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51.
SUMMARY. We describe a model of zooplankton population dynamics that accounts for differences in mortality and physiology among animals of different ages or sizes. The model follows changes in numbers of individuals and changes in individual and egg biomass through time and it expresses mortality and net assimilation as functions of animal size.
We investigated the effect of egg size, age at first reproduction, and size at first reproduction on the per capita growth rates of populations growing under different conditions. In the absence of predation or when exposed to vertebrate predators that prefer large prey, populations achieve maximum growth rates when animals hatch from small eggs and reach maturity quickly at small sizes. Populations exposed to invertebrate predators that concentrate on small animals may increase r in two different ways. One way is for animals to increase juvenile survivorship by hatching from large eggs and by shortening the juvenile period. An alternative strategy is for animals to hatch from small eggs and to postpone maturity until they grow beyond the range of sizes available to their predators. Certain life history strategies maximize r if animals continue to grow after they reach maturity. By growing larger, non-primiparous females are able to hatch larger clutches and thereby increase the overall rate of population growth.
The model analysis shows how to assess age-dependent mortality rates from field data. The net rate of population increase and the age distribution of eggs together provide specific, quantitative information about mortality. 相似文献
We investigated the effect of egg size, age at first reproduction, and size at first reproduction on the per capita growth rates of populations growing under different conditions. In the absence of predation or when exposed to vertebrate predators that prefer large prey, populations achieve maximum growth rates when animals hatch from small eggs and reach maturity quickly at small sizes. Populations exposed to invertebrate predators that concentrate on small animals may increase r in two different ways. One way is for animals to increase juvenile survivorship by hatching from large eggs and by shortening the juvenile period. An alternative strategy is for animals to hatch from small eggs and to postpone maturity until they grow beyond the range of sizes available to their predators. Certain life history strategies maximize r if animals continue to grow after they reach maturity. By growing larger, non-primiparous females are able to hatch larger clutches and thereby increase the overall rate of population growth.
The model analysis shows how to assess age-dependent mortality rates from field data. The net rate of population increase and the age distribution of eggs together provide specific, quantitative information about mortality. 相似文献
52.
Temperature Regulation of Anthocyanin Accumulation in Apple Skin 总被引:1,自引:0,他引:1
The regulation of anthocyanin accumulation in apple skin (cv.Jonathan) by temperature was studied. In the field the increasein anthocyanin in the skin before harvest coincided with decreasingtemperatures and with increasing ethylene production by thefruit. In detached apples held under continuous white light,the optimum temperatures for anthocyanin accumulation were 12°C in unripe apples and 1624 °C in ripe apples.These effects were explained by corresponding changes in thelevel of phenylalanine ammonia-lyase (PAL), a key enzyme offlavonoid synthesis. PAL levels were higher at low than at hightemperatures and higher in ripe than in unripe apples. In intermittentlight the effects of temperature were similar but levels ofPAL and anthocyanin were lower, particularly in unripe apples.It is concluded that temperature, in conjunction with ripeningand light, is an important factor regulating anthocyanin accumulationand that its effects are mediated by effects on the level ofPAL activity. Key words: Apple, Anthocyanin, Phenylalanine ammonia-lyase 相似文献
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55.
Synthesis of Substance P 总被引:3,自引:0,他引:3
GEOFFREY W. TREGEAR HUGH D. NIALL JOHN T. POTTSJUN. SUSAN E. LEEMAN MICHAEL M. CHANG 《Nature: New biology》1971,232(29):87-89
SUBSTANCE P has been synthesized by the solid-phase procedure of Merrifield1,2 according to the sequence H-Arg-Pro-LysPro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2 reported in the previous letter. 相似文献
56.
SEVERAL mouse alloantigens described recently are expressed solely or chiefly on lymphoid cells. This indicates that the programme of selective gene action responsible for cyto-differentiation includes instructions for the synthesis of unique cell surface conformations characteristic of particular cell types1, 2. 相似文献
57.
EXCESS fat deposition after castration is thought to be a response to hyperinsulinism induced by an increased level of adrenal glucocorticoids1. Two mutant alleles, lethal yellow (Ay) and viable yellow (Avy), at the agouti locus in the mouse induce excess fat deposition; the Ay allele has also been found to induce insulin resistance2. Katsh3 concluded that in adrenalectomized KL strain mice a considerable portion of the high insulin tolerance depended on normal adrenal function. In the inbred YS/Wf and VY/Wf strains, both yellow pheno-types modulate serum insulin concentration4. Castration of inbred YS strain males causes a large decrease in serum insulin4, suggesting a possible relationship1 to the concentration of adrenal glucocorticoids. Growth of allogeneic tumour cells has different effects on the serum insulin concentrations of YS and VY strain mice4. 相似文献
58.
At least ten major mineral licks occur along the Gaji River which bisects the Yankari Game Reserve. The licks show varying degrees of utilization and have craters of up to 1200m3 in volume. Three hundred and eighty five hours of observation were kept at one of the licks. Its use by different species was highly variable. Some species occurring in the reserve made no visits to the lick, whereas Western hartebeest (51 groups), waterbuck (53 groups) and warthog (276 groups) made frequent visits. Lick use was much greater in the daytime than at night, with the peak occurring around 1400 h. A limited number of soil samples (29) were analysed but no single element can be considered as the factor attracting wildlife. However, the lick areas were richer in most minerals than were samples of soil from drier habitat in the open woodland. It is possible that significantly increased lick use might in the future indicate degrading environmental conditions in the drier parts of the reserve. If hides are constructed, the commonly utilized licks will provide excellent opportunities for behaviour observatioii and photography by tourists. 相似文献
59.
PRECIPITATION of collagen from solution in the presence of chondroitin sulphates (or certain other glycosaminoglycans), followed by dialysis against water, leads to the formation of the fibrous long spacing (FLS) polymorphic form of collagen. Electron microscopy shows this to consist of banded fibrils with a periodicity in the range 1200–2400 Å and a symmetric intraperiod structure, in contrast to the 600–700 Å periodicity and polarized fine structure characteristic of native collagen fibrils. FLS was originally reported by Highberger et al.1–3 and has been observed in three principal modifications, FLS I, FLS II and FLS III, differing in periodicity and arrangement of dense bands4–6. This letter reports a new form of FLS, denoted FLS IV. 相似文献
60.
Admissible clustering procedures 总被引:3,自引:0,他引:3