全文获取类型
收费全文 | 6839篇 |
免费 | 666篇 |
国内免费 | 503篇 |
专业分类
8008篇 |
出版年
2024年 | 16篇 |
2023年 | 140篇 |
2022年 | 142篇 |
2021年 | 201篇 |
2020年 | 243篇 |
2019年 | 276篇 |
2018年 | 226篇 |
2017年 | 263篇 |
2016年 | 259篇 |
2015年 | 264篇 |
2014年 | 303篇 |
2013年 | 462篇 |
2012年 | 234篇 |
2011年 | 283篇 |
2010年 | 242篇 |
2009年 | 314篇 |
2008年 | 330篇 |
2007年 | 363篇 |
2006年 | 350篇 |
2005年 | 298篇 |
2004年 | 247篇 |
2003年 | 224篇 |
2002年 | 251篇 |
2001年 | 208篇 |
2000年 | 197篇 |
1999年 | 147篇 |
1998年 | 131篇 |
1997年 | 111篇 |
1996年 | 119篇 |
1995年 | 114篇 |
1994年 | 135篇 |
1993年 | 75篇 |
1992年 | 102篇 |
1991年 | 83篇 |
1990年 | 72篇 |
1989年 | 59篇 |
1988年 | 49篇 |
1987年 | 50篇 |
1986年 | 42篇 |
1985年 | 55篇 |
1984年 | 59篇 |
1983年 | 28篇 |
1982年 | 39篇 |
1981年 | 36篇 |
1980年 | 46篇 |
1979年 | 20篇 |
1978年 | 17篇 |
1977年 | 26篇 |
1976年 | 16篇 |
1973年 | 14篇 |
排序方式: 共有8008条查询结果,搜索用时 15 毫秒
111.
112.
Six successive summer generations of the alder aphid (Pterocallis alni (DeGeer) (Homoptera: Callaphididae)) were reared simultaneously in the field and under controlled temperature conditions in the laboratory. The growth and reproduction of each generation were recorded. The available food quality for the aphids was measured by weekly analysis of foliar soluble nitrogen concentrations. Although there was a significant change in leaf soluble nitrogen during the season, the decline was only 23% and this did not appear to have an adverse effect on the performance of this aphid. Instead, the major environmental factor affecting the aphid is temperature. This is evidenced by the facts that when aphids were reared at constant temperatures, there were no differences in generation performance, even though food quality varied seasonally. In addition, all generations of the aphid posess the same number of ovarioles, indicating that there is no pre-programmed anticipation of a seasonal deterioration in food quality in this aphid species.
Résumé P. alni DeGeer (Homopt. Callaphididae) vit sur Alnus glutinosa Gaertner pendant toute l'année. L'aulne étant un arbre fixant l'azote, cette étude a été entreprise pour examiner la croissance et la reproduction de ce puceron sur cet arbre afin d'expliquer les changements de la dynamique de sa population. 6 générations successives estivales ont été élevées dans la nature et en température constante au laboratoire. La qualité de l'aliment a été mesurée chaque semaine par une microkjeldahl analyse des concentrations foliaires en azote soluble. La qualité de l'aliment varie saisonnière, mais ne diminue que de 23%; elle semble n'avoir que peu d'effets sur le puceron, puisque la fécondité dans la nature n'est réduite que pendant une génération. Quand la température est maintenue constante, il n'y a pas de différences entre les performances des différentes générations de pucerons. Ainsi, les changements saisonniers dans la croissance et la reproduction des pucerons sont provoqués par la température et non par la qualité de l'aliment.相似文献
113.
Summary An ultrastructural and morphometric analysis was made of the nucleolar components in pinealocytes of 40 male Fischer rats sampled at eight times in an LD 1212 photoperiod cycle. Comparisons of results from the eight times showed variation in estimated mean volume of the granular component of ±29%, and of the fibrillar component ±11%, in relation to daily means. Peaks in mean volume of total nucleolus and its granular component occurred at 1 h of light. Near maximal and minimal mean volumes of the fibrillar component both occurred during both light and dark. Fibrillar centers (nucleolar organizer regions) of different sizes were found at all sampling times. It is concluded that temporal patterns in 24-h changes in the nucleolar components are most prominent in the granular component, and are more complex than suggested by changes in total nucleolar size or mean dimensions, and than represented by a simple biphasic circadian rhythm. Examples of different stages in the migration of the granular component, and of possible sites of nucleo-cytoplasmic transfer of nucleolar material, are described. 相似文献
114.
When varanid lizards, Varanus niloticus, were allowed to select their preferred body temperature (c. 34°C) they exhibited swifter growth, larger food intake and superior efficiency of conversion when compared with animals restricted to a maximum body temperature of 24°C. At the higher temperature the animals also exhibited a higher metabolic rate and increased thyroid activity, while those at the lower temperature lostproportionately more energy via respiration. A complete energy budget over a period of 6 weeks is presented. 相似文献
115.
