全文获取类型
收费全文 | 3587篇 |
免费 | 253篇 |
国内免费 | 4篇 |
出版年
2023年 | 9篇 |
2022年 | 17篇 |
2021年 | 52篇 |
2020年 | 30篇 |
2019年 | 44篇 |
2018年 | 48篇 |
2017年 | 55篇 |
2016年 | 93篇 |
2015年 | 141篇 |
2014年 | 152篇 |
2013年 | 219篇 |
2012年 | 271篇 |
2011年 | 207篇 |
2010年 | 176篇 |
2009年 | 127篇 |
2008年 | 226篇 |
2007年 | 231篇 |
2006年 | 174篇 |
2005年 | 195篇 |
2004年 | 187篇 |
2003年 | 181篇 |
2002年 | 169篇 |
2001年 | 46篇 |
2000年 | 42篇 |
1999年 | 40篇 |
1998年 | 50篇 |
1997年 | 31篇 |
1996年 | 31篇 |
1995年 | 39篇 |
1994年 | 45篇 |
1993年 | 37篇 |
1992年 | 29篇 |
1991年 | 26篇 |
1990年 | 22篇 |
1989年 | 20篇 |
1988年 | 23篇 |
1987年 | 32篇 |
1986年 | 20篇 |
1985年 | 32篇 |
1984年 | 24篇 |
1983年 | 21篇 |
1982年 | 26篇 |
1981年 | 26篇 |
1980年 | 23篇 |
1979年 | 21篇 |
1978年 | 12篇 |
1977年 | 11篇 |
1975年 | 17篇 |
1974年 | 12篇 |
1973年 | 8篇 |
排序方式: 共有3844条查询结果,搜索用时 556 毫秒
991.
The classical theory of the ideal free distribution (IFD) predicts that the spatial distribution of consumers should follow the distribution of the resources they depend on. Here, we study consumer–resource matching in a community context. Our model for the community is a food chain with three levels. We study whether the primary consumers are able to match resources both under predation risk and in its absence. Both prey and predators have varying degrees of knowledge of the global and local resource distribution. We present two versions of the model. In the "resource maximising" model, the consumers consider the availability of their resource only. In the "balancing" model, individual consumers minimise predation risk per unit of resource that they can gain access to. We show that both models can lead to perfect matching of consumers on resources and predators on consumers, assuming that individuals have full knowledge of the whole environment. However, when the consumers' information and freedom of movement are greater than those of the predators, then the predators generally undermatch the consumers. In the opposite case, we observe overmatching and high consumer movement rates. Furthermore, undermatching of predators on consumers tends to induce overmatching of consumers on resources. 相似文献
992.
Several hundred specimens of a tiny olenid trilobite, Ctenopyge ceciliae sp. nov., have been found in stinkstone nodules in the upper Cambrian Peltura scarabaeoides Zone in southern Sweden. This exceptionally spinose form is known only from disarticulated specimens, but is quite well preserved, and all growth stages are represented. The early ontogenetic stages are exceptionally small, the protaspis being only half the size of that of the associated Peltura species. There may have been no more than three thoracic segments. Thus the whole ontogeny was compressed, and this together with the very small size of the adult indicates a true miniaturisation. Whereas the likely control of the origin of the tiny C. ceciliae was basically progenesis, the extreme spinosity had a different origin; allometric growth or possibly peramorphosis. C. ceciliae is small enough for the spines to have appreciably retarded sinking through frictional effects, and this small trilobite is interpreted as a free-swimming or floating form. 相似文献
993.
The l,3-ß-glucan synthase (callose synthase, EC 2.4.1.34) was solubilized from cauliflower ( Brassica oleracea L.) plasma membranes with digitonin, and partially purified by ion exchange chromatography and gel filtration [fast protein liquid chromatography (FPLC)] using 3-[(cholamidopropyl)dimethylammonio]-1-propane-sulfonate (CHAPS) in the elution buffers. These initial steps were necessary to obtain specific precipitation of the enzyme during product entrapment, the final purification step. Five polypeptides of 32, 35, 57, 65 and 66 kDa were highly enriched in the final preparation and are thus likely components of the callose synthase complex. The purified enzyme was activated by Ca2+ , spermine and cellobiose in the same way as the enzyme in situ, indicating that no essential subunits were missing. The polyglucan produced by the purified enzyme contained mainly 1,3-linked glucose. 相似文献
994.
Gibberellins Al (GA1 ), GA3 , GA4 , GA9 , and after enzymatic hydrolysis of GA-conjugate-like fractions, GA9 and GA15 , were identified in shoots of Sitka spruce [ Picea sitchensis (Bong.) Carr.] of different ages by combined gas chromatography-mass spectrometry (GC-MS). The purification and separation of the GAs involved the use of reverse phase and normal phase high performance liquid chromatography (HPLC). The Tan-ginbozu dwarf rice bioassay and binding to antibodies raised against GA1 , GA4 and GA9 were used for detection of GA-like substances. The qualitative differences between the three ages of plant material were the presence of GA3 and GA1 in the 48-year-old material and the absence of detectable amounts of GA4 in the same material. This indicates a difference in GA metabolism which may reflect the difference in ability to form reproductive buds. 相似文献
995.
