodium berghei Vincke & Lips, 1948, was studied in vivo using transmission electron microscopy. Livers of Brown Norway rats were examined 30 and 60 min after intraportal injection of 15 million sporozoites each. Sporozoites found after incorporation into vacuoles in hepatocytes were often located near a bile canaliculus at the lateral cell surface, surrounded by hepatocyte lysosomal structures; however, degradation of sporozoites caused by lysosomal digestion inside hepatocytes was never observed. Due to the crescent shape of sporozoites, serial sections were necessary to demonstrate the actual process of invasion of the hepatocyte. The hepatocyte's plasmalemma appeared to invaginate due to the sporozoite's action, thereby creating a parasitophorous vacuole. It was suggested that the sporozoite actively penetrated the hepatocyte; however, no visible depletion of rhoptries and micronemes was observed. 相似文献
78.
The rate of decarboxylation of DL-tryptophan-carboxyl-14C in homogenates of cotyledons, hypocotyls and roots of sterile and non-sterile cucumber seedlings of 4, 8 and 11 days was measured. Tryptophan decarboxylating activity is highest in hypocotyls, lowest in cotyledons. In all organs the activity decreases with age. This enzyme activity does not parallel the IAA level in the organs during ageing. 相似文献
79.
80.
Interactions between saprotrophic and ectomycorrhizal fungi have been largely ignored, although their mycelia often share the same microsites. The mycelial systems show general similarity to each other and, although the enzymatic potential of the saprotrophic fungi is generally considered to be higher, the importance of organic nutrient sources to ectomycorrhizal fungi is now widely accepted. In the experiments described here, nutritional interactions involving transfer of elements from one mycelium to the other have been monitored dynamically using radioactive tracers and a non-destructive electronic autoradiography system. Microcosms were used in which mycelial systems of the ectomycorrhizal fungi Suillus variegatus and Paxillus involutus , extending from Pinus sylvestris host plants, were confronted with mycelia of the saprotroph Hypholoma fasciculare extending from wood blocks. The fungi showed a clear morphological confrontation response. The mycorrhizal mycelium often formed dense patches over the Hypholoma mycelia. Up to 25% of the 32 P present in the Hypholoma mycelium was captured by the mycorrhizal fungi and translocated to the plant host within 30 d. The transfer of 32 P to the saprotroph from labelled mycorrhizal mycelium was one to two orders of magnitude lower. The significance of this transfer as a 'short cut' in nutrient cycling is discussed. 相似文献