Rates of nitrogen and phosphorus release from individualDaphnia magna were determined by measuring ammonia and soluble reactive phosphorus in successive 10-min incubations in small (0.05 ml) vessels after the animals were removed from their food. Release rates of both nutrients were generally highest initially and decreased with time after removal. The ratio of nitrogen to phosphorus released increased with time after animals were removed from an artificial detritus/bacterial food; ratios were lower and changed with time less for animals fed algae. These data suggest errors may be introduced by assumptions of constant stoichiometry for nutrient release in varying environments.GLERL Contribution No. 268 相似文献
116.
The late Miocene North African mammalianassemblage is considered here from three viewpoints: survivals, extinctions, and immigrations. The Eurasiatic affinities of the large mammals slightly prevail over the Ethiopian affinities. Amongst the North African large mammals, 4 to 8 taxa are Eurasiatic immigrants, while 4 to 6 are of Subsaharian origin. Contrarily, the micromammalian fauna is highly endemic, with only one species, a murid (Paraethomys miocaenicus), considered here as being related to an Asiatic form (Karnimata darwini). Our study of Eurasian and African Miocene faunas reveals that during the late Astaracian-early Turolian interval, the Saharo-Arabic belt permitted very little latitudinal faunal exchanges. However, during the middle and late Turolian such faunal exchanges became frequent. The micromammal record unequivocally indicates that a brief period of faunal exchange occurred between Africa and western Europe at the end of the Miocene, corresponding with the Messinian Salinity Crisis. The increased intercontinental faunal exchange between Africa and Eurasia during the late Miocene coincides with, and counterbalances the extinction of more than 10 taxa at the Mio/Pliocene boundary. 相似文献
117.
Against the background of cercarial fine structure, ultrastructural changes were compared in schistosomules of Schistosoma mansoni 30 min and 1 hr after their production in vivo by skin penetration and in vitro by shear pressure. The same developmental pattern was observed in schistosomules of both derivations. In vitro schistosomules, however, developed more slowly, resembled cercariae more closely, and varied less among organisms than did in vivo schistosomules. The greatest morphological changes were observed in the 1-hr in vivo schistosomules. These were as follows: (1) in tegument, formation of transient microvilli, a hepatalaminate outer membrane and accented surface invaginations, loss of glycocalyx, movement outward of cyton vesicles via bridges, accumulation of multilaminate bodies around bridge openings; (2) in the anterior organ (oral sucker), movement of head gland vesicles via the ducts into tegument followed by collapse of the gland fundus, disappearance of the circumfundal cells and two large support cells, and the appearance in these areas of membranes and parenchymal cells; (3) secretion of the acetabular gland contents, collapse of the glands and replacement by membranes and parenchymal cells; (4) peristaltic activity of the digestive tract as shown by alternate areas of lumen constriction and dilation; (5) loss of bladder and contraction of the small aboral collecting tubules; and (6) conversion of heterochromatic parenchymal cell nuclei to euchromatic. In contrast, the 1-hr in vitro shear schistosomules resembled 30-min in vivo schistosomules, retaining many cercarial features. 相似文献
118.
K. S. Chung 《Hydrobiologia》1981,78(2):177-181
The acclimation rates of temperature changes in Cyprinodon dearborni, collected from Laguna Los Patos, Cumana, Venezuela, were determined by the critical thermal maximum method.
At an increase in temperature (from 24 to 31°C) fish started to gain acclimation level after 3 hours and took 3 days to fully
get up to a higher level of resistance to heat death; however, at a decrease in temperature (from 3 t to 24°C) fish began
to lose its acclimation level after 12 days and required 39 days to reach a lower level of resistance to thermal death. 相似文献
119.
James W. Moore 《Hydrobiologia》1981,83(3):505-510
Factors influencing the species composition, distribution and abundance of benthic invertebrates were determined in a eutrophic subarctic lake from April 1978 to April 1979. Collections were made at five stations located at depths of 4 to 13 m. The largest populations of up to 5 × 103 animals m–2 were found in the deepest part of the lake. of the 24 species recorded in this area, the chironomidsProcladius denticulatus, Dicrotendipes modestus, Chironomus decorus andGlyptotendipes barbipes were most common. The strong development of benthos in the profundal zone was attributed to a consistently large supply of food and warm (4 °C) winter temperatures on bottom. Slightly smaller populations (up to 4 × 103 animals, m–2), composed of 19–23 species, occurred in shallower water, a reflection of lower (1.5 °C) winter temperatures. In the anoxic northern part of the lake, only 4–8 species were found in low numbers (400–1 000 animals m–2). This was likely due to low (<5% saturation) oxygen levels in water and high organic content (18.5%) of the sediments. 相似文献
120.
Bilateral eyestalk ablation in the freshwater prawn Macrobrachium lanchesteri results in high mortality, while unilateral eyestalk ablated prawns exhibited a high survival rate. There was marked increase in the growth of bilateral eyestalk-ablated prawns (47.70 mg/prawn) as compared to those that were unilaterally ablated (19.19 mg/prawn). 相似文献