全文获取类型
收费全文 | 9501篇 |
免费 | 1465篇 |
国内免费 | 2483篇 |
专业分类
13449篇 |
出版年
2024年 | 103篇 |
2023年 | 348篇 |
2022年 | 268篇 |
2021年 | 258篇 |
2020年 | 495篇 |
2019年 | 527篇 |
2018年 | 585篇 |
2017年 | 557篇 |
2016年 | 549篇 |
2015年 | 526篇 |
2014年 | 560篇 |
2013年 | 711篇 |
2012年 | 470篇 |
2011年 | 586篇 |
2010年 | 411篇 |
2009年 | 562篇 |
2008年 | 538篇 |
2007年 | 557篇 |
2006年 | 524篇 |
2005年 | 434篇 |
2004年 | 392篇 |
2003年 | 365篇 |
2002年 | 359篇 |
2001年 | 317篇 |
2000年 | 270篇 |
1999年 | 242篇 |
1998年 | 223篇 |
1997年 | 186篇 |
1996年 | 177篇 |
1995年 | 156篇 |
1994年 | 149篇 |
1993年 | 119篇 |
1992年 | 130篇 |
1991年 | 78篇 |
1990年 | 88篇 |
1989年 | 74篇 |
1988年 | 64篇 |
1987年 | 48篇 |
1986年 | 58篇 |
1985年 | 60篇 |
1984年 | 52篇 |
1983年 | 26篇 |
1982年 | 60篇 |
1981年 | 47篇 |
1980年 | 31篇 |
1979年 | 38篇 |
1978年 | 19篇 |
1977年 | 9篇 |
1976年 | 17篇 |
1958年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Giuseppe Bagatto Louise C. Paquette Joseph D. Shorthouse 《Entomologia Experimentalis et Applicata》1996,79(1):111-117
We examined how leaf galls, induced by the cynipid wasp Phanacis taraxaci, influence the partitioning of photoassimilates within the host, the common dandelion, Taraxacum officinale. Galled and ungalled plants were exposed to 14CO2 and the labelled photoassimilates accumulating within galls and other parts of the host were measured. During the growth phase of the gall they were physiological sinks for photoassimilates, accumulating 9% to 70% of total carbon produced by the host, depending upon the number of galls per plant. High levels of 14C assimilation in the leaves of galled plants compared to controls, suggest that galls actively redirect carbon resources from unattacked leaves of their host plant. This represents a significant drain on the carbon resources of the host, which increases with the number and size of galls per plant. Active assimilation of 14C by the gall is greatest in the growth phase and is several orders of magnitude lower in the maturation phase. This finding is consistent with physiological and anatomical changes that occur during the two phases of gall development and represents a key developmental strategy by cynipids to ensure adequate food resources before larval growth begins. 相似文献
52.
Hydrilla verticillata (L. f.) Royle tubers from monoecious plants andPotamogeton gramineus L. winter buds were sprouted and allowed to grow in the dark for 120 days. We measured plant length and counted the number
of leaves at 2–3 day intervals.Hydrilla grew most rapidly during the first 16–17 days andPotamogeton grew most rapidly during the first 16–25 days. Measurement of propagule carbon content over time indicated that cessation
of rapid growth coincided with depletion of tuber carbon by one-half forHydrilla. ForPotamogeton, growth was reduced after 16 to 25 days while the winter bud C half-life was 37 days. Calculations indicated thatHydrilla mobilized 49% andPotamogeton 39% of the initial propagule carbon to support growth. In a second experiment, in which plants were grown in substrate the
plants grew taller and produced slightly more leaves per plant.Potamogeton removed from darkness after specified time periods, and allowed to grow for 21 days in a greenhouse recovered from 20–30
days in the dark. Similarly treatedHydrilla plants recovered from up to 80 days in the dark.Potamogeton had mobilized 79% of initial C by the time it was unable to recover from the dark treatment. Combined results for both species
indicate that the majority of propagule C was utilized in the first 16 to 30 days following sprouting. In conjunction with
an understanding propagule sprouting requirements, this information will be useful in the timing of application for management
techniques.
The U.S. Government right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged.
The U.S. Government right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged. 相似文献
53.
