全文获取类型
收费全文 | 6318篇 |
免费 | 704篇 |
国内免费 | 1699篇 |
出版年
2024年 | 46篇 |
2023年 | 203篇 |
2022年 | 270篇 |
2021年 | 324篇 |
2020年 | 286篇 |
2019年 | 344篇 |
2018年 | 308篇 |
2017年 | 312篇 |
2016年 | 365篇 |
2015年 | 365篇 |
2014年 | 458篇 |
2013年 | 497篇 |
2012年 | 378篇 |
2011年 | 371篇 |
2010年 | 285篇 |
2009年 | 333篇 |
2008年 | 359篇 |
2007年 | 387篇 |
2006年 | 350篇 |
2005年 | 273篇 |
2004年 | 255篇 |
2003年 | 225篇 |
2002年 | 202篇 |
2001年 | 157篇 |
2000年 | 149篇 |
1999年 | 130篇 |
1998年 | 117篇 |
1997年 | 168篇 |
1996年 | 86篇 |
1995年 | 92篇 |
1994年 | 56篇 |
1993年 | 73篇 |
1992年 | 54篇 |
1991年 | 44篇 |
1990年 | 47篇 |
1989年 | 45篇 |
1988年 | 29篇 |
1987年 | 42篇 |
1986年 | 39篇 |
1985年 | 32篇 |
1984年 | 27篇 |
1983年 | 24篇 |
1982年 | 29篇 |
1981年 | 9篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 12篇 |
1977年 | 10篇 |
1976年 | 9篇 |
1975年 | 13篇 |
排序方式: 共有8721条查询结果,搜索用时 734 毫秒
151.
The objective was to determine the effect of ACTH 1-17, an adrenocorticotropin analogue, on the mitotic index in the corneal epithelium of mice standardized in 12 hr of light alternating with 12 hr darkness. A question asked was whether the time of administration along the 24-hr time scale influenced any response found. The findings showed that ACTH 1-17 could, depending upon when it was administered, bring about a statistically significant decrease, an increase or even no such change in the mitotic index. The greatest responses found were increases, especially when ACTH 1-17 was administered during the dark span. Also the time after injection when the responses occurred varied. The greatest response recorded was at 12 hr after injection when ACTH 1-17 was given at 2 hr into the dark with a 641% and a 718% increase with a low (0.02 IU/kg) and a higher (20 IU/kg) dose, respectively. A 3-way analysis of variance supported the conclusion that the kind-of-treatment, time-of-treatment and treatment-to-kill interval (sampling time) are important factors when determining any response to ACTH 1-17 on the mitotic index. 相似文献
152.
The interspecific interactions of Meloidogyne javanica with races 1, 2, 3, and 4 of M. incognita on tomato were determined. Impacts of the interactions on fecundity and morphometrics of females were also examined. Mutually inhibitory interactions occurred between M. javanica and the races of M. incognita, but the negative interactions did not reflect in plant growth. Numbers of root galls, egg masses, mature females, total population, fecundity, and reproduction factor declined in concomitant treatments, but the morphometrics of the females remained unaltered. In general, mutual suppressive effects in all parameters were smaller for M. javanica than M. incognita, but some variations occurred among the races of M. incognita. Race 2 appeared to be more competitive than other races. The interaction between the species was not intense; therefore, the species coexist in mixed populations in agricultural fields. 相似文献
153.
154.
Structural properties of two types of mangrove stands on the northwestern coast of Sri Lanka 总被引:2,自引:1,他引:1
Mangrove stands in Puttalam lagoon and Dutch bay, two interconnected lagoons situated on the northwestern coast of Sri Lanka have been classified broadly into two groups, i.e., estuarine and island/mainland-fringing stands. Structural diversity of six mangrove stands, representing the two types was studied in terms of floristic composition, density, basal area, mean stand diameter, tree height, standing above-ground biomass and leaf-area index.Rhizophora mucronata andAvicennia marina were the dominant species. Higher mean stand diameters for the mangrove stands in Puttalam lagoon indicated greater maturity than the estuarine mangrove stands in Dutch bay. Nevertheless, estuarine stands in Dutch bay were structurally more complex (complexity indices 8.11–22.7) than the island/mainland-fringing mangrove stands (complexity indices 1.38–6.78). Higher number of species present in the estuarine mangrove stands is the major element that contributes to the higher values for the complexity indices for those stands. This appears to mask the contribution of stand-age to the complexity of a mangrove stand. Therefore complexity indices alone may not be used to explain adequately the structural diversity among mangrove stands. 相似文献
155.
Vertical profiles were measured in soil cores taken from flooded rice fields in the Po valley during July and August 1990.
Methane concentrations generally increased with depth and reached maximum values of 150–500 μM in 5–13 cm depth. However,
the shape of the profiles was very different when studying different soil cores. The CH4 content of gas bubbles showed a similar variability which apparently was due to spatial rather than temporal inhomogeneities.
