全文获取类型
收费全文 | 1937篇 |
免费 | 209篇 |
国内免费 | 800篇 |
专业分类
安全科学 | 200篇 |
废物处理 | 68篇 |
环保管理 | 184篇 |
综合类 | 1467篇 |
基础理论 | 504篇 |
污染及防治 | 302篇 |
评价与监测 | 65篇 |
社会与环境 | 80篇 |
灾害及防治 | 76篇 |
出版年
2024年 | 4篇 |
2023年 | 27篇 |
2022年 | 69篇 |
2021年 | 87篇 |
2020年 | 73篇 |
2019年 | 99篇 |
2018年 | 78篇 |
2017年 | 90篇 |
2016年 | 122篇 |
2015年 | 99篇 |
2014年 | 142篇 |
2013年 | 196篇 |
2012年 | 187篇 |
2011年 | 216篇 |
2010年 | 163篇 |
2009年 | 160篇 |
2008年 | 140篇 |
2007年 | 147篇 |
2006年 | 141篇 |
2005年 | 115篇 |
2004年 | 96篇 |
2003年 | 81篇 |
2002年 | 60篇 |
2001年 | 68篇 |
2000年 | 70篇 |
1999年 | 49篇 |
1998年 | 34篇 |
1997年 | 16篇 |
1996年 | 25篇 |
1995年 | 18篇 |
1994年 | 24篇 |
1993年 | 12篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1978年 | 2篇 |
1975年 | 1篇 |
排序方式: 共有2946条查询结果,搜索用时 562 毫秒
341.
342.
By scoring the chromosome number of developing embryos, we show that the sex ratio bias of the African social spider Stegodyphus dumicola Pocock is the result of an overproduction of female embryos. Only 17% of 585 embryos sexed from 14 egg sacs were male, a
significant departure from a 1:1 sex ratio. We also explored the possibility of direct control of the sex of individual offspring
in this species by examining the variance in the number of males per sac and the spatial distribution of male and female embryos
within the sacs. We postulated that a variance in the number of males per sac lower than binomial (i.e., underdispersed or
precise sex ratios) or a non-random distribution of male embryos within the sacs would suggest direct control of the sex of
individual offspring. We found that the variance in the number of males per sac was indistinguishable from binomial and significantly
larger than expected under exact ratios. Likewise, the spatial distribution of male embryos within three sacs examined was
no more clustered than expected by chance. The sex ratio biasing mechanism in this species, therefore, apparently only allows
control of the mean sex ratio but not of its variance. We present randomization and Monte Carlo methods that can be applied
to test for departures from a random spatial arrangement of male and female embryos in an egg mass and for departures from
binomial or exact ratios when not all members of a clutch have been sexed.
Received: 21 October 1998 / Received in revised form: 23 March 1999 / Accepted: 26 April 1999 相似文献
343.
Variance in female quality, operational sex ratio and male mate choice in a bushcricket 总被引:8,自引:0,他引:8
Male bushcrickets, Kawanaphila nartee, exercise mate choice when nutrients are limited. Male mate choice is associated with a female-biased operational sex ratio
(OSR) that arises from an increased relative paternal investment under nutrient limitation. However, increased male choosiness
could be attributable to the fact that females vary more in fecundity, and consequently in mate quality, when nutrient limited.
Our objective was to experimentally partition the influences of OSR (male or female bias) and variance in mate quality (high
or low) and to assess their relative influence on the intensity of mate choice by male bushcrickets. Female quality was manipulated
by controlled feeding regimes that directly affected female fecundity. We found that males and females engaged in sexual interactions
sooner under a male-biased than a female-biased OSR. Males were more likely to reject females on their first encounter when
variance in female quality was high. However, the effect of quality variance on the total number of rejections during a 4-h
observation period was dependent on the perceived OSR. A male's prior experience of variance in female quality did not influence
male choosiness. Our observed rates of mate rejection conformed well with those predicted from recent theoretical models of
sexual differences in choosiness. In conclusion, our results show that the opportunity for selection via male mate choice
is influenced by an interaction between OSR and the variance in mate quality that arises within nutrient-limited populations
of females.
Received: 5 January 1998 / Accepted after revision: 25 October 1998 相似文献
344.
Contradictory findings in studies of sex ratio variation in roe deer (Capreolus capreolus) 总被引:3,自引:0,他引:3
A. J. Mark Hewison Reidar Andersen Jean-Michel Gaillard John D. C. Linnell Daniel Delorme 《Behavioral ecology and sociobiology》1999,45(5):339-348
Patterns of sex ratio variation and maternal investment reported in the literature are often inconsistent. This could be
due to intra- and inter-specific variation in social systems, but may also be a result of the a posteriori nature of much
of this type of analysis or the testing of models which are inappropriate. Two recent papers reported directly opposed results
concerning variation in offspring sex ratios in relation to maternal condition in roe deer, interpreting the results as support
for the Trivers and Willard model and for the local resource competition hypothesis, respectively. In this paper, we present
data on offspring sex ratios and early juvenile body weight from two long-term studies of this species to test predictions
arising from these two models concerning sex biases in litter composition and maternal care. First, we observed no consistent
pattern of sex differences in an index of weaning weight or body weight at 1 month old in either population, indicating a
lack of sex bias in maternal care. However, in one population, higher maternal body weight was associated with higher juvenile
body weight of daughters, but not of sons. Secondly, we found a negative, but not statistically significant, relationship
between maternal body weight and litter sex ratio such that heavier females tended to produce more daughters and lighter females
to produce more sons. These results indicate that roe females which have additional investment potential available do not
invest it in sons, as predicted by the Trivers and Willard model. Our results may provide some support that roe deer are subject
to local resource competition acting at the level of the individual mother; however, the fact that particular trends in sex
ratio data can be explained in functional terms provides no indication that they are actually adaptive.
