全文获取类型
收费全文 | 15622篇 |
免费 | 1007篇 |
国内免费 | 1345篇 |
专业分类
17974篇 |
出版年
2024年 | 48篇 |
2023年 | 277篇 |
2022年 | 312篇 |
2021年 | 502篇 |
2020年 | 438篇 |
2019年 | 524篇 |
2018年 | 403篇 |
2017年 | 437篇 |
2016年 | 519篇 |
2015年 | 569篇 |
2014年 | 737篇 |
2013年 | 905篇 |
2012年 | 547篇 |
2011年 | 645篇 |
2010年 | 561篇 |
2009年 | 702篇 |
2008年 | 754篇 |
2007年 | 846篇 |
2006年 | 716篇 |
2005年 | 676篇 |
2004年 | 652篇 |
2003年 | 571篇 |
2002年 | 508篇 |
2001年 | 421篇 |
2000年 | 422篇 |
1999年 | 372篇 |
1998年 | 410篇 |
1997年 | 319篇 |
1996年 | 338篇 |
1995年 | 302篇 |
1994年 | 251篇 |
1993年 | 232篇 |
1992年 | 249篇 |
1991年 | 219篇 |
1990年 | 177篇 |
1989年 | 174篇 |
1988年 | 142篇 |
1987年 | 119篇 |
1986年 | 109篇 |
1985年 | 148篇 |
1984年 | 114篇 |
1983年 | 74篇 |
1982年 | 109篇 |
1981年 | 94篇 |
1980年 | 66篇 |
1979年 | 72篇 |
1978年 | 55篇 |
1977年 | 41篇 |
1976年 | 26篇 |
1973年 | 22篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
Calli of P. argentatum were grown on a newly designed liquid nutrient flow-through system which facilitated the subculturing of calli and delayed
browning for 6 weeks. Friable calli were obtained on half-strength Gamborg B5-medium supplemented with 0.05 mgl−1 2,4-dichlorophenoxyacetic acid. Shoots developed on media supplemented with 0.2 mgl−1 benzylaminopurine but lacking 2,4-dichlorophenocyacetic acid. 相似文献
83.
Summary Two pairs of ganglia are found in the propodial region of the veliger of Onchidoris bilamellata: the anterolateral pair is located at the foremost corners of the propodium, and the frontal pair is located beside the propodial midline. Both sets of ganglia are positioned below the epidermis, and they are joined to the cerebral ganglia by large, common connectives. Each ganglion possesses sensory cells, nerve cells and sheath cells, and the frontal pair contains a complement of secretory cells. Externally, the propodial ganglia are manifested as sensory fields. The fields of the anterolateral pair are elliptical in shape, and each appears as a band of cilia bordering an unciliated zone. The region devoid of cilia is composed of ordinary epidermal cells, whereas the ciliated portion is comprised of dendritic endings originating from cells in the ganglion. Dendrites arise from one type of sensory cell and pass through the epidermis in bundles. Each dendrite terminates as a single cilium at the epidermal surface. Sensory fields of the frontal ganglia are key-shaped and oppose one another on the anterior end of the foot. Each field appears as a flat, circular, unciliated region which extends into a ciliated groove that runs dorsally toward the mouth. The groove contains the terminals of secretory cells, ciliated sensory cells, and the cell bodies of nonciliated sensory cells. The nonciliated sensory cells, characterized by a microvillous apex, are the dominant cells in the flattened circular zone. The space between the frontal ganglia and the epidermis is bridged by bundles of processes which are similar to those of the anterolateral ganglia. However, these tracts contain collections of the apical processes of secretory cells, the dendrites of ciliated sensory cells, and the axons of nonciliated sensory cells. Morphological and behavioral evidence indicates that the propodial ganglia serve a chemosensory function during settlement and metamorphosis. 相似文献
84.
A classification scheme for those population models which allow variation in development rates is proposed, based on two ways of modifying standard age-structured models. The resulting classes of models are termed development index models and sojourn time models. General formulations for the two classes of models are developed from two basic balance equations, and numerous specific models from the literature are shown to fit into the scheme. Concepts from competing risks theory are shown to be important in understanding the interplay between mortality and maturation. Relationships among the classes are investigated both for the most general forms of the models and for the simpler forms often used. The scheme can provide guidance in developing appropriate insect population models for specific modelling situations.Contribution 3878871 相似文献
85.
The effect of temperature on the development times of eggs,naupliar and copepodite stages of five species of cyclopoid copepods 总被引:2,自引:1,他引:1
Gerhard Maier 《Hydrobiologia》1989,184(1-2):79-88
The duration times of eggs, combined naupliar instars and of the different copepodite stages of five species of cyclopoid
copepods — Acanthocyclops robustus, Cyclops vicinus, Diacyclops bicuspidatus, Mesocyclops leuckarti, and Thermocyclops crassus — were investigated at five different temperatures. The five species can be divided in two groups: two species, C. vicinus and D. bicuspidatus, adapted to cold water conditions and three species, A. robustus, M. leuckarti and T. crassus adapted to warm water conditions. The cold water species showed a faster egg development than M. leuckarti and T. crassus at 5–15 °C. The eggs and instars of the warm water species M. leuckarti tend to develop faster than those of the former two species at higher temperatures. A. robustus showed the shortest egg and instar development at 10–25 °C. The warm water species T. crassus produced no eggs at 10 °C and temperatures below. At higher temperatures (20, 25 °C) the egg and instar duration times were
similar or longer than those of the other species. When cultured in total darkness a great part of the CIV respectively CV copepodites of the summer forms entered arrest and the percentage of copepodites that showed an arrest of development was
highest at lowest temperatures. The present results are compared with data from literature and differences are discussed. 相似文献
86.
