全文获取类型
收费全文 | 1890篇 |
免费 | 128篇 |
国内免费 | 169篇 |
专业分类
2187篇 |
出版年
2024年 | 13篇 |
2023年 | 15篇 |
2022年 | 21篇 |
2021年 | 25篇 |
2020年 | 19篇 |
2019年 | 26篇 |
2018年 | 33篇 |
2017年 | 40篇 |
2016年 | 26篇 |
2015年 | 41篇 |
2014年 | 54篇 |
2013年 | 48篇 |
2012年 | 43篇 |
2011年 | 48篇 |
2010年 | 39篇 |
2009年 | 117篇 |
2008年 | 107篇 |
2007年 | 112篇 |
2006年 | 113篇 |
2005年 | 103篇 |
2004年 | 73篇 |
2003年 | 57篇 |
2002年 | 49篇 |
2001年 | 39篇 |
2000年 | 59篇 |
1999年 | 67篇 |
1998年 | 64篇 |
1997年 | 60篇 |
1996年 | 47篇 |
1995年 | 43篇 |
1994年 | 54篇 |
1993年 | 52篇 |
1992年 | 49篇 |
1991年 | 53篇 |
1990年 | 48篇 |
1989年 | 37篇 |
1988年 | 43篇 |
1987年 | 37篇 |
1986年 | 29篇 |
1985年 | 33篇 |
1984年 | 33篇 |
1983年 | 10篇 |
1982年 | 23篇 |
1981年 | 29篇 |
1980年 | 20篇 |
1979年 | 15篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有2187条查询结果,搜索用时 11 毫秒
251.
Nitrogen distribution and leaf area indices in relation to photosynthetic nitrogen use efficiency in savanna grasses 总被引:3,自引:0,他引:3
Leaf photosynthetic characteristics, distribution patterns of nitrogen content per unit leaf area (nL) and leaf area production per unit nLwere measured in natural stands of a C4 grass (Hyparrhenia rufa) from the seasonal savannas and of a C4grass (Paspalum fasciculatum) and two C3grasses (Leersia hexandra and Hymenachne amplexicaulis) from the flooded savannas in central Venezuela. Daily rates of canopy photosynthesis (PcD) as well as the optimal leaf area production per unit nLat which PcDfor a given total amount of nitrogen in the canopy (i.e., canopy-PNUE) is maximized were also calculated. The C3and C4species from the flooded savannas had similar light saturated rates of photosynthesis per unit nL(i.e. leaf-PNUE) and similar canopy-PNUEs which was in strong contrast with previous studies. Especially H. rufa but also L. hexandra and H. amplexicaulis had leaf- and canopy-PNUEs which were considerably higher than the values calculated for most other species with the same photosynthetic pathway (i.e., C3or C4). In contrast to previous studies, differences in the light gradient in the canopy between stands only partially explained differences in N distribution. Measured leaf area indices were greater and the average nL values were consequently smaller than the calculated optima. There was, however, a very strong linear correlation between the optimal and actual average nLindicating that even though the model overestimated average nL, it did predict the differences in leaf area production per unit nitrogen – the inverse average nL– very well. This result strongly indicates that leaf area production per unit of leaf nitrogen increases with leaf-PNUE and decreases with the extinction coefficient for light. Grass species from seasonal savannas have extremely high leaf-PNUEs and thus optimally produce large amounts of leaf area per unit nL. This helps explain how stands of these species may have high leaf area indices and achieve high photosynthetic productivity despite the very low nutrient availability at which they grow. 相似文献
252.
Questions: How are leaf attributes and relative growth rate (RGR) of the dominant tree species of tropical deciduous forest (TDF) affected by seasonal changes in soil moisture content (SMC)? What is the relationship of functional attributes with each other? Can leaf attributes singly or in combination predict the growth rate of tree species of TDF? Location: Sonebhadra district of Uttar Pradesh, India. Methods: Eight leaf attributes, specific leaf area (SLA); leaf carbon concentration (LCC); leaf nitrogen concentration (LNC); leaf phosphorus concentration (LPC); chlorophyll concentration (Chl), mass‐based stomatal conductance (Gsmass); mass based photosynthetic rate (Amass); intrinsic water use efficiency (WUEi); and relative growth rate (RGR), of six dominant tree species of a dry tropical forest on four sites were analysed for species, site and season effects over a 2‐year period. Step‐wise multiple regression was performed for predicting RGR from mean values of SMC and leaf attributes. Path analysis was used to determine which leaf attributes influence RGR directly and which indirectly. Results: Species differed significantly in terms of all leaf attributes and RGR. The response of species varied across sites and seasons. The attributes were positively interrelated, except for WUEi, which was negatively related to all other attributes. The positive correlation was strongest between Gsmass and Amass and the negative correlation was strongest between Gsmass and WUEi. Differences in RGR due to site were not significant when soil moisture was controlled, but differences due to season remained significant. The attributes showed plasticity across moisture gradients, which differed among attributes and species. Gsmass was the most plastic attribute. Among the six species, Terminalia tomentosa exhibited the greatest plasticity in six functional attributes. In the step‐wise multiple regression, Amass, SLA and Chl among leaf attributes and SMC among environmental factors influenced the RGR of tree species. Path analysis indicated the importance of SLA, LNC, Chl and Amass in determining RGR. Conclusion: A mass, SMC, SLA and Chl in combination can be used to predict RGR but could explain only three‐quarters of the variability in RGR, indicating that other traits/factors, not studied here, are also important in modulating growth of tropical trees. RGR of tree species in the dry tropical environment is determined by soil moisture, whereas the response of mature trees of different species is modulated by alterations in key functional attributes such as SLA, LNC and Chl. 相似文献
253.