Received: 9 December 1997 / Accepted after revision: 11 November 1998 相似文献
345.
The host size model, an adaptive model for maternal manipulation of offspring sex ratio, was examined for the parasitoid
wasp Spalangia endius. In a Florida strain, as the model predicts, daughters emerged from larger hosts than sons, but only when mothers received
both small and large hosts simultaneously. The pattern appeared to result from the mother's ovipositional choice and not from
differential mortality of the sexes during development. If sex ratio manipulation is adaptive in the Florida strain, it appears
to be through a benefit to daughters of developing on large hosts rather than through a benefit to sons of developing on small
hosts. Both female and male parasitoids were larger when they developed on larger hosts. For females, developing on a larger
host (1) increased offspring production, except for the largest hosts, (2) increased longevity, (3) lengthened development,
and (4) had no effect on wing loading. For males, development on a larger host had no effect on any measure of male fitness
– mating success, longevity, development duration, or wing loading. In contrast, a strain from India showed no difference
in the size of hosts from which daughters versus sons emerged, although both female and male parasitoids were larger when
they developed on larger hosts. These results together with previous studies of Spalangia reveal no consistent connection between host-size-dependent sex ratio and host-size-dependent parasitoid size among strains
of S. endius or among species of Spalangia.
Received: 28 October 1998 / Received in revised form: 20 May 1999 / Accepted: 30 May 1999 相似文献
346.
研究并探讨了蔬菜作物在施用耐氨固氮菌后的生长、生理反应及增产的效果和原因。试验结果表明: 施用耐氨固氮菌显著增加了蔬菜苗期生物量、叶面积和叶片的叶绿素含量, 同时还增强了植株的根系活力, 并有明显的增产效果。耐氨固氮菌促进增产的作用不仅与其本身的固氮作用有关, 还可能与其分泌出某些生长刺激物 ( 或激素) 有关。 相似文献
347.
从产于粤北山区的野生建兰报中分离出4株内生真菌,并对这些真菌在兰根上的感染特征及生理学特性进行了研究。结果表明,菌根真菌多数浸染植株根段离很尖3~18cm的根毛区,而根尖和新长出的根则很少被感染;几乎所有的老根都受过真菌的感染。真菌菌丝从兰报表皮侵入,通过表皮层进入皮层薄壁细胞内部形成菌丝团,而在表皮层并不进入细胞内,也不形成菌丝团。真菌感染极状整后第9天,皮层细胞内的菌丝团开始消解。所分离的菌株能利用大部分简单的有机碳源和氮源,能在营养成分不明确的天然介质中生长良好,但对硝酸盐的利用较差。 相似文献
348.
小麦-玉米轮作制度下潮土硝态氮的分布及合理施氮肥研究 总被引:1,自引:0,他引:1
利用田间小区试验研究了小麦-玉米轮作中在不同施氮量和不同时期潮土1m土层中硝态氮的计中状况结果表明:3年不施氮肥的土壤仍有6~16kg/hm2的硝态氮,其中0~20cm土层占20%~34%.80%~100cm上层占10%~18%;每季施氨量小于225kg/hm2时.1m上层中各时期硝态氮含量变化不大,在11.4~41.3kg/hm2之间;当施氮量增加到375kg/hm2时,1m土层的硝态氮含量增加1.5~7.4倍,有7.4%~29.9%分布在0~20cm上层中,10%~16%分布在80~100cm上层中225kg/hm2对作物斋求及对土壤环境来说是一个合理的施只量。 相似文献
349.
江津市紫色土中N、P养分元素区域空间变异性研究 总被引:49,自引:2,他引:47
利用地统计学,结合GIS研究紫色土土壤表层(0~20cm)的全氮、碱解氮、全磷和速效磷4种养分含量的空间分布特征结果表明,全氮和速效氮为正态分布,全磷和速效磷为对数正态分布;通过半方差函数分析,发现全氮存在纯块金效应,块金值为0.2,其它养分在一定间距内(50m)存在空间相关性,且为中等强度的空间自相关性(块金值与基台值之比在25%~75%之间);用普通克立格法和对数正态克立格法进行最优内插,做各种养分含量的分布图.结合GIS可充分了解土壤中N、P养分的空间变异性规律,并可进一步应用于精确施肥和农业非点源污染预测和控制. 相似文献
350.