Dissolved organic carbon (DOC) dynamics were examined over five years (1989–1993) in Sycamore Creek, a Sonoran Desert stream, specifically focusing on DOC concentration in surface and hyporheic waters, and rates of export. In 1989 and 1990, the years of lowest stream discharge (0.08 and 0.04 m3 s–1 annual mean of daily discharge, respectively), DOC was high, averaging 7.37 and 6.22 mgC l–1 (weighted annual means). In contrast, from 1991 through 1993, a period of increased flow (1.1, 1.2 and 4.3 m3 s–1), concentration was significantly lower (P<0.001) with annual mean concentrations of 3.54, 3.49 and 3.39 mgC l–1. Concentration exhibited little spatial variation between two sampling stations located 6 km apart along the mainstem or between surface and hyporheic waters. Annual export of DOC from Sycamore Creek varied 100-fold over the five-year period from a mean rate of only 24 kgC d–1 in 1990 to 2100 kgC d–1 in 1993. Ninety percent of DOC was exported by flows greater than 2.8 m3 s–1, and 50% during flows greater than 27 m3 s–1; flows of 2.8 and 24 m3 s–1 occurred only 9 and 1% of the time. The export of organic matter in Sycamore Creek appears to be coupled to El Niño-Southern Oscillation phenomena. The years of highest export, 1991–1993, had El Niño conditions while 1989 and 1990 had medial conditions. 相似文献
54.
Fine root demography in alfalfa (Medicago sativa L.) 总被引:1,自引:1,他引:0
In perennial forages like alfalfa (Medicago sativa L.), repeated herbage removal may alter root production and mortality which, in turn, could affect deposition of fixed N in soil. Our objective was to determine the extent and patterns of fine-diameter root production and loss during the year of alfalfa stand establishment. The experiment was conducted on a loamy sand soil (Udorthentic Haploboroll) in Minnesota, USA, using horizontally installed minirhizotrons placed directly under the seeded rows at 10, 20, and 40 cm depths in four replicate blocks. We seeded four alfalfa germplasms that differed in N2 fixation capacity and root system architecture: Agate alfalfa, a winter hardy commercially-available cultivar; Ineffective Agate, which is a non-N2-fixing near isoline of Agate; a new germplasm that has few fibrous roots and strong tap-rooted traits; and a new germplasm that has many fibrous roots and a strongly branched root system architecture. Video images collected biweekly throughout the initial growing season were processed using C-MAP-ROOTS software.More than one-half of all fine roots in the upper 20 cm were produced during the first 7 weeks of growth. Root production was similar among germplasms, except that the highly fibrous, branch-rooted germplasm produced 29% more fine roots at 20 cm than other germplasms. In all germplasms, about 7% of the fine roots at each depth developed into secondarily thickened roots. By the end of the first growing season, greatest fine root mortality had occurred in the uppermost depth (48%), and least occurred at 40 cm (36%). Survival of contemporaneous root cohorts was not related to soil depth in a simple fashion, although all survivorship curves could be described using only five rates of exponential decline. There was a significant reduction in fine root mortality before the first herbage harvest, followed by a pronounced loss (average 22%) of fine roots at the 10- and 20-cm depths in the 2-week period following herbage removal. Median life spans of these early-season cohorts ranged from 58 to 131 days, based on fitted exponential equations. At all depths, fine roots produced in the 4 weeks before harvest (early- to mid-August) tended to have shorter median life spans than early-season cohorts. Similar patterns of fine root mortality did not occur at the second harvest. Germplasms differed in the pattern, but not the ultimate extent, of fine root mortality. Fine root turnover during the first year of alfalfa establishment in this experiment released an estimated 830 kg C ha–1 and 60 kg N ha–1, with no differences due to N2 fixation capacity or root system architecture. 相似文献
55.
The effects of mycorrhizal roots on litter decomposition, soil biota, and nutrients in a spodosolic soil 总被引:1,自引:0,他引:1
We studied the effects of mycorrhizal pitch pine (Pinus rigida) roots on litter decomposition, microbial biomass, nematode abundance and inorganic nutrients in the E horizon material of a spodosolic soil, using field microcosms created in a regenerating pitch pine stand in the New Jersey Pinelands. Pine roots stimulated litter decomposition by 18.7% by the end of the 29 month study. Both mass loss and N and P release from the litter were always higher in the presence of roots than in their absence. Nutrient concentrations in decomposing litter were similar, however, in the presence and absence of roots, which suggests that the roots present in the with-root treatment did not withdraw nutrients directly from the litter. The soil was slightly drier in the presence of roots, but there was no discernible effect on soil microbial biomass. The effects of roots on soil extractable inorganic nutrients were inconsistent. Roots, however, were consistently associated with higher numbers of soil nematodes. These results suggest that, in soils with low total C and N contents, roots stimulate greater activity of the soil biota, which contribute, in turn, to faster litter decomposition and nutrient release.Contribution No. 95-22 from the Institute of Marine and Coastal Sciences.Contribution No. 95-22 from the Institute of Marine and Coastal Sciences. 相似文献
56.