Maintaining and restoring biodiversity in European boreal forests by developing natural disturbance regimes 总被引:1,自引:0,他引:1
Per K. Angelstam 《植被学杂志》1998,9(4):593-602
Abstract. A conceptual model is presented as a guide to the maintenance and restoration of ecologically sustainable boreal forest. The model is based on the hypothesis that self-sustained forest ecosystems can be (re-)created, and their biodiversity developed, if forest management can simulate the composition and structure of boreal forest landscapes by introducing and maintaining disturbances leading to naturally dynamic spatial and temporal patterns of forest regeneration. The major explanatory variable in the model is the effect of wildfire on sites with different fuel characteristics and climates found in the European boreal forest. Four levels of fire intensity are distinguished, based on mean fire frequencies. These range from extremely low in some wet tall-herb sites or sites at high altitudes or latitudes with a humid climate, where fire is absent or rare, to dry lichen-rich sites where fire occurs often. The model is called ASIO, after the words Absent, Seldom, Infrequent and Often, indicating the four levels. Three main disturbance regimes are distinguished in the European boreal forest, based on the complex interactions between probabilistic (e.g. mean fire intervals at different site types) and random events (e.g. where and when a fire occurs): (1) gap-phase Picea abies dynamics; (2) succession from young to old-growth mixed deciduous/coniferous forest; and (3) multi-cohort Pinus sylvestris dynamics. The model stems mainly from studies in Fennoscandia, but some studies from outside this region are reviewed to provide support for a more general application of the model. The model has been implemented in planning systems on the landscape level of several large Swedish forest enterprises, and is also used as an educational tool to help private land owners with the location and realization of forest management regimes. Finally, the model can be used to develop an administrative system for the monitoring of biodiversity in boreal forest. 相似文献
996.
997.
Abstract: The release of radiolabeled material from regenerating frog sciatic nerves was studied using a multicom- partment chamber, in which the ganglia and the outgrowth region, respectively, were separated from the rest of the nerve. The nerves were incubated with radioactive amino acids in the ganglionic compartment, and the material transported to and released at the outgrowth region was collected and analyzed. Approximately 10% of the transported radioactivity was released over a 24-h incubation period. Of the released materials, 84% had a molecular mass of < 1,000 daltons [the low-molecular-mass (LM) fraction] as determined by exclusion chromatography. The presence of LM material could not be explained by leakage, nor was it due to intracellular or extracellular degradation of radiolabeled, transported proteins. It was reduced by cold and was shown by the use of vinblastine to be dependent on axonal transport. According to TLC, both the original precursor and metabolites thereof could be detected among the released LM material. The present results demonstrate the existence of a transport system for LM material in peripheral axons. The preferential release of LM over high-molecular-mass material at the outgrowth region suggests that it could serve specific functions during regeneration. 相似文献
998.
Hans Henrik Saxild Jan Hassan Jacobsen Per Nygaard 《Molecular & general genetics : MGG》1994,242(4):415-420
We have found that Bacillus subtilis possesses a second 5-phosphoribosyl-1-glycinamide (GAR) transformylase catalysing the first one-carbon transfer reaction in the purine biosynthetic pathway. Inactivation of the purN gene encoding the N10-formyl tetrahydrofolate-dependent enzyme did not result in purine auxotrophy. However, growth of a purN strain was stimulated when either purine or formate was added to the growth medium. In cell-free extracts GAR could be formylated, provided formate was added to the assay mixture. From the purN strain, purine-requiring mutants were isolated. One of these mutant strains was defective in the formate-dependent formylation of GAR in vitro. The gene containing this second mutation was designated purT, and was mapped to approximately 20° on the genetic map between the cysA and aroI markers. 相似文献
999.
Stimulation of somatic embryogenesis in carrot by ethylene: Effects of modulators of ethylene biosynthesis and action 总被引:6,自引:0,他引:6
Per Nissen 《Physiologia plantarum》1994,92(3):397-403
Endogenous levels of ethylene appeared to he suhoptimal for somatic embryogenesis in a suspension culture of carrot. Low concentrations of 1-aminocyclopropane-1-carboxylic acid (ACC). 2-chloroethylphosphonic acid (ethephon) and elhylene stimulated embryogenesis whereas higher concentrations were inhibitory. The stimulation by ACC was through its conversion to ethylene. whereas the inhibition by ACC was not. Low concentrations of AgNO3 . an inhibitor of ethylene action, inhibited embryo-genesis but stimulated ethylene production. Aminoethoxyvinylglycine (AVG) and aminooxyacetic acid (AOA). commonly used inhibitors of ACC synthase. inhibited both embryogenesis and ethylene production. However, the inhibition of embryogenesis was not related to the inhibition ote ethylene production. Very low concentrations of AVG stimulated embryo production in a way unrelated to its effect on ethylene production. Salicylic acid and CoCl2 . inhibitors of ACC oxidase in other systems, inhibited embryogenesis but. again, in way(s) unrelated to their inhibition of ethylene production. In fact, low concentrations of salicylic acid stimulated rather than inhibited ethylene production. The results show that in suspension-cultured cells, caution is warranted in the interpretation of results obtained with agents presumed to inhibit ethylene biosynthesis. The stimulation of somatic embryogenesis by ethylene unequivocally shows that the inhibition of embryo development by 2.4-dichlorophenoxyacetic acid (2.4-D) and other auxins cannot be through their stimulatory effect on ethylene production. 相似文献
1000.
Per Milberg 《Nordic Journal of Botany》1994,14(1):27-29
Old dry-stored seeds of the annual pyrophyte Geranium bohemicum from seed collections and herbaria were tested to determine their germinability. Ten samples of seed, ranging in age from 3 to 200 years, were used. The oldest seeds to germinate (three out of ten) were from a 129-year old collection. All three seedlings developed into healthy plants. This finding supports the assumption that G bohemicum can emerge from a bank of long-lived seeds after forest fires. 相似文献