In this paper, twenty eight species of the genus Salix, with similar phenotypic characters, were selected to study the leaf epidermal microfeatures under Scanning Electronic Microscope (SEM). Seven types of wax layer are totally detected in those species, including smooth layer, crust, fissured layer, non entire platelets, membraneous platelets, conicoids and scale like conicoids. Among those types, conicoids and scale like conicoids are exclusively found in Salix, while non entire platelets and membraneous platelets in Salix are depicted for the first time. The microfeatures of leaf hairs of Salix plants under SEM are less diversified than their macrofeatures, with detectable microfeature variations on hair length, density and degree of curve (straight, slightly curved and deeply curved). In conclusion, our results showed leaf epidermal microfeatures of wax layer types and stomatal apparatus are relatively unchangeable, which are valuable in the discrimination of similar Salix species. The Salix species from colder region and/or higher elevation usually bear diverse microfeatures, which might be adaptive evolution to colder environments. 相似文献
254.
The effects of temperature on photosynthesis of a rosette plant growing at ground level, Acaena cylindrostachya R. et P., and an herb that grows 20–50 cm above ground level, Senecio formosus H.B.K., were studied along an altitudinal gradient in the Venezuelan Andes. These species were chosen in order to determine
– in the field and in the laboratory – how differences in leaf temperature, determined by plant form and microenvironmental
conditions, affect their photosynthetic capacity. CO2 assimilation rates (A) for both species decreased with increasing altitude. For Acaena leaves at 2900 m, A reached maximum values above 9 μmol m−2 s−1, nearly twice as high as maximum A found at 3550 m (5.2) or at 4200 m (3.9). For Senecio leaves, maximum rates of CO2 uptake were 7.5, 5.8 and 3.6 μmol m−2 s−1 for plants at 2900, 3550 and 4200 m, respectively. Net photosynthesis-leaf temperature relations showed differences in optimum
temperature for photosynthesis (A
o.t.) for both species along the altitudinal gradient. Acaena showed similar A
o.t. for the two lower altitudes, with 19.1°C at 2900 m and 19.6°C at 3550 m, while it increased to 21.7°C at 4200 m. Maximum
A for this species at each altitude was similar, between 5.5 and 6.0 μmol m−2 s−1. For the taller Senecio, A
o.t. was more closely related to air temperatures and decreased from 21.7°C at 2900 m, to 19.7°C at 3550 m and 15.5°C at 4200 m.
In this species, maximum A was lower with increasing altitude (from 6.0 at 2900 m to 3.5 μmol m−2 s−1 at 4200 m). High temperature compensation points for Acaena were similar at the three altitudes, c. 35°C, but varied in Senecio from 37°C at 2900 m, to 39°C at 3550 m and 28°C at 4200 m. Our results show how photosynthetic characteristics change along
the altitudinal gradient for two morphologically contrasting species influenced by soil or air temperatures.
Received: 5 July 1997 / Accepted: 25 October 1997 相似文献
255.
256.
研究了西双版纳热带季节雨林6种乔木幼树在林窗中的叶生长与叶虫食动态。6种幼树叶生长主要在雨季(5~10月),在旱季雾凉期(11~2月)叶停止生长。阳性树种旱季干热期(3~4月)开始叶生长,顶极树种至5月雨季初才开始叶生长。表明雾凉期低温抑制两类树种叶生长,干热期水分不足抑制顶极树种叶生长。叶生长同步性(每两个月的叶生长量变异系数C.V.)顺序为:望天树(Shorea chinensis,1.42)>金钩花(Pseuduvaria indochinensis,1.41)>八宝树(Duabanga grandiflora, 1.02)>云南石梓(Gmelina arborea,0.98)>团花树(Anthocephalus chinensis,0.84)和铁刀木(Cassia siamea,0.84)。旱季造成一些种类幼树出现落叶高峰,严重叶虫食也导致云南石梓在7~8月出现落叶高峰。叶虫食主要出现在雨季,团花树和云南石梓在干热期叶生长能减少昆虫取食,但此时铁刀木叶同步生长却不能降低食叶昆虫危害。6种幼树叶生长量年进程与叶虫食量年进程间存在正相关,其中望天树、金钩花、八宝树和团花树分别达到显著(p<0.05)或极显著(p<0.01)水平。望天树和金钩花在雨季初叶同步生长能减轻叶虫食。食叶昆虫偏爱取食幼叶,6种幼树平均幼叶的虫食量占总虫食量的72.9%,幼叶虫食速率平均为成熟叶的4.3倍。 相似文献
257.