Patricia Lyons Johnson 《Human ecology: an interdisciplinary journal》1990,18(4):403-416
The European demographic transition of the nineteenth century is often proposed as a model for demographic change in twentieth century developing nations, and economic development is seen as leading to an inevitable reduction in total fertility in these nations. This paper examines data from the Gainj of Papua New Guinea, a natural fertility population with very low reproductive output, and suggests that the effects of development on fertility change are much more complex than a simple demographic transition model would suggest. Looking at two variables known to contribute significantly to low total fertility among the Gainj, late age at first birth for women and long interbirth intervals, the paper suggests that households, in their recruitment and allocation of labor, may exert a mediating influence in the relationship between economic development and fertility. 相似文献
87.
Sertoli and Leydig cell numbers and gonadotropin receptors in rat testis from birth to puberty 总被引:4,自引:0,他引:4
Marino Bortolussi Renato Zanchetta Paola Belvedere Lorenzo Colombo 《Cell and tissue research》1990,260(1):185-191
Summary In testes of rats from 2 to 60 days of age, we examined the number of Sertoli cells (SC) and Leydig cells (LC) as well as the binding of radioiodinated gonadotropins to frozen sections and homogenates. The number of SC per testis increased only during the first 2 postnatal weeks, whereas that of LC was stable up to days 7–10 and increased thereafter. The uptake of 125I-labelled human follicle-stimulating hormone (125I-FSH) to frozen sections was confined to sex cords or seminiferous tubules, while that of 125I-labelled human choriogonadotropin (125I-hCG) matched the distribution of LC in the interstitium. High affinity receptors for FSH and hCG were found in homogenates at all stages studied. The number of FSH receptors per testis increased steadily, whereas that of hCG receptors was low until days 7–10 and rose afterwards. Thus, SC in rat testis appear to proliferate in the presence of fetal LC during the first 2 postnatal weeks and to differentiate concomitantly with the emergence of the adult LC generation after day 10. The complement of FSH receptors in SC remains constant as they proliferate and increases after day 21 as they differentiate. The hCG receptor number is relatively fixed in each LC generation, being higher in adult compared to fetal LC. 相似文献
88.
Hair morphology was exammed on developing seedlings and mature plants of ten species from five of seven series inPhysalis. The taxonomic importance of hair development and the distribution of hair types then was evaluated with respect to other comparative data. Two classes of hair types were observed, as found in earlier investigations ofSolanum andRhododendron. The first hair class comprises a sequence from unbranched hairs to dendroid-stelliform hairs. Hairs of this class show considerable variability among taxa in size, presence or absence of glandular tips, and presence and degree of branching, and so were useful in making taxonomic comparisons. The second hair class consists of short hairs, each tipped with a multicellular gland. This second hair class was not useful taxonomically because of its lack of variability in morphology and its often rare occurrence on a plant. The taxa studied exhibit distinctive trichome features which serve to distinguish series and, in some cases together with other data, provided new insight into previously enigmatic species relationships. 相似文献
89.
Summary The ability of embryos at different developmental stages to form plants in vitro has been studied in cultivated Cucumis sativus L. and in the wild species C. zeyheri 2 x Sond. and C. metuliferus Naud. On MS medium containing 3.5% sucrose, 0.1 mg 1–1 kinetin (Kn) and 0.01 mg 1–1 indoleacetic acid (IAA), proembryos (0.03–0.05 mm) and early globular embryos (0.05–0.08 mm) from the wild species developed into plants in low frequencies of 8% and 21%, respectively. These embryos should be surrounded by the embryo sac tissue. On the same medium late globular (0.08–0.1 mm) and early heart-stage embryos (0.1–0.3 mm) developed into plants in moderately high and high frequencies of 48% and 83%, respectively. The presence of the embryo sac at these stages was still beneficial, but no longer a prerequisite. Late heart-stage embryos (0.3–0.8 mm) also showed high frequencies of plant formation, 63%, if Kn was applied at a concentration of 1 mg 1–1. From the early cotyledon stage onwards, the frequency of plant formation gradually decreased, reaching a minimum at the late cotyledon stage. Subsequently it began to increase again up to the late maturation stage. The poor plant formation shown by the intermediate-aged embryos could be improved slightly by lowering the sucrose concentration to 0.5% and by increasing the Kn concentration to 10 mg 1–1. Relative to the wild species, embryos of C. sativus showed lower percentages of plant formation. The optimum sucrose concentration was 2% for the heart-stage C. sativus embryos. In all three species the ability to form plants strongly decreased with increasing embryo age, from early to late cotyledon. This is thought to be caused by the increasing tendency of the embryos at these stages to continue in vitro the normal embryo development. 相似文献
90.
Summary We raised monoclonal antibodies against a membrane fraction ofXenopus neurulae in order to detect tissue-specific cell-surface markers. Here we describe a monoclonal antibody that recognizes an epithelial membrane-associated antigen (EMA) in immunohistological stainings. The tissue-specific and membrane-associated antigen detected in immunohistological stainings could serve as useful marker in epithelium differentiation and membrane organization of the early embryo. In tadpoles and adults EMA was found in specific epithelial tissues derived from different germ layers such as kidney, skin, gut, pancreas, epiphysis and choroid plexus. In the cleaving embryo this antibody stained newly formed membranes between blastomeres from the two-cell stage onwards. Cytoplasmic staining in large oocytes and early embryos was also observed. The possibility that the cytoplasmic signal represents a maternal store of membrane material is discussed. 相似文献