全文获取类型
收费全文 | 1658篇 |
免费 | 170篇 |
国内免费 | 1篇 |
专业分类
1829篇 |
出版年
2023年 | 6篇 |
2022年 | 16篇 |
2021年 | 28篇 |
2020年 | 23篇 |
2019年 | 25篇 |
2018年 | 19篇 |
2017年 | 30篇 |
2016年 | 45篇 |
2015年 | 69篇 |
2014年 | 85篇 |
2013年 | 77篇 |
2012年 | 116篇 |
2011年 | 129篇 |
2010年 | 69篇 |
2009年 | 69篇 |
2008年 | 95篇 |
2007年 | 99篇 |
2006年 | 93篇 |
2005年 | 84篇 |
2004年 | 67篇 |
2003年 | 70篇 |
2002年 | 78篇 |
2001年 | 30篇 |
2000年 | 30篇 |
1999年 | 37篇 |
1998年 | 23篇 |
1997年 | 19篇 |
1996年 | 10篇 |
1995年 | 11篇 |
1994年 | 8篇 |
1993年 | 9篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 16篇 |
1989年 | 10篇 |
1988年 | 10篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 17篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1980年 | 9篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1975年 | 6篇 |
1974年 | 10篇 |
1972年 | 8篇 |
1967年 | 6篇 |
1957年 | 5篇 |
排序方式: 共有1829条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
5.
Addition of fruit cell wall extracts from two muskmelon cultivars into liquid media affected mycotoxin production by a strain of Myrothecium roridum pathogenic to muskmelon. Cell wall extracts from a susceptible cultivar (Iroquois) significantly increased toxin production while cell wall extracts from a resistant cultivar (Hales Best) significantly inhibited toxin production. Media containing 0.1 or 1.0 mg ml–1 stimulated toxin production more than media containing 10 or 100 mg ml–1 of cell wall extracts. Previous studies in our laboratory suggest that roridin E may be involved in virulence or pathogenicity of M. roridum; the present study indicates that cell wall polysaccharides as well as other materials present in cell wall preparations from susceptible host tissue provide a better substrate for toxin production than cell wall preparation from resistant host tissue. 相似文献
6.
Summary Root hairs have been shown to enhance P uptake by plants growing in low P soil. Little is known of the factors controlling root hair growth. The objective of this study was to investigate the influence of soil moisture and P level on root hair growth of corn (Zea mays L.). The effect of volumetric soil moistures of 22% (M0), 27% (M1), and 32% (M2) and soil (Raub silt loam, Aquic Argiudoll) P levels of, 0.81 (P0), 12.1 (P1), 21.6 (P2), 48.7 (P3), and 203.3 (P4) mol P L–1 initially in the soil solution, on shoot and root growth, P uptake, and root hair growth of corn was studied in a series of pot experiments in a controlled climate chamber. Root hair growth was affected more by soil moisture than soil P. The percentage of total root length with root hairs and the density and length of root hairs on the root sections having root hairs all increased as soil moisture was reduced from M2 to M0. No relationship was found between root hair length and soil P. Density of root hairs, however, was found to decrease with an increase in soil P. No correlation was found between root hair growth parameters and plant P content, further suggesting P plays a secondary role to moisture in regulating root hair growth in soils. The increase in root hair growth appears to be a response by the plant to stress as yield and P uptake by corn grown at M0 were only 0.47 to 0.82, and 0.34 to 0.74, respectively, of that measured at M1 across the five soil P levels. The increase in root hair growth at M0, which represents an increase of 2.76 to 4.03 in root surface area, could offset, in part, the reduced rate of root growth, which was the primary reason for reduced P uptake under limited soil moisture conditions.Journal Paper No. 10,066 Purdue Univ. Agric. Exp. Stn., W. Lafayette, IN 47907. Contribution from the Dep. of Agron. This paper was supported in part by a grant from the Tennessee Valley Authority. 相似文献
7.
A preliminary study of aquatic insect communities and leaf decomposition in acid streams near Dorset,Ontario 总被引:10,自引:10,他引:0
Some streams near Dorset in south-central Ontario suffer from acid precipitation via run-off and seepage from thin soils with little buffering capacity. A spring-summer survey of eight headwater streams revealed some characteristics of their insect communities which could be correlated with pH. The streams could be divided into three groups according to pH and community structure. In the most acid group (annual pH range 4.3–4.8), Ephemeroptera were absent from two streams although mature Leptophlebia were collected just after spring thaw from the most acid one (pH 4.3–4.5). One of these three streams also lacked Plecoptera but the others had two or three genera, all shredders. The second group of three streams (pH 5.0–6.3), with one exception, did support Ephemeroptera (3–4 genera) and Plecoptera (1–4 genera), most of the latter being shredders. In all six of these acid streams, Trichoptera were more diverse and more dense than Ephemeroptera and Plecoptera; again, shredders were clearly dominant, especially the limnephilid caddisfly, Frenesia difficilis (Walker). These six streams also had similar chironomid communities (densities were an order of magnitude higher than other insects). Dominance by Chironomini and abundant Tanypodinae typified the most acid streams. In contrast, the two streams in the third group (pH 5.3–6.7) had richer and more balanced communities in general with relatively fewer shredders (no Frenesia), more collectors, and fewer Chironomini and Tanypodinae. As a field experiment showed that autumn-shed leaves decomposed more slowly in acid than in non-acid streams, summer-growing shredders may benefit from the pulse of acidity at snowmelt. 相似文献
8.
D B Watson G F Burns I R Mackay 《Journal of immunology (Baltimore, Md. : 1950)》1983,130(5):2442-2447
Epstein Barr virus was used to transform the B lymphocytes infiltrating metastatic tumor tissue from seven patients with melanoma. In this way it was possible to establish continuously growing B lymphoblastoid cell lines (LCL) derived from the tumor-infiltrating B lymphocytes from each of the seven patients. Antibody production of up to 50 micrograms/ml could be achieved by such cultures, and the lymphoblastoid cells could be cloned readily by limit dilution on a feeder layer of irradiated fetal fibroblasts. Preliminary analysis of the antibodies produced by lymphoblastoid cell lines established from tumors from two of the patients indicated that most were of IgM type and bound to a panel of melanoma cell target cells, as well as to some nonmelanoma tumor cell lines. Cloned LCL were produced from the tumor-infiltrating B cells from one of the patients, and of 100 such clones tested, 9% secreted antibody that bound to autologous tumor cells, and one of these clones produced antibody that appeared to be melanoma specific. 相似文献
9.
10.
G. M. Mackay 《BMJ (Clinical research ed.)》1969,4(5686):799-801
Some aspects of urban and rural traffic accidents have been studied at the scene of some accidents in Birmingham and the county of Worcestershire. Accidents to pedestrians are essentially an urban problem, occur mainly at low speed, and most of the serious injury comes from the initial contact with the vehicle, rather than from secondary impacts with the road surface. The characteristics of motor-cycle accidents are more varied; in urban areas there are many side impacts, with consequent injury to the lower limbs, while rural collisions are predominantly front on, with a high incidence of head injury. Accidents to car occupants vary according to the environment. 相似文献