Tetsuko Noguchi 《Protoplasma》1994,180(1-2):29-38
Summary The formation and the decomposition of vacuoles in a member of Xanthophyceae,Botryococcus braunii, were examined by light and electron microscopy. Particles around the nucleus were identified as vacuoles from their stainability with neutral red. These particles disappeared during cell division. They reacted positively in an activity test for acid phosphatase. Electron microscopy revealed the presence of spherical vacuoles around the nucleus. During cell division, these vacuoles seemed to be decomposed by the ER which surrounded the vacuoles. Soon after this decomposition, many immature multivesicular bodies (MVBs) appeared to develop from the trans-Golgi network (TGN) and were pinched off from the TGN. These immature MVBs took up small vesicles in them as they grew into the mature MVBs. Mature MVBs took up and digested the surrounding cytoplasm, fused with one another, and eventually became the vacuoles.Abbreviations MVB
multivesicular body
- TGN
trans-Golgi network 相似文献
57.
Speculations on carbon dioxide starvation, Late Tertiary evolution of stomatal regulation and floristic modernization 总被引:2,自引:0,他引:2
J. M. ROBINSON 《Plant, cell & environment》1994,17(4):345-354
Ambient atmospheric CO2 concentration ([CO2]a) has apparently declined from values above 200μmol mol−1 to values below 200μmol mol−1 within the last several million years. The lower end of this range is marginal for C3 plants. I hypothesize that: (1) declining [CO2]a imposed a physiological strain on plants, and plant taxa evolving under declining [CO2]a tended to develop compensating mechanisms, including increased stomatal efficiency; (2) angiosperms were better able to adjust to declining [CO2]a than were gymnosperms and pteridophytes; and (3) angiosperm adjustment has been uneven. Fast-evolving taxa (e.g. grasses and herbs) have been better able to adapt to CO2 starvation. If these propositions are true, stomatal adjustment mechanisms should show patterned variation, and a single pattern of stomatal regulation cannot be assumed. 相似文献
58.
We report here on the characterization and isolation of two ecotypes of Chlorella vulgaris Beyerinck that coexist in wastewater reservoirs. One ecotype (C1) contains high amounts of chlorophyll b, is capable of autotrophic growth, and can utilize only a few organic solutes for growth. The second ecotype (C2) contains low amounts of chlorophyll b, requires vitamin B12, and can support its growth with a broad range of organic compounds. Of the two ecotypes, the latter showed slower growth rates when light was the sole source of energy. Cells of C2-type Chlorella attained higher photosynthetic activities than C1-type cells at saturating irradiances. However, their low chlorophyll b content and lower light utilization efficiency suggest that C2-type Chlorella contains relatively low amounts of light-harvesting antennae, a disadvantage in severely light-limited ecosystems like wastewater reservoirs. We hypothesize that the two Chlorella types coexist by adopting different lifestyles: C1-type cells rely largely on their photosynthetic potential for energy conservation and growth, whereas C2-type cells may exploit their heterotrophic properties for this purpose. 相似文献
59.
60.
Soil pCO2, soil respiration,and root activity in CO2-fumigated and nitrogen-fertilized ponderosa pine 总被引:2,自引:0,他引:2
Dale Johnson Donn Geisinger Roger Walker John Newman James Vose Katherine Elliot Timothy Ball 《Plant and Soil》1994,165(1):129-138
The purpose of this paper is to describe the effects of CO2 and N treatments on soil pCO2, calculated CO2 efflux, root biomass and soil carbon in open-top chambers planted with Pinus ponderosa seedlings. Based upon the literature, it was hypothesized that both elevated CO2 and N would cause increased root biomass which would in turn cause increases in both total soil CO2 efflux and microbial respiration. This hypothesis was only supported in part: both CO2 and N treatments caused significant increases in root biomass, soil pCO2, and calculated CO2 efflux, but there were no differences in soil microbial respiration measured in the laboratory. Both correlative and quantitative
comparisons of CO2 efflux rates indicated that microbial respiration contributes little to total soil CO2 efflux in the field. Measurements of soil pCO2 and calculated CO2 efflux provided inexpensive, non-invasive, and relatively sensitive indices of belowground response to CO2 and N treatments. 相似文献