By ammonium sulfate fractionation and gel filtration an enzyme preparation which catalyzed NAD+-dependent L-lactate oxidation (10-4 kat kg-1 protein), as well as NADH-dependent pyruvate reduction (10-3 kat kg-1 protein), was obtained from leaves of Capsella bursa-pastoris. This lactate dehydrogenase activity was not due to an unspecific activity of either glycolate oxidase, glycolate dehydrogenase, hydroxypyruvate reductase, alcohol dehydrogenase, or a malate oxidizing enzyme. These enzymes could be separated from the protein displaying lactate dehydrogenase activity by gel filtration and electrophoresis and distinguished from it by their known properties. The enzyme under consideration does not oxidize D-lactate, and reduces pyruvate to L-lactate (the configuration of which was determined using highly specific animal L-lactate dehydrogenase). Based on these results the studied Capsella leaf enzyme is classified as L-lactate dehydrogenase (EC 1.1.1.27). It has a Km value of 0.25 mmol l-1 (pH 7.0, 0.3 mmol l-1 NADH) for pyruvate and of 13 mmol l-1 (pH 7.8, 3 mmol l-1 NAD+) for L-lactate. Lactate dehydrogenase activity was also detected in the leaves of several other plants.Abbreviation FMN
flavin adenine mononucleotide 相似文献
258.
Measurements of net CO2 exchange, malate accumulation, properties and capacity of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) in leaves of different ages of two short-day dependent Crassulacean acid metabolism (CAM) plants (Kalanchoe blossfeldiana v. Poelln. Tom thumb and K. velutina Welw.) show that, in both species: a) young leaves from plants grown under long days display a CO2 exchange pattern typical of C3 plants; b) leaf aging promotes CAM under long-day conditions; c) short-day treatment induces CAM in young leaves to a higher degree than aging under long days; d) at least in K. blossfeldiana, the PEPC form developed with leaf aging under long days and the enzyme form synthetized de novo in young leaves grown under short days were shown to have similar properties. Short days also promote CAM in older leaves though at a lesser extent than in young leaves: The result is that this photoperiodic treatment increases the general level of CAM performance by the whole plant. The physiological meaning of the control of PEPC capacity by photoperiodism could be to afford a precisely timed seasonal increase in CAM potentiality, enabling the plant to immediately optimize its response to the onset of drought periods.Abbreviations CAM
Crassulacean acid metabolism
- PEP
phosphoenolpyruvate
- PEPC
phosphoenolpyruvate carboxylase (EC 4.1.1.31)
- LD
long day
- SD
short day 相似文献
259.
We studied the spatial distribution of fruits and plants, mortality and growth rates ofScaphium macropodum (Sterculiaceae) in four 1-ha plots in a tropical rain forest in West Kalimantan, Indonesia. The species is a large deciduous
tree and produces wind-dispersed fruits on defoliated twigs. The density of dispersed fruits on the ground decreased with
increasing distance from a parent tree. The area under the parent's crown had the highest density of the fruits and the highest
mortality of the seedlings immediately after germination. Consequently, the density of the established seedlings peaked 14
m from the tree which is outside its crown. Thick litter mainly from the parent tree seemed to physically prevent the seedlings'
root from reaching the soil surface and caused the high mortality. Juvenile and mature trees distributed exclusively, suggesting
that regeneration is the most successful outside of the crown of mature trees. Saplings under canopy shade did not grow well.Scaphium macropodum is hypothesized to require a gap for seedling growth and successful regeneration, whereas it can germinate and last under
closed canopies as suppressed seedlings or saplings. 相似文献
260.
《Saudi Journal of Biological Sciences》2022,29(1):296-303
A 105-day experimental trial was conducted to assess different levels of dietary Aleo vera extract supplementation on water quality parameters, proximate composition, growth performance and haematological parameters of fry Oreochromis niloticus. Four different percentages of dietary leaf extract powder of Aleo vera (ALE) with a basal feed, designated as, i.e., T0 (Control group; without ALE), T1 (1% ALE), T2 (2% ALE), and T3 (3% ALE). Fish fry was reared in concrete tanks (7.0 m, 1.6 m, 1.0: L, W, H; water volume 11.2 m3/tank), with an average initial weight 4.04 ± 0.03 g/ fry, and each treatment was triplicated. Fry was randomly distributed at a stocking rate of 450 individuals/ tanks. The water quality parameters revealed that temperature, pH, salinity, dissolved oxygen (DO) and nitrates were found in a promising range as given by FAO/WHO limits. However, the record values obtained for Electric Conductivity (EC), Total dissolved solids (TDS), and alkalinities were not found in all tanks' suitable range according to FAO/WHO limits. The results revealed a significant impact of different percentages of dietary ALE supplementation on fry's body composition and haematological parameters. Moreover, the final body weight, final body length, average daily weight gain (g), net weight gain (g) and specific growth rate (%) were significantly higher (p < 0.05) in T1 and T2 compared with T0 and T3 treatments. The poorest feed conversion ratio was recorded in the T2 group compared with other treatments. Thus, the current study provides information about the nutritional quality of Nile tilapia culturing in Pakistan. 相似文献