Similar inhomogeneities were observed in the vertical profiles of acetate, propionate, lactate, and formate which showed maximum
values of 1500, 66, 135, and 153, μM, respectively. However, maxima and minima of the vertical profiles of the different substates
usually coincided in one particular soil core. Large inhomogeneities in the vertical profiles were also observed for the rates
of total CH4 production, however, the percentage contribution of H2/CO2 to CH4 production was relatively homogeneous at 24 ± 7% (SD). Similarly, the H2 content of gas bubbles was relatively constant at 93.3 ± 9.6 ppmv when randomly sampled in the rice field at different times
of the day. A small contribution (6%) of H2/CO2 to acetate production was also observed. Vertical profiles of the respiratory index (RI) for [2-14C] acetate showed that acetate was predominantly degraded by methanogenesis in 5–11 cm depth, but by respiration in the surface
soil (3 cm depth) and in soil layers below 13–16 cm depth which coincided with a transition of the colour (grey to reddish)
and the physical characteristics (porosity, density) of the soil. The observations indicate that the microbial community which
degrades organic matter to CH4 is in itself relatively homogenous, but operates at highly variable rates within the soil structure.
Author for correspondence 相似文献
156.
Using a combination of mathematical modeling and field studies we showed that in dense stands of growing herbaceous plants the vertical pattern of leaf nitrogen distribution resembles the pattern of mean light attenuation in the stand and hence tends to maximize total daily photosynthetic carbon gain of the whole stand. Flowering represents a strong sink of nitrogen away from the photosynthetic apparatus and in herbs like Solidago altissima it induces leaf shedding. We studied both the effect of nitrogen reallocation and leaf shedding on the whole canopy photosynthesis and changes in leaf nitrogen distributions in stands moving from the growing to the flowering stage. Despite a decrease in leaf area index and total nitrogen available for photosynthesis in the flowering stand, the leaf nitrogen distribution here also leads to an almost maximum canopy photosynthesis. In both the growing and the flowering stands the leaf area index was higher than calculated optimum values. It is pointed out that this should not necessarily be interpreted as non-adaptive. 相似文献
157.
Thomas Nogrady 《Hydrobiologia》1982,89(3):277-284
The quantitative composition of the rotifer fauna of seven interconnected lakes located in southern California was studied in a preliminary investigation. The lakes are fed by reclaimed sewage, and the water quality increases rapidly from the first lake, barely able to support life, to a series of lovely lakes stocked with fish and utilized for recreation. The Shannon index of rotifer population diversity showed an appropriate increase. Up to 24 – 28 different rotifer taxa in each lake provided a rich community dominated by Brachionids, Keratella and Trichocerca. The diversity and quantity of the rotifer association then declined again in the last lake. Comparison of the lakes using the Bray and Curtis index of similarity and cluster analysis supported the picture of a lake-chain evolution, a biotic development showing a maximum curve. 相似文献
158.
159.
David G. Mann 《Journal of phycology》1982,18(1):162-176
Rhoicosphenia Grun. has been placed by some authors in the monoraphid group with Achnanthes Bory and Cocconeis Ehrenb., and by others near Gomphonema Ehrenb. In order to clarify the systematic position of the genus, the morphology and anatomy of the vegetative cells of Rh. curvata (Kütz.) Grun. were investigated using light and electron microscopy. The structure and formation of the two types of valve are described, and the heterovalvy shown to be of a different type from that of the monoraphids; on the basis of raphe, valve and girdle structure a close relationship between these and Rhoicosphenia is unlikely. Rhoicosphenia shows many resemblances to Gomphonema but the types of pore occlusion present, coupled with apparently slight differences in the mucilage-secreting structures and the girdle, suggest that classification in the same family is unwise. The cryptic asymmetry of the valves, and in particular of the raphe system, is noted and explained with reference to their formation; with respect to this asymmetry two configurations of the valves can occur (named cis and trans types) and the distribution of these in raphid genera is discussed briefly. In view of the lack of evidence in raphid diatoms supporting a classification of bands into copulae and pleurae, it is recommended that this practice be suspended. 相似文献
160.
J. Zavitkovski 《Plant and Soil》1981,60(3):409-422
Litterfall was sampled in 3- to 7-year-old irrigated and fertilized plantings of Populus Tristis#1 in northern Wisconsin. Leaves accounted for more than 90 per cent of the total litter and reached an asymptotic value of about 4,000 to 4,200 kg/ha. Leaf litterfall culminated in late September or first half of October. Inadequate light appeared to be responsible for an early (June) leaf fall in dense plantings. The maximum cumulative leaf area index ranged from 8.4 to 8.7. The pattern of foliation and defoliation was similar: it started with small leaves in the lower canopy and ended with large leaves of the upper canopy. Longevity of the small leaves was shorter than that of the large leaves. Leaf size was positively affected by fertilization but was not affected by a discontinuation of irrigation by mid-July during the droughty 1976. The specific leaf weight ranged from 2.6 mg/cm2 in the lower canopy to 10.2 mg/cm2 in the upper canopy. The specific leaf weight of litter leaves was 4 to 38 per cent lower than that of green leaves of the same size. 相似文献